Fuses are not blown so what is next to check?

Tiny
CADEPHILLIPS
  • MEMBER
  • 1966 CHEVROLET TRUCK
  • 100,000 MILES
Several issues but maybe all are connected because they all started last week. Noticed no dash/panel lights or interior light one day, the next day noticed no brake lights or tail lights. However, the headlights, front parking lights and signal lights (front and rear) still work. Tested all fuses in the block and only the panel (3A) did not work even though it did not appear to be blown. Here's where it gets odd for me: All the fuses (except the panel) make my tester light up until I pull the switch to turn on the headlights/tail lights, then the tail and brake light fuse do not make the tester light up. So what is next to check? Switch?
If the switch has gone bad I could see how it would shut those lights down but then why would it also stop the brake light fuse from lighting up? And if not the switch then what? The truck had run fine and had just gotten a state inspection the previous week.
any help would be greatly appreciated.
Cade
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Wednesday, March 3rd, 2021 AT 6:44 PM

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Tiny
CARADIODOC
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I don't have access to diagrams for your truck, but I think we can figure this out. First of all, you said you checked the fuses, but you didn't say how. It sounds like you were checking for voltage on both sides. That will work with glass fuses. When it comes to measuring voltages at other places in the circuits, put the digital voltmeter aside if that's what you're using, and use a test light instead. When you find confusing results, as you mentioned, it is usually due to a very high resistance in the circuit. No current can get through that resistance to run the circuit, (bulbs and motors in particular), but if the circuit is switched off, and then you take a measurement with a voltmeter, those require almost no current flow to do their thing. The tiny tickle that's needed can get through that resistance, so a voltmeter will incorrectly say you have 12 volts there, but if you measure with a test light, those do require current to operate. That makes them more accurate for this type of problem.

Find the fuse for one of the dead circuits, then check for voltage on it with the test light. While holding the probe there, turn that circuit on with the switch. If you find no voltage all the time, we have to work back toward the battery. Same if you do find voltage that drops to nothing when the circuit is switched on. If the fuse for your dead circuit has 12 volts all the time or when it is switched on, the defect is after that point.

Another good place to find that high resistance is in the contacts of the head light switch or in the connector terminals for it. That can also occur with dimmer switches, especially those mounted on the floor where they're susceptible to water. Tell me what you find up to this point.
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Wednesday, March 3rd, 2021 AT 7:17 PM
Tiny
CADEPHILLIPS
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Sorry I only have a short time at work to respond, I will get back again after I get off work. However I did use a cheap tester light to test the fuses from Walmart to test the fuses. And on that old C10 Chevrolet it is the headlight switch or knob that you pull out to engage the headlights and tail lights. If you twist that same knob it turns the interior light on or the dash lights down.
I will see if I can figure out how to send a picture when I get back to the truck.
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Thursday, March 4th, 2021 AT 12:37 PM
Tiny
CADEPHILLIPS
  • MEMBER
Thought. Is there a possibly of some kind of grounding issue? I know the headlight switch grounds by tightening it to the dashboard and it has always worked and it is not loose at the moment. Is there another grounding wire somewhere that I could look for to see if that may be loose.
But now that I think about it if there was a loose ground somewhere would the test light light up at all?
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Thursday, March 4th, 2021 AT 12:52 PM
Tiny
CARADIODOC
  • EXPERT
Back then typically all the lights were powered through one large wire that went through the bulkhead connector on the firewall, then it went to multiple fuses in the fuse box, for each circuit. The brake light switch feeds two circuits that first run through the signal switch, then two wires run to the back. Most of the time each rear corner has its own ground for all of the bulbs on that side. Next, the headlight switch turns on one circuit for the headlights, another circuit for the running / tail lights, and tapped off that is the rheostat for the dash lights. All of those circuits have their own dedicated grounds, so it is unlikely all of them corroded off at the same time. It would be different if it was just one circuit we were concerned with, but with so many dead circuits, we have to start with what they have in common, and that's the 12-volt supply.

