Combination Meter fuse blows

If I understand correctly, you always had one of the three connectors plugged into the cluster, so you are assuming the problem is in the cluster. What happens to the fuse when all three are unplugged? If it still blows, you need to do something other than buying fuses in bulk.

A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a twelve volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it is not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.

No. To be sure you understand what I meant, you are installing a twelve volt light bulb instead of a fuse. How you do that is up to you, but for convenience, I use two small jumper leads connected to a 2057 brake light bulb. The other end of the leads can be clipped right to the terminals in the fuse box, if you can get them to stay on. Plugging in a spade terminal into each one just makes it easier to connect the jumper leads.

Now imagine you use those two leads to connect a light bulb right to the two battery terminals. The bulb will light up full/normal brightness and its resistance is what limits current to about one amp. Now, if you add anything into that simple circuit that adds resistance, current must go down, and the bulb will not be quite as bright. Well, suppose what you add is a piece of wire. For purposes of this sad story, it adds no resistance to the circuit, so the bulb will still be full brightness. Instead of a piece of wire, suppose you add an instrument cluster with a short, to the circuit. Current is still limited by the bulb's resistance, and it will still be full brightness. The point is current is still limited to one amp. You are not blowing more and more fuses. The brightness of the bulb tells you current is still flowing through something in the circuit that is shorted. Nothing will be damaged, but when you do something that isolates the short from the circuit, the additional resistance of anything else in the circuit will cause current flow to go down, and the bulb's brightness will decrease.

My guess is the bulb will go out completely when you unplug the cluster. It all depends on what else is protected by that fuse. There really is not much on an instrument cluster than can short and draw enough current to blow a fuse. It is more likely current is going into the cluster on the fused wire, through some circuitry, then out on a different wire to something else that is shorted. What is surprising is that short would appear when more than just one connector is plugged in. Reply back when you have the test bulb hooked up and it's indicating a short, then I will dig up a wiring diagram to see if I can identify any good suspects.

Absolutely. I just typed for over an hour, and with one magic keystroke with this laptop pile, it's all gone! There's no "undo" button, and I'm much too frustrated right now to start typing all over again.

Do not try to use your test light. It's too small and will limit current to too low a value. Even with no short in the circuit, the bulb will be near full brightness, so you won't have the indication you're looking for when you do something that isolates the short.

Dandy. Now unplug the cluster and see what happens.

I've been retyping my wondrous reply from yesterday. I'll post that when I'm done.

Okay; I'm calm now. Lets try this again.

There must be a complete path from and back to the battery for current to flow through the circuit. That includes the positive battery cable, wires, fuses, connector terminals, switches, the load, (bulb, motor, etc.), More wires, then back to the battery's negative post. The ground you asked about is the part of that path that comes after the load. It includes wires that bolt to the body sheet metal, the negative battery cable, and it can include switches and connectors. Anything that breaks that circuit anywhere will cause current flow to stop. That could be an open, (turned-off) switch, a corroded connector terminal, a cut wire, blown fuse, or burned-out bulb.

A circuit that is protected by a ten-amp fuse can be expected to draw perhaps two to five amps. A brake light bulb has close to 12 ohms of resistance. If we use one of the twelve Ohm's Law formulas, 12 volts divided by 12 ohms equals one amp of current flow. To show that more clearly, imagine connecting the bulb right across the two battery posts. The bulb's 12 ohms is what limits current flow. The fact I impressed you with one of the Ohm's Law formulas is to make this description easier to follow. In practice, we never sit and calculate things when looking for a problem.

So now you have a 12-volt source, (the battery), and we use up that 12 volts across all of the loads. In this case the light bulb is a load, and it uses up all that 12 volts. To be technically correct, each of the connecting wires is also a load, sort of, but since they each have 0 ohms, none of the 12 volts is dropped across them. It's all used up across the bulb.

Now suppose we add another piece of wire in series. That's the same as making one of those wires longer. We still haven't added any resistance, and the bulb is full brightness. That wire has 0 ohms, but so does a short circuit. Instead of inserting that additional piece of wire, we inserted something that was shorted to the negative battery post, (ground). Now the entire circuit consists of the battery's positive post, a wire, the bulb, a wire, another wire that is touching the battery's negative post, and it is attached to that post. Everything in this circuit has 0 ohms of resistance, ... Except for the bulb. The bulb is the only thing limiting current flow to one amp.

Now lets move this to your vehicle. We still have the battery, the wire that goes to the bulb, (which is in the fuse socket), another wire, then to the unintentional short to ground, and then back to the battery's negative post. The bulb is the only thing with resistance, so it is the only thing that is limiting current flow to one amp. All of the battery's 12 volts is being used up, or "dropped" across the resistance, (the bulb).

