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94 Chevy Camaro LT1 350 Auto

posted Mar 27, 2016 01:13:01 by Noah
1994 Camaro v8 auto. The way the problem was described to me was that the car would shut down at random and would sometimes restart and sometimes not. The driver of the car says he thinks the car only runs when the CEL is on.
I figured I was going to go over there and find a 5v ref wire shorting to ground, get paid and get out.
They tell me this car has had 5 optispark distributors and as many ignition control modules and coils! Supposedly Jegs is refusing to warranty another distributor assembly, so they bought a GM unit for $400.
It's also on it's 4th PCM. There's only a few feet of harness between the distributor and the PCM, I've opened the loom and wiggled the whole thing around to no avail.
Plus, as soon as I started working on it, I discovered the clamp was off the water pump bypass hose and the distributor took a nice coolant bath...
Great, now the plug wires I can't even see or reach are wet with coolant, this should help!
The way I understand it is that the PCM supplies 5v on the High Res and Low Res signal lines and the optispark distributor pulls them to ground. The PCM then commands the ICM to fire to coil based on that input.
Codes:
42 EST grounded
36 High res pulse failure
16 Low res pulse failure
29 secondary injection pump circuit fault
18 injector circuit
66 a/c pressure switch
All the connectors look nice, don't think there's a pin fitment or corrosion issue on the PCM side or 4 pin distributor connector on the intake. There's a jumper connector for the distributor, guess the new unit isn't plug and play, kind of like those Delphi fuel tank units.
It USUALY fires after a long crank and will stay running pretty well, (until the wires got wet), then it starts breaking down and stalls. The last time it stalled on me it puffed a big cloud of smoke from the intake
I'm charging the Verus and will post my captures after I review them, but I wanted to get this up here before I forgot something. Maybe someone has an idea???
Got some ugly looking low res waveforms. There's one KOEO where I was looking at something else but still had the low ref wire backprobed. The signal was 0v, then there were 5v spikes, then it jumped up to steady 5v. When ohmed to Bat Neg with the sensor and PCM unplugged early on during the diag, I get no continuity.
Honestly, I think I'm just going to pass on this job, I've got more time into it than I wanted to have already and I hate working on Camaros. Everything is just too crowded and cramped.

I think the optispark is wiped out again, or there's some kind of internal contamination issue but this guy doesn't want to pull the trigger on #6. I don't blame him.
Massachusetts, USA
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10 replies
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Noah said Mar 27, 2016 23:27:28
Going over my scope captures, I'm seriously considering the possibility of a poor pcm or distributor ground.
Also at idle my feed voltage looks like crap, real choppy and never going much above 12v.
This is a screen shot of a capture koeo, on the low resolution input to the pcm. The optical sensor isn't moving, so the value should be a steady 0v or 5v. Note the glitches and the rise to steady high voltage. Also, the low voltage portion of this signal is around 200mv.
I'm going to go back and unplug the sensor to verify the PCM is giving a good steady 5v to start with and check the computer grounds.
The second capture is of the ignition feed to the distributor at idle. Not impressed with this alternator! come to think of it, it was making some noise and putting off some heat. Another piece of the puzzle?

Massachusetts, USA
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Tyler said Mar 28, 2016 01:41:29
Weeeeeeee, another doozy! BTW, ask me about the '98 Dodge pickup I had in from the body shop last week. Engine wouldn't shut off, dash lights on with the key out, good times.

What's the spark and injection pulse doing during the long crank time? Wondering if the long crank is associated with these codes, or if the pump is weak/bad gas/other issues.

Good capture from the low res input on the distributor. The spikes and the voltage transition with no engine rotation is bad, but what gets me is the 200mv 'ground'. This screams failing hall effect to me, or (like you said) failing ground. I understand why no one wants to re-distributor this again, but (obviously) these things don't play well with coolant.

The distributor 12V doesn't look amazing, either, and possibly says 'high resistance' to me. I don't know that an alternator problem explains all of your symptoms, but a failing diode certainly won't help matters! I'd suggest voltage dropping that power feed along with your PCM tests.
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Noah said Mar 28, 2016 02:19:14
This screams failing hall effect to me, or (like you said) failing ground.

The distributor 12V doesn't look amazing, either, and possibly says 'high resistance' to me.


Wondering if maybe they have more in common than I first thought, block ground perhaps?

Wondering if the long crank is associated with these codes, or if the pump is weak/bad gas/other issues.

Couldn't find a schrader valve. I know, poor excuse. I figured the long crank could be from the crappy high/low res signals, like an ugly cmp sensor signal. It started to belch black smoke from the exhaust before dying out on me once, so I just kind of assumed that it wasn't loosing injector pulse or fuel pressure. That and SCARY spark knock as it was dying

What else...

At times, when you first turn on the key, the low res stays low, even during cranking. Then when you turn the key off and you get that GM key off fuel prime, the signal line goes to 5v, and the car will usually start. Forgot all about that, looks like another night on All Data for me!

Dodge sounds AWESOME! Don't spill the beans just yet, I want to mull that one over.
Look like your job is exciting btw.
Massachusetts, USA
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Noah said Apr 10, 2016 01:07:10
UPDATE: The owner felt like changing the ignition control module again, so he did that to no avail. He also changed the ignition switch. I'm going to head over this weekend after having some time to think about it.

I'm changing the BS battery terminals before anything. It's all bolts and washers and crap, and frankly, I'm not sure it was reliable scope ground.

Weeeeeeee, another doozy! BTW, ask me about the '98 Dodge pickup I had in from the body shop last week. Engine wouldn't shut off, dash lights on with the key out, good times.