I'm trying to locate a wiring diagram because for now, I can't remember if the fuses come before the the 12 volts gets to the switches or after. That will determine whether or not there's 12 volts on a fuse before you turn that circuit on. This is where I should be able to figure it out when you tell me what you find on those fuses.
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Thursday, March 4th, 2021 AT 1:26 PM
Tiny
CADEPHILLIPS
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Hopefully you can see on the video that when I pull the knob out for the headlights the tester light goes out and that also happens on the fuse directly above where I have the tester now which reads tail. So something about pulling the one knob kills 2 circuits at once.
I may have mentioned this before but that 1 knob turns on and operates many things. The headlights the tail lights front parking lights also the panel lights with a twisting dimmer.
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Thursday, March 4th, 2021 AT 4:56 PM
Tiny
CARADIODOC
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This is typical of a high-resistance connection. Enough current can through to operate the tester, but not to run the lights. That's why the voltage will drop any place after the defect when you turn on the switch.

Think of a garden hose that you're standing on. If the nozzle is closed, you'll still have full pressure there, but open that nozzle and try to get water to flow, and the pressure will drop to almost nothing. Only a little water will dribble on your shoes.
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Thursday, March 4th, 2021 AT 7:12 PM
Tiny
CADEPHILLIPS
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Okay, so then what would be the next step to get this working? Replaced the whole switch or tried to figure out why only parts are working and other parts are not?
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Thursday, March 4th, 2021 AT 8:16 PM
Tiny
CADEPHILLIPS
  • MEMBER
Is is possible to use the tester on each of the wires that come out of the headlight switch and maybe that way see if something is loose or bad?
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Friday, March 5th, 2021 AT 9:34 AM
Tiny
CARADIODOC
  • EXPERT
I need voltage readings. We have to know the voltages at various places to figure out the location of the defect. To be valid, those have to be taken with the circuit turned on so there is a load on that circuit trying to cause current to flow. This is also why I want you to use a test light instead of a voltmeter. Test lights need current flow to operate, so they will give a more accurate result. We don't care about exact voltages like a digital voltmeter would provide. We're just interested in whether the test light is bright, dim, or off.

Remember, you found the brake lights aren't working. Those have nothing to do with the head light switch, so forget that for now. Unless you have two totally separate defects at the same time, we have to look for what all the dead circuits have in common, and that's the 12-volt feed wire coming through the bulkhead connector, and the "buss bar" connecting multiple fuses together in the fuse box. Check for voltage on both sides of those fuses and tell me which ones have 12 volts all the time, which ones never have 12 volts, and which have 12 volts until you turn the switch on for that circuit, or have it only when the switch is turned on.
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Friday, March 5th, 2021 AT 12:47 PM
Tiny
CARADIODOC
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I'm searching online for a wiring diagram I can read. In so doing, it looks like your truck uses a six-volt electrical system. If that's the case and it hasn't been modified, check for six volts where I previously said "12-volts".
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Friday, March 5th, 2021 AT 1:11 PM
Tiny
CADEPHILLIPS
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Alright I will probably only be able to do this tomorrow. Work ran late but so I'm clear you want me to test each side of the fuse or take out the fuse and then test each side and see what lights up the tester and what doesn't?
Oh also, to my knowledge Chevrolet switched over to 12 V in the middle fifties so I'm pretty sure this was a 12V system all its life.
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Friday, March 5th, 2021 AT 5:47 PM
Tiny
CARADIODOC
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Wonderful. One of the diagrams I found for a '66 Chevy truck showed a 6-volt battery, which really didn't seem right.

Yes, the fuses must be in place. I typed up a two-page explanation of what we're trying to do by comparing it to water flow in a hose. The comparison helped a lot of my students in the past. I'll post it if you want me to and you think it might help.
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Saturday, March 6th, 2021 AT 1:11 PM
Tiny
CADEPHILLIPS
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Sounds like a good idea. Anything will help.
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Saturday, March 6th, 2021 AT 4:21 PM
Tiny
CARADIODOC
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The reason for having you use a test light is all the voltage readings are only valid when current is trying to flow in that circuit. That means switches have to be turned on and fuses have to be in place. If it helps, everything electrical that you can't see can be compared to something with water that you can see or visualize. Think of your municipal water tower as the battery. It stores water under pressure from gravity. Batteries store electrical energy under pressure, chemically. The pipes are the wires and the valves are the switches. The only point of confusion is "open" and "closed" are exactly opposite. An open valve lets water flow, and an "open circuit", or "open in a circuit" is a break that stops current flow. A "closed circuit" is one that is turned on and there is a complete path for current to flow from the battery, through the wires, switches, connections, splices, loads, then back to the battery.