If we were to remove the bulb and replace it with the fuse, that has no resistance, so there would be 0 ohms in the entire circuit. Ohm's Law says 12 volts divided by 0 ohms equals infinite amps. That would certainly cause a ten-amp fuse to blow! This is the condition you have now.

Keep in mind these numbers are perfect and only good for explaining theory. In actual practice, the wires, the fuse, even the battery, have a very tiny amount of resistance, but knowing that won't help my story.

If your circuit was working properly, there would be no short after the fuse. Current would have to travel through the desired load, then back to ground and the battery negative post. The intended load has the resistance that is supposed to limit current to a safe value. For the sake of this sad story, lets assume the wire after the fuse socket, (light bulb), has rubbed through and is touching ground. That short bypasses the desired load, which at this point appears to be the instrument cluster. Only the bulb limits current flow. Since it has the full 12 volts dropped across it, the bulb is full brightness.

During your troubleshooting, you find a connector at the firewall where the wire between the fuse socket and instrument cluster passes through. When that connector is unplugged, the test bulb turns off. That tells us there is now no path for current to get back to the battery. Whatever is shorted has to be beyond that connector. The clue here is the bulb went out completely.

Plug that connector back in, then continue on. Unplug the instrument cluster, and behold, the test lamp goes off again. The short is either in the cluster and something is creating a direct path to one of its ground wires, or the cluster is feeding a wire that is going to something else that is shorted to ground. At this point there are a couple of ways to approach this. The fastest is to install a replacement cluster, ... If you have one in your back pocket! Without that luxury, we need to look at the wiring diagram to see if we can find what else that circuit feeds, and identify some possible suspects. Suppose, for example, a wire leaves the cluster and feeds, ... Oh, ... The generator and voltage regulator. You find that wire is bare where it is rubbing on a sharp metal bracket on the engine. That is where current is able to bypass the intended load, the regulator's turn-on circuit, and take a shortcut back to the battery.

Once you lift up that grounded wire, current has to go through the voltage regulator, but the test bulb is also still in the circuit. There are two loads that each now drop part of the battery's 12 volts. If the regulator drops four volts, the bulb will drop the other eight volts, so it will be dim. THAT is the indication you're looking for. A dim bulb proves there is current flowing through it, so the rest of the circuit is not broken, and a less-than-full-brightness bulb proves there is no short further down the line.

Finally, ... To illustrate how this works in real life, when I worked at the dealership, I had a right rear tail lamp harness from a Dodge Viper. It had three sockets for a total of five filaments, so I could plug in as many bulbs as the circuit needed. The individual wires were tied together, then soldered to a blown fuse. That was just so it could be plugged in quickly to save time fiddling with jumper leads.

I had a full-size Dodge van come in with a blown fuse for the tail lights. The customer said they had been replacing fuses, then they'd blow again in a minute or a day, but not always right away. I plugged in the test harness, then drove around the parking lot until the bulb went full brightness. Luckily it stayed that way while I carefully drove into the shop. It did flicker a few times. So I knew this wasn't a solid, or dead short. I ran around the van with a rubber hammer, and when I got near the left rear tail light, the test light flickered quite a bit. When I pushed on that lens just right, the test light went dim, and at the same time I could see a faint glow from the tail lamp bulb.

My test bulb was limiting current to one amp. When the short was gone, the current also had to go through the resistance of all the other bulbs. Since it was dividing up among all the van's bulbs that were in "parallel", only some of that current went through each bulb. The low current through each one of those bulbs is why they were so dim. Since there were so many bulbs on that circuit, their total resistance in the circuit was very low. Relatively speaking, the test bulb presented more resistance, so more of the 12 volts was dropped across it, and it was still fairly bright. The change between "fairly bright" and full brightness was enough to show when I was doing something that made the short go away.

Upon removing the lens, I found that someone had spliced in a trailer wiring harness. They caught the wire under a screw when they put it back together. Simply cutting off the damaged part, and reconnecting the wire solved the short.

If I had not learned this light bulb trick at a tv servicing class in the '70s, how many fuses would I have had to blow before I figured out there must be some better way to diagnose this?

Getting back to your test light that you want to use in place of the fuse, that bulb is much too small and has too much resistance. Most test lights draw less than a quarter amp. Given their high resistance in relation to the circuit you're testing, most of the 12 volts will be dropped across it, even when there is no short further down in the circuit. The bulb will be near full brightness when the circuit has no defect, so you won't get the desired indication of when a defect is removed.