So, what's the story with the Dodge man? I've been waiting to see a new post from you about it!
Massachusetts, USA
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Tyler said Apr 10, 2016 15:04:37
UPDATE: The owner felt like changing the ignition control module again, so he did that to no avail. He also changed the ignition switch. I'm going to head over this weekend after having some time to think about it.

I'm changing the BS battery terminals before anything. It's all bolts and washers and crap, and frankly, I'm not sure it was reliable scope ground.


Because clearly, more parts will fix it, right? ;-) I understand that he's probably desperate at this point, though...

So, what's the story with the Dodge man? I've been waiting to see a new post from you about it!


I know, I apologize for not being around recently. I gotta catch up on all the cars that got fixed around here!

Okay, the Dodge... The short version is that the ASD relay was getting power when it shouldn't have, causing it to stay on with the key out, and the engine to keep running like everything's A-OK. It was getting power due to a melted harness behind the drivers seat, which contained circuits that got power at all times, as opposed to ignition switch controlled. The power-at-all-times circuit backfed power into the ASD circuit (among other thing), and boom, wacky symptoms.

The harness was melted when the body guy welded some new panels in, and didn't know there was wiring close by :-/ Honest mistake, it happens, but I had to build whole new harness to fix it.
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Noah said Apr 10, 2016 22:54:38
Man, don't see many melted wires inside the car! Nice job.

I went back to the camaro today and left just as confused. It's starting to look more like a PCM issue... I think.

I started at the 4 wire connector for the distributer. There's high res, low res, power and ground. All from the pcm. Unplugged, both signal wires have 5 volts. The high res is like the crank signal and producers 5v square waves, like the name implies, with one notch for every degree of cam rotation. The low res makes 8 pulses, one for each cylinder.
I watched them both on the old vantage, and was getting the appropriate signals.
As soon as you clear the cel running, the code for loss of high resolution signal comes right back, even though I'm watching it at the PCM connector. Strike one for PCM.
The distributer power feed from the PCM shows system voltage. When I load it with my load pro, voltage drops one volt. Strike 2 PCM.

The next couple I'm not sure about, and at the risk of sounding stupid, I'll post any way and hope for some guidance.

After the car warmed up and stalled and became a no start, I checked for spark and there was none. So I checked for the PCM control of the ignition module on the white wire, there was nothing. 0v while cranking. So I unplugged the PCM and icm to ohm the circuit, and check for shorts to ground (not shorted). With one lead on the ICM connector, I accidentally touched the other on the PCM case and heard my meter start to auto range. .208kohms, why is there continuity between the PCM case and the ignition control circuit? Possible strike 3 PCM?

I move on to checking power and grounds at the pcm with the load pro, every circuit can handle current flow until I get to b6 and b16, both are sensor grounds. B16 shows ground, load the circuit, no drop. B6 however can't take a load. Again, at the risk of sounding stupid, this should just be a ground path, right? It shows 0v running, but I gave it a momentary ground while the car was running and it wanted to die immediately. I don't see any indication that this is a signal circuit.

I'm also losing fuel pump relay power side control from the PCM when it starts dying as well as ignition control from PCM.
Once it stalls, it's a no com, no ses lamp.

If anyone can shed some light on the loaded sensor ground failing, and the continuity between the PCM case and the ignition control circuit, I'd certainly value the input and learning experience.
And I'm posting from my phone, so please excuse any weird auto correct misspellings.
Massachusetts, USA
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Noah said Apr 10, 2016 23:00:21
While it's a cranking no start, some times the tach doesn't move, sometimes it bounces around. I didn't see any mention of a crank sensor on this system. I believe the two signals from the distributer is all this system uses.
Massachusetts, USA
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cheryl hartkorn said Apr 10, 2016 23:35:55
wonder when it stalls and becomes a no communication a sensor on the 5 volt ref circuit isn't crapping out and pulling the 5 volts ref to ground. just a thought. and there for losing fuel pump control.
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Noah said Apr 11, 2016 00:08:47
I was working under that assumption early on, but im not losing the two 5v feeds to the distributer so I don't think that's the case.
Thanks for the input Cheryl.
Massachusetts, USA
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Tyler said Apr 11, 2016 00:55:19
As soon as you clear the cel running, the code for loss of high resolution signal comes right back, even though I'm watching it at the PCM connector. Strike one for PCM.


Agreed.

The distributer power feed from the PCM shows system voltage. When I load it with my load pro, voltage drops one volt. Strike 2 PCM.


Loaded at the PCM, or at the distributor? I'm glad you did brought this up, because I *thought* I saw a voltage drop issue in one of the captures you posted previously.

So I unplugged the PCM and icm to ohm the circuit, and check for shorts to ground (not shorted). With one lead on the ICM connector, I accidentally touched the other on the PCM case and heard my meter start to auto range. .208kohms, why is there continuity between the PCM case and the ignition control circuit? Possible strike 3 PCM?


So, .208K ohms is... 208 ohms? I think? Might have meant to say 208K, phone posting is tricky.

Anyway, I honestly can't think of why there'd be continuity there, in the hundreds OR thousands of ohms. Possible internal PCM short?

B16 shows ground, load the circuit, no drop. B6 however can't take a load. Again, at the risk of sounding stupid, this should just be a ground path, right? It shows 0v running, but I gave it a momentary ground while the car was running and it wanted to die immediately. I don't see any indication that this is a signal circuit.


I went and dug up the diagram just to get my own eyes on, and you're absolutely right, B6 should be a simple signal ground. Not a stupid question, especially when the engine reacted the way it did to ground application. You should be able to ground that all day long and never see a change, IMO.

B16 having an issue would have made a bit more sense, given the A/C pressure transducer code you started out with, and that the MAP sensor shares the same ground. Ah well, it looked nice on paper anyway.
[Last edited Apr 11, 2016 00:57:57]
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