Water also needs a complete path, as in current flows down a river into the ocean where it evaporates, floats back up in a cloud that gets ripped on a church steeple, resulting in rain. It trickles back to puddles, storm drains, creeks, and back to the river. If you put a giant sheet of plastic over the ocean, evaporation stops, current flow stops, and you have an "incomplete circuit". No current flows anywhere.

Now visualize a water pipe running to your house, then to an outdoor faucet. A garden hose is connected to the faucet, and a second hose is added to make it longer, then there's a nozzle at the end. With the nozzle closed and no water flowing, you'll have full system pressure everywhere up to that nozzle. Lets say that's 50 psi. If you were to stand on the hose and block it 99 percent, you'd still have 50 psi at the nozzle. That is the voltage you would read with a digital voltmeter and with a test light. In your truck, the typical scenario is you'd find 12 volts at both sides of every fuse, all the way up the various switches, and possibly a number of test points after the switches when they're turned on.

Now open the nozzle while you're still standing on the hose. With it 99 percent blocked, you can see not much water will flow out. Pressure at any point after your foot will be real low, as in almost 0 psi. Pressure before your foot will still be a nice strong 50 psi. In this sad story we already know where the restriction is. It's caused by your foot. What if you don't know where the restriction is located? It could be caused by the hose is kinked where it goes around a planter, the leg of a picnic table, or it just had a twist in it, then you tugged on it. The same pressures apply. If you don't mind poking a lot of holes in the hose, you could stick a pressure gauge in it at various places. Remember though, the nozzle has to be open so water is trying to flow. Then, if you find 50 psi at a test point, the blockage is after that point. Keep working your way down the hose until you come to a point where pressure is near 0 psi. The blockage is between that point and the last one where you found 50 psi. If the nozzle is turned off, you'll again have 50 psi before and after the blockage, so the test is useless.

Removing the fuse is equivalent to turning the faucet off, or closed. I have to stretch my story a little here. We know some water pressure will be stored in the hose. Forget that. Pretend that doesn't happen. With the faucet turned off, no water is trying to flow, and there will be 0 psi everywhere in the hose, before and after the blockage, whether the nozzle is open or closed, so pressure tests will be invalid.

Now lets get back to the electrical circuit. Amp meters, or "ammeters" measure current flow and they have to be placed in "series" in the circuit so current flows through them. This is similar to your water meter on your house. All of the water you use flows through that meter. Those have no place in this story. I only mentioned them to point out their difference. We're using a voltmeter which measures electrical pressure. As such, no current flows through the meter. If you live in the country and have your own well and water pump, there will be a pressure gauge on the storage tank. No water flows through that gauge. If it did, you'd have a wet floor. The same type of gauge is used on an air compressor. No air flows through that gauge.

You have blockage in the circuit running to your brake lights. We know that because they don't light up when the brake light switch is turned on. As with the hose, we're going to find that blockage with pressure readings; in this case electrical pressure, or voltage readings. The faucet has to be turned on. The battery and fuse must be connected. The nozzle has to be open. The switch has to be turned on. Water must be trying to flow. Current must be trying to flow. Water pressure will be much lower after the blockage. Voltage will be much lower after the defect.

People who like working with their hands learn best by observing, and by manipulating things to see what happens. As such, we have a very hard time learning electrical theory because we can't see it. That's why so many mechanics run from electrical problems. Comparing everything to something corresponding with water, I never had a single student I couldn't get to understand basic electrical diagnosis. Simply understanding that people learn best in different ways takes a lot of the fear out of this subject.

To finish this story, the faucet is the fuse. The nozzle is the switch, and it's turned off. No water is trying to flow. There's still that blockage somewhere in between. If you were to connect a pressure gauge right at the nozzle, it will show 50 psi, so you'd incorrectly think everything is okay up to that point. That's what a voltmeter does. It will show 12 volts even though there's a spot of very high resistance somewhere that is preventing the circuit from working. The test light is different. Unlike the pressure gauge and the voltmeter, current does have to flow through it to make the light bulb glow. A bright test light indicates we have something close to 12 volts, and current is able to get through that far. This tells us the blockage is after that point. The voltmeter can't do that.

