Car won't start, U0100, Bad ECM?!

[flht99b]

One thing to look into it the wiring harness that passes through the front of the door jam into the door for the power windows/locks and power mirror. It is common for these wires to get pinched and or break from the flexing of this harness inside the rubber boot. Issue can be at either end, door jam or door. Since you hear a clicking when opening the door and are blowing fuses, you could have a broken/scuffed power wire in this harness going to ground.

[gtstorey]

The BCM issue I was helping diagnose was being done remotely on a kit car forum and we eliminated everything upstream to the BCM so the BCM was replaced. It still shot the fuse afterwards. Unfortunately the person I was helping dropped off the forum when the BCM didn't resolve the problem so no final answer.

It probably won't be a sensor grounding since the current is so low and many are designed to ground. It will probably be something the ECM is actually powering. I think this will limited number, but I haven't studied the wiring diagram to confirm. And the ECM is probably different from a BCM in this regard.

At some point you might have to try the replacement as a next step. I'm not sure how you could test the ecm itself for shorts to ground. Maybe check for 0 resistance from a power feed to the an ecm ground circuit?

[silversleeper]

Originally Posted by gtstorey

The problem with the ECM replacement route is it could be something after the ECM that is shorting. At least I've seen something after a BCM cause a power feed to the BCM cause a fuse to shoot.

That's the problem when someone has two threads going on the same topic. I repeated what you said in the other thread before I read this.

[acutron42]

Originally Posted by silversleeper

That's the problem when someone has two threads going on the same topic. I repeated what you said in the other thread before I read this.

I did see that & it does get confusing. Here is a power distribution diagram but it is hard to post a full page & probably wont help much.

[Gunn]

Originally Posted by silversleeper

That's the problem when someone has two threads going on the same topic. I repeated what you said in the other thread before I read this.

Yes, sorry about that. At first I assumed it must be a ground wire according to that thread I posted in as there were several users with my exact issue, so I asked a question about finding a particular ground wire. But once I checked all grounds, I didn't want that thread to go off-track, so started a new one. Probably should have started a new one the whole time, my apologies.

Originally Posted by acutron42

I did see that & it does get confusing. Here is a power distribution diagram but it is hard to post a full page & probably wont help much.

This is helpful. Didn't realize there was an ECM BATT fuse in the rear trunk that feeds forward to the ECM. Though I'm guessing that's ok since I'm blowing the fuse in the engine compartment. I need to find this diagram in the Service Manual PDF I have so I can zoom in better.

I'm out to go exploring more wiring harnesses...

[Gunn]

Originally Posted by hesster

I considered this as a aid to resolve my starting issue.
https://www.camaro5.com/forums/showthread.php?t=539796

Ok, more messing around, more clues, but still no fix. Just as many of you have stated, I think it is a wiring issue, so still continuing down that path, but still unsure where I'm going.

That ScannerDan Youtube channel referenced in the link above is great. I should trailer my car up to Pittsburgh and have him and his students have at it. He'd probably figure it out in an hour. Anyway...

I watched one of his videos where the main power train relay was the culprit. In my studying, the only thing listed as upstream of this ECM FUSE blowing was that very Powertrain relay (K26) in the engine compartment fuse block. So I decided to test it. The relay tested out fine on the bench. 80ohms across the control terminals, and with 12V applied across control terminals, relay clicks and continuity is confirmed on load terminals.

Originally Posted by 102SS

My G8 LY7(3.6L) makes those noises.

It sounds like relays/solenoids clicking or something like that.

My V8 Camaro does not

It turns out, that clicking I mentioned in Post #2 (2-3 second series of about 10-15 clicks, at about 5Hz) is that very relay. With it removed, no clicking at all.

• With the #5 ECM Main fuse blown, relay K26 ticks quickly several times when door is opened (car "waking up"?).
• With new fuse, just a single light click when door is opened. No 5Hz clicking.
• With new fuse, no voltage on ECM fuse #5 is measured with car asleep, key off.
• With car awake (open the driver door), fuse now shows 12.8V on left side leg (towards rear of car).
• Right side of fuse shows basically 0V...?!
• ~90seconds later, a click is heard (relay) and fuse blows
• With fuse blown each side measure 0 volts to ground with door immediately opened (car awake)

I'm not sure what to make of all of this. Note that I have learned that the key position makes no difference. It's whether the car is awake or asleep. And it seems to blow the fuse at the point it decides to go to sleep, strange!

I would also think that when the fuse is good, I would see battery voltage on each terminal of the fuse. Why am I only seeing it on one leg? Doesn't make sense to me, as each leg is only separated by a 1/4" of fuse material. I'm measuring the voltage using the contact on the top of each leg of the fuse, with it installed. Multimeter ground is on a good ground.

I even swapped the K26 fuse with the K55 fuse (rear defogger). Same issue, so I'm pretty certain the relay is OK.

