truck stopped running, pulled off onto a side street. towed home & began troubleshooting. Sprayed starting fluid while cranking - no effect, and... no start.

quickly found no spark from coil to distributor. Truck is needed badly right now, so threw an ignition coil at it. No change. A few minutes on youtube and a forum or 2, changed the ignition control module. no difference. welp, I've gone this far shooting off the parts cannon, might as well replace the crank position sensor. yep: no start.

found a diagnostic guide, 32 page pdf. very informative and helpful. a series of 10 electrical tests, no problem.

The various coil, icm and ckp wires check out whether it's key on or cranking - except for a few differences, causing me to think I have a bad ckp or a bad pcm.
the signal wire at the coil and icm has power, but does not flash on and off while cranking. Checking the ckp - while hand cranking the engine, a volt meter displays 9.3v or zero as the engine is cranked. diag guide says it should be 5 volts.

connected to wire 31 on the pcm blue connector. key on: 9.3v. crank the engine, and voltage reads 2.5v no flashing.

interested to learn ideas as to root cause

 

Can you tell me more about this truck and its engine? What model is it exactly? I need to get the correct wiring diagram in front of me to help you fix your truck.

George

 

hello George.

any help you can provide would be outstanding. I'm in the middle of a residential construction project as an owner-builder. Getting the structure closed in before cold weather is the top priority, so the truck has been sitting with occasional attention. I'm grateful to have a friend who will loan me his truck when I need building material... I need to get this thing fixed before my friend tells me to go away.

The truck is a 1997 Chevrolet C2500 Silverado, 2 wheel drive with the 5.7 Vortec engine.

Brad

 

Alright, let me find some wiring diagrams then we can get you fixed up. I think I’m almost done dealing with this hurricane and can get on this for you.

George

 

I think I have the correct wiring diagram for your truck.

You may have done some of this already but here is where we start:

1) Key on Battery voltage at terminals
2) Key on voltage at coil pink wire to battery ground everything connected, back probed
3) Key on voltage on ICM pink wire to icm blk/wht ground wire all at connector everything connected, back probed
4) Key on voltage at CKP pink wire to battery ground everything connected, back probed

Also, please describe how you performed this test:

"the signal wire at the coil and icm has power, but does not flash on and off while cranking. "

Report back and if all good then we find out where the signal pulses for ignition has failed.


George

 

hello George.

1) Key on Battery voltage at terminals.
yes. all tests were accomplished with the key on. the same tests were also accomplished while cranking the starter

2) Key on voltage at coil pink wire to battery ground everything connected, back probed.
yes - the pink wire at the coil has 12volts with VOM connected as requested.

3) Key on voltage on ICM pink wire to icm blk/wht ground wire all at connector everything connected, back probed.
wait - I get 12volts using battery ground. see the answer to your last question - asking how I accomplished the signal wire test.

4) Key on voltage at CKP pink wire to battery ground everything connected, back probed.
yes - pink wire has 12volts connected using battery ground as requested,

Also, please describe how you performed this test: "the signal wire at the coil and icm has power, but does not flash on and off while cranking. "

the coil and the icm were done the same way, w/coil and icm harnesses connected, I back probed the wht/blk wires to connect the test light alligater clip, then touched the positive battery terminal as the engine was being cranked. for both the coil and icm - the test light was fully illuminated with key on and remained illuminated during cranking.

 

3) The reason for referencing the meter negative probe to the ICM ground instead of the battery ground is to measure both the quality of the 12v power on the pink wire and the quality of the ICM ground connection. This is the best way to do it because it tells you the net voltage to the module under a load, the most accurate real life measurement. Because you referenced battery ground, the ICM ground still needs to be tested.

Testing for control pulses:

There are three control pulses involved. First the CKP hall effect (3 wire) sensor sends a square wave timing signal to the VCM as the engine rotates. This is best monitored with a scope but since most here dont have one then you can either measure AC voltage or duty cycle (this is the GM manual approach) or watch a DC voltage when rotating the engine hand . Some hall effect sensors output 0 - 5 volts and some output 0 - 10 ish volts. Your technique is the least accurate but at least suggests a working CKP sensor. Does the original sensor give the same result?

Second the VCM sends low current control pulses to the ICM on the VCM to ICM white wire. Did you test that line and if so, how?

Third the ICM converts those PCM pulses to high current control switching that can handle the load of the coil primary circuit on the wht/blk wire. When you tested this was the connector unplugged? What type of test light? LED or old school?

This is ground side control which means that you have to be very careful because the modules or computers are sinking the control current and switching it on and off with unprotected transistors to ground. If you present high enough current to those ground side switching lines you can brick the computer or module. Anytime you are testing such lines an scope, DVM or LED test light is best unless you really know what you are doing because the extra load of an old school test light is high enough to cause damage. Sometimes a high current incandescent test light is preferred for certain testing because it can substitute for the high current load so you are testing a fully loaded circuit. An example would be the O2 sensor heater circuit.


George

 

A few thoughts:

Whenever you have a system failure and especially in an older vehicle, wiring problems are responsible at least 50% of the time. I just finished with a member who had headlight problems for over a year. Shops couldn't find it, parts were dispensed from the proverbial cannon and ultimately is was a corroded wiring harness.

Our goal is to follow the signal path: CKP>wiring>VCM>wiring>ICM>wiring>coil>... When we find the first place with no signal then we have to isolate that wire and connector to make sure thats not the problem before condemning components. There are opens, shorts to power, shorts to ground and shorts to other signal wires. That and confirming all powers and grounds. Easy peazy.


George

 

good afternon George.

I hear ya. I'm retired now, but made a career with GE Aircraft Engines as a military field service tech rep. Young USAF jet mechanics were always ready to change parts - stated another popular way: loading up the parts cannon.

When I saw power to the pink wire at the coil, and knowing the truck has 240K miles, figured the original coil had packed up. Haste makes waste which is another way of saying - that's why I R&Red the ICM after seeing 12V at the pink wire.

anyway... just returned from testing the blk/wht ground wire (number 3 on your post above) It's a good ground. Got a solid test light illumination with key on and everything connected. Followed up with a couple cranks on the starter - still fully illuminated.

Brad

 

So now you have to test both signal wires (before and after ICM) for pulses and if dead, test both ends or continuity. Be careful with the VCM sink current.

George