The only time the test light and the voltmeter will give the same correct readings is when there's a total break in the circuit. That break might equate to the second garden hose being disconnected from the first one. There would be 0 psi everywhere in the second hose. Same with a cut wire or a switch that's turned off. You'd find 0 volts everywhere after those points whether you used a voltmeter or a test light.

The reason I have you start with a test light is at first we don't know if you have a solid break or a high-resistance connection, as in a corroded splice or arced and pitted switch contacts. We aren't interested in the exact voltages a voltmeter will give us. A bright or dim test light is good enough, plus it puts a load on the circuit that causes current to want to flow. That is why they can be more accurate. Part of your description of the symptoms included finding different voltages at a point depending on what you did with some switches. That is exactly what points to a high-resistance point in the circuit. That is what we have to find and repair. Now you see why the test light works for that and the voltmeter doesn't.

To add one more chapter to this story, a lot of vehicles use "fuse link wires" instead of really large fuses. Those are simply a small section of smaller-diameter wire spliced into a larger wire. That makes it the weak link in the chain. It's a slow-acting fuse, and when the wire does melt, the insulation is designed to not burn or melt. A good one will act like a piece of wire when you tug gently on it. One that's burned open will act like a rubber band. The confusion they can cause is when they burn open, the arcing leaves a carbon track behind on the inside of the insulation, similar to what can develop inside a distributor cap. If a short occurs, the wiring is protected by that fuse link. No current can get through to run the circuit, but once the short is repaired, that carbon can still pass just enough current for a voltmeter to incorrectly say there's 12 volts there. I was involved with a very experienced transmission specialist who couldn't understand why he had 12 volts at the connector, but when he plugged in the brand new radiator fan, it wouldn't run. Simply switching to a test light showed he did not, in fact, have 12 volts there. The fuse link was burned open from the old fan motor which drew high current due to tight bearings. Replacing the fan motor was only half of the repair. It ran fine once the fuse link wire was replaced.
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Monday, March 8th, 2021 AT 1:38 PM
Tiny
CARADIODOC
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All of the diagrams I've found so far have been very disappointing. Most are too blurry to read, and some don't lend themselves to understanding where current flows and what is connected to what. This is the best one I found so far. I'll explain how I figured some things out up to this point. If it's too hard to read, try copying it, then pasting it into an MS Word typing program. I selected "200%" to make it big enough to read.

We know the license lamp is tied in with the running lights. That's the brown wire, (brown arrow). It is spliced in at the left rear light socket. This will be the common 1157 socket with two contacts. The outer brass shell is the ground. They show it as a very tiny ground symbol at the bottom of the socket. Since we know the top contact is the tail lights, the bottom one has to be for the left turn signal and brake light. That's a yellow wire, (yellow arrows). If you follow that wire to the left, it comes to the two halves of a three-wire connector, (red arrows). From there it continues toward the cab, but I can't find that part of the diagram. Naturally that's the one we need.

The brown wire for the tail lights also goes through that connector. Then there's the third wire; a dark green one, (green arrow). That goes up to the right rear socket for the brake and signal light. They don't say where that connector is located, but if you can find it, that would be a good place to take some voltage readings. Remember, it has to remain connected so the circuit isn't broken. Probes can usually be poked in on the back side next to a wire to touch the terminal and take the reading.

For your particular truck, it doesn't pay to find that connector because we know the defect has to be well before it. You're having issues with the dash lights, and you've already figured out voltage is dropping at the fuse box when a switch is turned on. All of that comes before that connector, and before the rear lights.

Go back to the fuses with your test light. Find one where the test light turns on bright, then goes dim when you turn on a switch. With it dim, check the other side of that fuse, and I'm pretty sure you're going to find the same thing there. See how many fuses that applies to and see if you can read the labels under them.

What I didn't do a good job of explaining earlier is I'm hoping with your voltage readings I can figure out which comes first in the circuits. Most commonly a 12-volt wire comes through the bulkhead connector, then into the fuse box where it feeds multiple fuses. The other side of one fuse will go to the brake light switch. The second side of a different fuse will go to the head light switch. The disadvantage of doing it that way is if you have simply a shorted wire going to a rear tail light, that will blow the fuse, then everything including the head lights will be dead.