Anyone see any clues from this?

[gtstorey]

If you aren’t getting the same voltage on each side of the fuse (voltage above zero) then the fuse is bad or your measurement is wrong.

[silversleeper]

Originally Posted by Gunn

• With the #5 ECM Main fuse blown, relay K26 ticks quickly several times when door is opened (car "waking up"?).
• With new fuse, just a single light click when door is opened. No 5Hz clicking.
• With new fuse, no voltage on ECM fuse #5 is measured with car asleep, key off.
• With car awake (open the driver door), fuse now shows 12.8V on left side leg (towards rear of car).
• Right side of fuse shows basically 0V...?!
• ~90seconds later, a click is heard (relay) and fuse blows
• With fuse blown each side measure 0 volts to ground with door immediately opened (car awake)

Anyone see any clues from this?

That is not possible with a fast blow fuse unless it's already blown in a place where you can't see it which is unlikely but can happen. I'd double check your work here before moving on. Probably you had your ground clip wiggle loose but figure out that.
Ooops beaten to the post again.
Then if it's not your imagination and the fuse blows precisly when a relay clicks.
Carefully label each relay with masking tape sharpie and take a pic. THEN pull them out and replace them one by one. If what you noted is true then when it blows stop. Swap out a different same type relay and see if that still blows. If no its a bad relay. If yes you'll know the circuit that has a short. We still lean towards a short somewhere.

[acutron42]

Here is the ignition flow sequence. If you need ant diagrams let me know.

Circuit Description (Key Start)

When the ignition switch is placed in the Start position, a discrete signal is supplied to the body control module (BCM) notifying it that the ignition is in the Start position. The BCM then sends a message to the engine control module (ECM) notifying it that CRANK has been requested. The ECM verifies that the transmission is in Park or Neutral. If it is, the ECM then supplies 12 V to the control circuit of the crank relay. When this occurs, battery positive voltage is supplied through the switch side of the crank relay to the S terminal of the starter solenoid.

[bsn]

If you have an old school (non LED) test light put it in series in place of the fuse. It will glow brightly when there is a short with less danger of burning something up with a fuse. It's doubtful it's a relay, start disconnecting connectors when your test light goes out you know what circuit/area to look at.

[Gunn]

Originally Posted by gtstorey

If you aren’t getting the same voltage on each side of the fuse (voltage above zero) then the fuse is bad or your measurement is wrong.

Thanks everyone, good stuff.

So I was thinking about these measurements and while I didn't get a chance to head out to the car today after work, I think I realized what my measurements were telling me.

I'm betting the fuse blew as soon as I opened the door to wake the car up. By the time I got back to the fuse box and multimeter, I checked voltage, and only saw it on one side of the fuse...the battery side. Then, 90 seconds later the car went back to sleep, the relay clicked off, and my voltage went to zero. That's when I assumed the fuse blew. But it had been blew all along, I'm thinking now.

I need to get a helper out to help me and test it again.

[Gunn]

Originally Posted by bsn

If you have an old school (non LED) test light put it in series in place of the fuse. It will glow brightly when there is a short with less danger of burning something up with a fuse. It's doubtful it's a relay, start disconnecting connectors when your test light goes out you know what circuit/area to look at.

That's a good idea, and I see what you're saying (the bulb will give resistance to the circuit, unlike an LED with such low current draw) but unfortunately the two test lights I have are both LED's.
Actually, I'm not sure if they are or not. I kinda assumed that test lights were incandescent to create a safety net sort of speak which will limit the amount of current that can flow through...

[silversleeper]

Originally Posted by Gunn

That's a good idea, and I see what you're saying (the bulb will give resistance to the circuit, unlike an LED with such low current draw) but unfortunately the two test lights I have are both LED's.

I've made my own by buying a cheap tail light bulb and soldering a couple wires to it. The brake light would be more ideal for your application...headlight bulb...
Some generic, is it 1157 IIRC ?

[PaulsSS]

I'm not up on all you have done so far but I did skim through things. I'd be disconnecting and testing injectors coils etc before taking a chance on putting a ECM in there because if something is shorted other than the ECM itself it may instantly fry the new ecm. At least follow the diagrams and disconnect everything from the ecm. You should be able to find a dead short like this pretty quickly, I know I could using a ohm meter and a test light. Unfortunately I've been down this road many times and it can be very costly if you don't think things through carefully. If you know of a good tech in your area it may be worth it to let them diagnose it. I know finding any good knowledgeable techs today is a really tough. Last time I had to use the local dealer to simply reprogram the key fobs because they stopped working they were stumped and tried to chase their tails on my money. I stopped their crazed frenzy of blaming this and that by simply requiring them to FOLLOW GM's procedure to reprogram the fobs, Fixed it. Good Luck