Twelve Volt (pink) wire that goes to all sensors tests good continuity to ground post on battery with key off

Tiny
JOHNNY CARDIN
  • MEMBER
  • 1998 CHEVROLET SILVERADO
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 181,000 MILES
I was recently testing sensors on my truck due to the fact it quit running going down the highway, far as I could tell, they checked out. I then discovered a spot that was bare, and pinched between the engine, and transmission. I spliced the wire back together, and just knew that I had fixed the problem, however it still does not start. I somehow figured out that when the key is off, this twelve volt wire, is in direct current with the ground on the battery. My question is, is the twelve volt sensor feed wire a direct ground with the key off?
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Monday, December 26th, 2016 AT 12:56 PM

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Tiny
CARADIODOC
  • EXPERT
Your last two sentences do not make sense and I do not know what you are trying to describe. This truck has pink wires all over the place but for different circuits. Most engine sensors run on 5.0 volts, not 12 volts.

If you have a pink wire shorted to ground, you should be blowing a fuse. There are two 20-amp fuses in circuits that have 12 volts on them when the ignition switch is in the "run" position. If you do indeed have a fuse that is blowing, I have a trick for working on that circuit.
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Monday, December 26th, 2016 AT 4:02 PM
Tiny
JOHNNY CARDIN
  • MEMBER
Ok. Let me clarify, according to the wiring diagram I have in the chiltons repair manual, it shows a lot of pink wires alright. But, they are all on the same circuit according to the diagram, they supply the sensors with 12 volts straight from the PCM. And the signal FROM the sensors is 5 volts. Generally speaking, my crankshaft position sensor however gives back a 11.53 volt signal. Either way, it doesnt start, been working on this thing for weeks, sure need to get it running. I was checking for continuity on various wires with the ol' ohm meter, when somehow I noticed that the 12 volt feed wire to the sensor is a ground when the key is in the off position. I then checked for continuity to the ground post on the battery from this pink wire, and sure enough, my ohm meter beeps.I just thought that was kinda weird. Never figured a hot wire to be a direct ground to the battery with the key off, I have read several threads about the possibility of it being in the ignition switch, but I can't afford to blindly throw money at this thing anymore. The coil has a strong spark when cranking it over, good compression, the top dead center mark is dead-on on the compression stroke. Ol lady turn the engine over while I checked for a spark, and compression, everything is dead on the money, even the distributor is right on the little 8 stamped into the body of the distributor, even put new cap and rotor, but spark is weak at the plugs. I used to build old school 350s all the time, and never had any problems getting them to crank right up after a fresh rebuild. But this thing has had me bumming rides and walking for almost 3 weeks now. Looking for a miracle now.
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Tuesday, December 27th, 2016 AT 8:39 PM
Tiny
HMAC300
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I've already answered this question
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Wednesday, December 28th, 2016 AT 6:48 AM
Tiny
CARADIODOC
  • EXPERT
Ahhh. You aren't measuring a ground wire. You're using your meter's continuity test to check for a complete path TO ground. Most meters will beep with anything less than 200 ohms. You're reading through the numerous "loads" on the circuit. You'll find the same thing if you measure the 12 volt feed wire to the heater fan motor, or the 12 volt feed wire to the head lights. All of those circuits have loads of less than 200 ohms, so your meter would beep, but that doesn't mean they're ground wires. If you did NOT have that resistance to ground, you wouldn't have any current flow when the circuit is turned on, and it would be dead.

Forget continuity readings. Those get people way too "wrapped around the axle" with misleading results. Mechanics never do these tests, except occasionally to verify their diagnosis, because it is a waste of time, and therefore, their customer's money. There are much more effective ways to diagnose problems.

Your understanding of electrical theory is also getting you into trouble. My head is spinning from trying to follow your logic with sensor readings. You're confused too as to what a sensor's signal voltage should be. That is to be expected before you get some help from geniuses like us!

12 volts and 5.0 volts is not a signal voltage. If that were the case, you wouldn't need a sensor. You could just connect that wire to the battery.

Sensors react to a condition that varies, and their signal is the result. The computer looks at that signal voltage to know what has taken place with that variable. 5.0 volts and 12.0 volts is not a variable and is not a signal voltage. The easiest example to understand of a signal voltage is perhaps the throttle position sensor. Imagine a garden hose connected to your house faucet, and the nozzle is open, so water is flowing. You have 50 pounds of pressure at the faucet, but 0 pounds after the nozzle. There's your 5.0 volt supply and ground for the throttle position sensor. Half way along the hose you would find 25 pounds of pressure. At half throttle, the movable contact inside the throttle position sensor would find a spot that has 2.5 volts. THAT is the signal voltage. The further the throttle is opened, the higher the signal voltage goes.