On newer models, while there is usually a very large fuse under the hood, just in case, the 12-volt wire still goes inside the cab, but it goes to the switches first, then each circuit coming out goes to its own fuse. Today you'll often see separate fuses for each head light, and others for the tail lights. This way regardless which circuit is shorted, only that fuse will blow, and all the other lights will still work. Overall it's more complicated, but the blown fuse tells you which circuit and which wire to follow to find the defect.

The last two photos show two different fuse boxes for your truck. Can you see the bulkhead connector and how to remove the fuse box? Voltage readings haven't verified this yet, but I have a suspicion that's the area we're going to find the defect.
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Monday, March 8th, 2021 AT 2:26 PM
Tiny
CADEPHILLIPS
  • MEMBER
Well I had a long response typed out and now it's disappeared. If its allowed, I can always give you my phone number, and this will be so much quicker. Thank you for that long response it really does explain a lot. In my long, somehow deleted response, I explained about taking my truck to my brothers so he could tried to help diagnose it also. He agrees that there is some kind of "load being drawn" when you pull the light switch. But we still can't determine where it starts. He thinks it is the switch but I agree that I need to take the fuse Box off to see behind it maybe that will shed more light. I will need to get a different test light because mine seems to have an LED light so it is always the same brightness. But I will work on getting you that information about the fuses.
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Monday, March 8th, 2021 AT 5:53 PM
Tiny
CARADIODOC
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I used to have a computer that would lock up on occasion or I'd hit the wrong key and lose everything when it went to a different page. That was real frustrating after typing for a long time. It appears the site owners changed something some time ago, because now, if I just hit "page back", after a few seconds all my typing comes back. Even if I turn the entire page off, then go to my "History" page to reopen it, the typing is usually still there. So don't fret; I know what you're feeling. Sometimes pages fail to load due to a poor wireless connection, so I'm still in the habit of copying every reply just before I post it. That way, if it gets lost, I just click "paste", and it's right there again. Let me know if you don't know how to copy and paste. It's real easy to do. That could save you lots of aggravation in the future.

I get the feeling you're making this more complicated than it needs to be. Lets do it this way. Lets start with the brake lights. If you can identify the fuse for that circuit, pull it out, then check for voltage on both terminals in the fuse box. You're either going to find 0 volts on both, or 12 volts on only one of them.

If you find 12 volts on one fuse terminal, that tells us it comes in there first, then goes to the brake light switch

If I'm wrong, and you find 0 volts on both fuse terminals, check again while holding the brake pedal down. Now if you find 12 volts on one terminal, that tells us the 12 volts comes in to the switch first, then it goes to the fuse, and then to the lights. I don't think that's what you're going to find because that would leave more of the circuit unprotected by that fuse.

Assuming the first way is right and you do find 12 volts on one fuse box terminal, put the fuse back in, then measure the voltage on it with a digital voltmeter. You should see 12 volts. Now watch what happens to that voltage when you press the brake pedal. If the voltage drops real low, the defect has to be in the back of the fuse box, or in the wire coming through the bulkhead connector and feeding those fuses.
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Tuesday, March 9th, 2021 AT 2:10 PM
Tiny
CADEPHILLIPS
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So I'm finally to make a small bit of progress. Progress because I found a big problem but I realized that fixing it might not solve the entire problem because I believe there are now more issues that will come up. But the big story as you can see from the picture is that there is definitely a problem with that burned up connection. That supplied the bottom 2 fuses and I can only assume that when it was screwed to the fire wall there was enough pressure to keep a connection to light up my test light. And make my Volt meter read 12.7 but when I applied the brake my test meter went down to 5.4.
I will get that fixed and hopefully everything will work as it should. The other problem/issue that may arise might come from if I use the correct amp fuse as listed on the fuse block. I discovered that both those bottom fuses instead of being 15 amps each were 30 each.
So if I simply fix the wire issue it seems apparent that it could happen again at some time. If there were a short somewhere else in the line would that have caused it to burn at this week point? Meaning that if I replaced the fuses with 15 amps as it should be and I plug everything back in they would instantly blow if there was some other problem? I'm assuming you'll say it needs to be whatever the fuse block says however I'm starting to not trust where everything was wired. There just seems to be a lot of non stock-ness in the wiring, but what do I know. Does a 15 amp fuse sound correct for a brake lamp? I guess I always thought it would be more than that. That's sort of why I'm 2nd guessing it. What do you think? Hope all this make sense didn't have a chance to proofread it.
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Thursday, March 11th, 2021 AT 6:03 PM
Tiny
CARADIODOC
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These are excellent photos, but it's hard to tell some things when you're not looking at a live 3D object. I copied them into a typing program where I could blow them up, but even before that I see some suspect areas. You're right that wire nuts and crimp terminals are not original.