Even my brake light bulb is too small for working in a high-current circuit like for a heater fan motor or head light circuit. This is where I plugged more bulbs into my Viper harness. All of them together would allow just over four amps to flow. If you need more than that, a low-beam head lamp bulb will pass five amps, and a high-beam is good for six amps. If you had a short in a heater fan circuit, for example, that motor might draw ten amps at a mid speed. A brake light bulb in place of the fuse would limit current to one amp. When the short is removed, the expected clue is the motor will run very slowly, but that tiny bulb limits current too much for that to happen. You need the head light bulb in place of the fuse. That will pass enough current for the motor to run fast enough for you to know it's running. If the short occurs again, the five amps the bulb will pass is not nearly high enough to overheat any wire in a car.

Keep in mind that the information I just posted took about six weeks to get to in my eight-week-long Automotive Electrical class. By that time we had covered basic electrical theory over and over until everyone got it. This is a very hard subject to learn for people who learn best by taking things apart. People who are good at visualizing abstract ideas learn this material easier, so don't sit in a corner and whimper if it doesn't make sense.

Okay, my fingertips are sore from pounding on this miserable keyboard. My next step, while waiting for you to try out this wondrous trick, is to look at the wiring diagram to see what else is involved with the instrument cluster. I'll be back tomorrow to see how you're doing.

I greatly prefer manufacturer's paper manuals because I can post bookmarks and notes, and open multiple pages at once. EBay is a good source for them. The next best thing is online manuals. Independent shops spend over a thousand dollars a year for access to all cars on Mitchell-On-Demand and AllData. You can buy a subscription for just your vehicle for about $28.00 per year or $48.00 for five years.

I typed for over a half hour a little while ago, then my computer locked up, ... Again, and everything was lost. To combat this pile, I'll be posting my replies in little pieces so I don't have to redo everything if this happens again.

Looks like I told you more than I knew at the time. I did find a 10-amp "Gauge" fuse on the left side of the dash that feeds a "Door lock control relay", which is a computer module. That fuse is listed as being live only in the "run" and "crank" positions. Inside that computer they show a "Key off power window" circuit, so there's your computer involved with the power windows and it gets current through a ten-amp fuse.

The only thing I can see that module does is turn on the "Main power relay" when the front passenger door is open, and that relay sends current to the power window master switch through a 30-amp fuse, not a circuit breaker. The module is listed as being on the right side of the instrument panel. It should have two red / blue wires, a white / red, a white / black, and a red / green wire. Unplug that module, then see if the test light still indicates there's a short.

If I'm following the correct wire, it does feed a lot of stuff on the cluster, but we can eliminate that since it's unplugged. I also followed it to the "variable speed sensor" listed as being at the rear of the engine, and to a "center diff lock control switch" at the center of the dash. I think that's for the front differential. Among other things, that circuit feeds a warning buzzer switch under the hood. Will that give you some more things to look at? Unplug various things again to see if the short is isolated. The speed sensor wires are red / blue, blue / black, and red / yellow.

If the short was in the wiring, it would still be there with the sensor unplugged, unless there were two wires shorted together very near the plug, and you disturbed them. My vote is for the sensor. It will be easy to prove. Just plug it back in and see if the short comes back.

At least it sounds like you're in the right area.

I've seen ads for those short finders but never tried any of them. There was one a few years ago called, "The Fox and the Hound", that I thought could be useful, but so far I've always found other methods just as effective. That tool, as I recall, was installed in place of the fuse, then the circuit was turned on. The tool limited current to a safe value, just like my light bulb trick does, but the second part, the "hound", relies on the fact that a magnetic field is set up around the wire when current flows through it. A steady magnetic field can't be differentiated between those from other wires, and a magnetic sensor can only detect a changing magnetic field. For that reason, the "Fox" in the fuse holder generates a pulsing signal that is constantly changing. You move the receiver along the wiring harness to follow the path of the current flow. The signal will stop at the point that wire is shorted to ground.

Logic dictates you could also set the receiver in one place where it can be heard, then you can unplug or move things to see what makes the short go away. The chirping sound would stop, just like the test bulb would go out.

This tool will not work for finding a break in a circuit, like a corroded connector terminal or splice. The receiver relies on the pulsing magnetic field, and that field relies on current flow. There won't be any current flow if there's a break in the circuit. What you COULD do is find that wire at various connectors, and ground it there to see if you cause a short to occur. If you can, that would prove the wire is okay up to that point. You can do the same thing with my test bulb trick.