That 5.0 volt supply comes from a very carefully regulated supply inside the Engine Computer. It can't be allowed to vary even a few tenths of a volt. If it did, the voltage at any point along the throttle position sensor would vary a proportional amount, and that would produce an incorrect signal voltage.

The crankshaft position sensor's signal is totally different. For these position sensors, voltage is unimportant. Timing is the critical information. As a magnetic field is disturbed by something moving past it, the sensor turns on and off. WHEN those events occur is what the computer needs to know so it can time the firing of ignition coils and injectors properly. Digital voltmeters react much too slowly to be of any use with these "square wave" signals. If you could slow it way down, as in turning the crankshaft by hand, you might see 5.0 volts, then close to 0.0 volts, then 5.0 volts again, as you continued to turn the crank. Slowing this down gives the meter time to take the reading, analyze it, and display it while it takes the next reading. During cranking this happens way too fast for the meter to catch it. There are some high-class digital meters that measure frequency, (hertz), but all that is good for is to verify a signal is being generated. You don't know if there's some gaps, or "dropouts", if it's the right frequency, or if the signal voltage is high enough for the computer to see. Even engine performance specialists rarely use this test.

The advantage we have is the Engine Computer does all the preliminary testing for you. The first step is it is constantly monitoring numerous circuits, and it will detect a problem and set a diagnostic fault code. It is important to understand that fault codes never say to replace parts or that one is bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. I can go into how codes are set related to circuit problems if it comes to that. The second step is setting codes related to operating conditions. One of the easiest to understand might be the computer knows engine speed and throttle position. It also knows the engine can't be idling at 800 rpm if the throttle is 3/4 of wide-open-throttle. Those two don't correlate. An even better example is the computer knows if the engine has been off for at least six hours, the intake air temperature sensor and the coolant temperature sensor had better be reading the same temperature. If they are wildly different, the computer has ways of figuring out which one is wrong.

The third step, if the computer hasn't detected a missing position sensor signal, is to view live data on a scanner, then you have to determine what is missing or wrong. Very often cam and crank sensors won't set a fault code simply from cranking the engine. They might only set while a stalled engine is coasting to a stop. That is where we would observe those sensors on the live data screen. I use a Chrysler DRB3 scanner because it works on other car brands too. Those sensors are listed with a "No" or "Present" during cranking. Aftermarket scanners have some similar way to show the same thing.

Regardless if you find a signal to be missing or you have a fault code stating that, you still have to figure out why yourself. That is when it's appropriate to start with individual voltage readings.

Now to get more specific to your truck, you do indeed have a crankshaft position sensor and a camshaft position sensor that run on 12 volts. That is very uncommon, but diagnosing the circuits is the same. Each sensor is fed 12 volts through a different fuse. Given your original observation of a grounded wire, be sure to check ENG1 fuse and ECM1 fuse. They are both 20-amp fuses in the under-hood fuse box. Next, check for 12 volts at both sensors. To be valid, these readings need to be taken by back-probing through the rear of the connectors while they're plugged in. If you unplug the connector, then measure right on the terminals, if the 12 volts is missing, it's missing. If you do find 12 volts, it is possible it is a false reading and will mislead you. I can explain that too if it becomes necessary. The ignition switch must be in the "Run" position for these tests.

To add to the confusion, the ground wire for both sensors is purple, and should have 0.2 volts. Don't confuse those as the pink wires when looking for the 12 volts. If you have the 12 volts and 0.2 volts for both sensors, they will develop their signals as long as they're good and have no mechanical issues related to them. This is the time it is appropriate to replace a suspect sensor. Way too many people just throw in a new sensor when they see a fault code. About 50 percent of the time the missing signal is caused by a wiring or connector terminal problem, not the sensor itself, but they'll keep on buying sensors, and will become more and more frustrated. Replacing a sensor as a first step is okay if you already have it on hand, as long as you understand there's only a 50 percent chance it will solve the problem.
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Wednesday, December 28th, 2016 AT 1:19 PM
Tiny
JOHNNY CARDIN
  • MEMBER
Thank you for your reply, yes turning the engine over by hand, I have done alot of that lately while reading the digital ohm meter for a change in voltage, as far as everything I could find to read, they are ok, except for the crankshaft sensor which is alot higher than the 5 volts they say it should send. I have followed all the directions given by the website diagnosemycar. Com I think it is, it was pretty helpful I suppose, except for the signal that travels through a trigger wire which is a white wire they speak of coming from the PCM to the ICM that particular wire has no apparent ground, or signal, which at some point they say either the PCM is bad, or the crankshaft position sensor from where it gets its signal is bad. I have touched that wire with a known ground, and it will make the coil fire. I sure dont know, never claimed to know anything about these vortec engines except the fact that they are hard to diagnose. I seriously dont want to have to buy a PCM, and have it programmed if there isnt a need for it Too many of the threads I've read the people had the same exact problem, and replaced the PCM, and it didnt fix it, unfortunately they never posted what they did to fix it Thanks again for your time and effort
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Wednesday, December 28th, 2016 AT 1:49 PM
Tiny
CARADIODOC
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It occurs to me I haven't seen you make any reference to actually reading the diagnostic fault codes. Have you done that? That is the place to start when diagnosing a crank / no-start problem.
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Wednesday, December 28th, 2016 AT 1:57 PM
Tiny
JOHNNY CARDIN
  • MEMBER
Yes, well actually it is showing no codes, it had showed a 1351 earlier on before I fixed the shorted wire
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Wednesday, December 28th, 2016 AT 2:11 PM
Tiny
CARADIODOC
  • EXPERT
P1351 - Ignition Coil Control Circuit High Voltage