If you look at the first photo, I added arrows to the suspect areas. The red arrow is pointing to what looks like a wire that's corroded off the terminal. That looks like tape residue at the blue arrow. Be sure that isn't two wires that overheated and melted together. Lots of rust at the rivet by the purple arrow. Where a wire / terminal is connected to metal strips with two and three rivets, those have to be the 12-volt feed sides of those fuses. The "load" side, or wires leaving to go to their respective circuits, will always have just one fuse terminal for each wire, never two or more. That tells us the purple arrow is pointing to one of three fuses all fed from the same wire. The green arrow is pointing to just such a single wire, the yellow one. To say that a different way, if the fuse is blown, you'll still have 12 volts by the purple arrow, and the two-rivet strip to its left, but not on the yellow wire, as an example.

That terminal for the yellow wire is badly rusted too. That should be addressed even if it isn't the cause of the current problem.

Most people would simply repair or replace the rusty terminals, then look to see if everything works. I'd rather know exactly what is causing the problem, then solve it. There's three ways to do this. I want to be sure to confuse you unnecessarily, but each way might be the best choice at different times. The least-favorite way is to start at a dead light, (this is always with the circuit turned on and trying to work), with the test light, and you'll find you do not have 12 volts. Now move it back one step which might be at the three-wire connector we looked at a while ago. You find less than 12 volts there, so move back another step. Keep moving back toward the battery until you find the spot where you do have 12 volts. The defect has to be between those last two points. The disadvantage here is it could take you all day to finally get to the defect when it's near the battery.

My preferred method is to find a spot in the middle of the circuit and start there. If you have 12 volts, you just eliminated half of the circuit, and the defect has to be in the other half. Now take that second half and split it in half again. Check for 12 volts at that point. The voltage reading will tell you which way to continue the search. This is the fastest method because you will jump over the good parts and not waste time looking for convenient test points where there's no need to test.

The easiest method to describe is the third one. Start at the battery where you'll find 12 volts, then work your way along the circuit until you find the spot where the voltage is too low. You're just locating the defect from the opposite end of the circuit as you did in the first method. In this case, look at the second photo. I might be using the wrong feed wire for this problem, but it lent itself best to this explanation. Once again the circuit has to be turned on for the voltage readings to be valid. The 5.4 volts you found is proof there's a high-resistance connection, and we're going to find it now.

When you take the following readings, you have to be very specific where you place the meter's probe. Start at the red arrow with the probe on the terminal that's crimped to the wire. What'cha got for voltage? Now move the probe to the mating terminal, (blue arrow) and see what you have there. I should point out that you may need to poke hard enough to insure the probe is making good contact with the part you're testing at, but we want to be gentle enough that we don't inadvertently cause something to start working. If the bad connection starts working, there won't be a defect to find, then we'll give up and the problem will occur again days or weeks later.

Next, we can move the probe to the metal strip, (green arrow), or the rivet head, (purple arrow). GM likes to put convenient accessory terminals in their fuse boxes, and I suspect that is where that metal strip is bent up by the green arrow. I think you'll find the rivet is holding the fuse terminal to the metal strip, with the plastic fuse box in between. That rivet is an excellent place to find a bad connection. Typically you'll find a black ring arced around the rivet head, just barely noticeable. Often the way we see them is some minor sparking occurs there when the parts are moved a little or have pressure put on them.

If the green arrow is simply pointing to a tab that isn't used, the rivet head would be the next place to test. If you still have 12 volts up to this point, double-check on the other side of the box right on that fuse terminal. You found low voltage there earlier. That means you will have had to cross over the defect by this time. When you hit the first place with 5.4 volts, the defect is between that point and the one you tested right before it.

On the odd chance you do have12 volts all the way to both sides of the fuse, and the lights still aren't working, the high resistance has to be someplace after that. For the brake lights, the next place to check would be both wires at the brake light switch. If one has 12 volts coming in and the other has 5.4 volts going out, that switch has burned or arced contacts inside and the switch must be replaced. Yu can prove that by using a stretched-out paper clip or piece of wire to jump those two wires together to bypass the switch.