I don't have a good answer for that one. On most cars the voltage on that wire will be near 0.0 volts because it's grounded by the computer. The computer switches the ground off, (open), forcing current flow to stop instantly. The resulting rapidly-collapsing magnetic field is what creates the high spark voltage in the ignition coil.

The point is, when switched off, the resulting high voltage is a normal condition. Someone smarter then me would have to describe what it takes to set that code. I suspect it means the computer saw 12 volts being applied to that wire while it was trying to ground it, but that isn't likely to occur while you were driving.
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Wednesday, December 28th, 2016 AT 3:25 PM
Tiny
JOHNNY CARDIN
  • MEMBER
Thank you again for your time. I was outside earlier scratching my head, and inspecting all my parts for premature wear, and noticed that the inside of the distributor cap, (which has less than 300 miles on it) is definitely showing signs of some arcing going on in there that I'm certain isn't supposed to happen. I guess I'm wondering now if that would cause the truck to just stall going down the road? It really bites that a new cap can fail like that if so.
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Wednesday, December 28th, 2016 AT 4:47 PM
Tiny
CARADIODOC
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The only tidbit I can offer related back to GM's first electronic ignition from '76, the "High Energy Ignition, (HEI) system. Those had the ignition coil built into the distributor cap. They were capable of developing up to 45,000 volts, about four times what it takes to fire a spark plug.

(It's important to understand that spark voltage will build up just to the point the spark jumps the gap, then it will not go any higher. Just like a dam on a river. Water can build up to the height of the dam, but no higher, even if you dump more water into the river).

A standard test for spark was to remove a plug wire at the spark plug, insert a screwdriver, then hold that 1/4" away from the engine to watch for spark. Someone always felt the need to pull the screwdriver further and further away to see how big a spark they could get, as though that had some meaning. Once you got to the point where the typical 15,000 to 20,000 volts was not sufficient, no spark occurred.

The problem with doing this with the HEI system was because the coil could develop such a high voltage, when you moved the screwdriver too far away, that spark was going to go somewhere. That turned out to be through the rotor and into the distributor shaft. That was called "punch-through". Wherever current flow makes a spark, carbon is left behind, as in the carbon-tracking inside a distributor cap that had moisture in it. That carbon also appeared in the hole where current went through the rotor. From then on, the rotor was shorted, and current would find it easier to follow that path to the shaft than to jump the gap in the spark plug. The result was no spark at the plugs, and a crank / no-start condition.

There is no reason to have an ignition coil capable of developing such a disastrously-high voltage, and I'm not aware of any coil today that can do that. That doesn't mean we don't have to look for those failures.
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Thursday, December 29th, 2016 AT 4:09 PM
Tiny
JOHNNY CARDIN
  • MEMBER
Very well said. My coil makes a pretty healthy spark at the end of the coil wire going to the distributor. Guess we can rule out the coil, move on to something else. Will eventually get that spark where it belongs, and then get on with putting milage on this thing, instead of my poor old feet, lol. Been too cold for me to go out, and stand around in the cold wind, we'll get back on it in a day or two, meanwhile, I got to pull a tranny in a 95 blazer, and find a way to get it to the shop to be rebuilt. At least then I'll have something to use until I can get this vortec figured out. Thanks again for your time, and consideration, have an awesome new year!
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Thursday, December 29th, 2016 AT 5:08 PM
Tiny
CARADIODOC
  • EXPERT
Where are you? I'm in the middle of Wisconsin, ... And I really hate winter!
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Thursday, December 29th, 2016 AT 5:45 PM
Tiny
JOHNNY CARDIN
  • MEMBER
I'm in Texas, we are having a bit of colder than normal weather, probably feels like summer here to you though. Lol
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Thursday, December 29th, 2016 AT 6:01 PM

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