The next place in the circuit is the wire from the brake light switch goes out to the signal switch. Those can also develop burned contacts inside, but that usually only affects the lights on one side, not both sides. After the signal switch, the brake / signal lights are on two different circuits, so assuming we only have one defect, we know the problem can't be after that switch.

Before you get to the signal switch, for the sake of putting the truck together on the assembly line, there is going to be an electrical connector near the base of the steering column. It used to be common to see that connector black and / or melted around a pair of mating terminals. Those connectors are always open on the back sides so you can slide the probe in alongside the wires. Check the voltage on both sides of the connector. The wire you want is the same color as the one that left the brake light switch, usually white on GM vehicles, but you don't even have to know that. Just check each pair of terminals. If a pair has 0 volts on both sides, that circuit is turned off. If they have 12 volts on both sides, there's no defect there, regardless what circuit they're for. You're looking for a pair that has 12 volts on one side and 5.4 volts on the other side. If you do find the defect is in that connector, standard practice was to cut both wires off, splice and solder them together, then seal the splice with heat-shrink tubing. You'll only have to cut them apart again if you need to replace the steering column, or possibly the signal switch.

Related to those overheated terminals, or when that happens in a switch connector, the wires will be hardened for about four inches from that heat. Solder won't adhere to that, so you'll want to cut that four inches off each affected wire, then splice in a short length of the same diameter if necessary.
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Friday, March 12th, 2021 AT 11:05 PM
Tiny
CARADIODOC
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To address your question on fuses, the common 1157 brake / signal light draws one amp, so there'd be two amps flowing through the fuse when the brake lights are on. Same for a left rear and left front signal light that are on at the same time. The smaller tail light filament draws around 3/4 amp. One in each corner, with no side marker lights, would draw close to a total of three amps, plus a little for the dash lights. Side marker lights, and the common 194 "peanut" bulbs used in instrument clusters and glove boxes draw close to a half amp each. A 15-amp fuse will be fine for those circuits, even if you connect a trailer.

If someone put in a larger fuse, it's still going to blow if there's a dead short in that circuit. The issue is what part of the circuit is going to be the weak link in the chain when too much current is flowing, but there's no dead short? Ever see one of the semi trucks with a gollyzillion orange lights all over it? The original circuit isn't going to handle that. Neither the fuse nor the wires can handle that much current, so that has to all be a custom setup, probably switched on and off with a relay. That would be an example of increasing the load on the circuit but there's no dead short. Suppose you had lots of additional lights on your truck, and in total they drew 25 amps. Obviously the 15-amp fuse would blow, but a 30-amp fuse would not. 14-gauge wire can handle 15 amps, and that is what is commonly used in cars and trucks for lighting circuits. That's why they get protected with a 15-amp fuse. If that circuit was asked to pass 25 amps, especially for a prolonged period of time, the wire would get hot enough to possibly melt the insulation. It would also tax the connector terminals and switch contacts. Those are also likely to overheat. Remember, that's if you added a lot of stuff to the circuit. If the circuit is left as designed originally, it will still work fine with a 30-amp fuse, and that fuse should blow to protect the wires from a dead short. What would be of concern is what if the wire has enough resistance in it that by itself would limit current to less than 30 amps? That would prevent the fuse from blowing and it would allow the wires to overheat. To put my mind at ease, I would want to put the 15-amp fuse in there if that's what was in there originally.

A better example of what can happen is when the bearings in the heater fan motor become tight. I can explain why if necessary, but for now, all we need to know is as a motor is loaded down, as tight bearings will do, it draws more and more current. We see that as the thermal fuse built into the speed control resistor assembly burns open, then the fan is dead. Many people replace the resistor assembly, then are confused and frustrated when it burns open again in a few days or weeks. The cause is that tight fan motor, but it is not shorted. Were it not for fuse devices of the appropriate current rating, that high current would stress the heater fan speed switch, and on most vehicles older than those from the late '90s, that current also passes through one section of the ignition switch. We used to see a lot of overheated connector terminals on ignition switches related to tight fan motors.

Most people will just tell you to put the right fuse in, but now you now why.
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Friday, March 12th, 2021 AT 11:14 PM

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