Car Not Starting - U0100 Code: SOLVED!

[Gunn]

Thanks! But no fix yet. I had assumed the starter is grounded to the engine block. But I don't think the starter is my issue. Made a little progress...

I did some electrical diagnostics. I checked the CAN bus since I was getting code U0100.

On the OBDII connector:
Pin 4 to pin 6 (can bus hi, key on): 2.67V
Pin 4 to pin 14 (can bus low, key on): 2.31V
Pin 6 and 14 (battery disconnected): 60.9 ohms

Those are all good, so CAN bus network checks out OK. Then I thought about the fuses. I went through the Service Manual and noted any fuse that mentioned ECM. Fuse 5 in the engine compartment fuse block is a 15A fuse for the ECM.

Sure enough, it was blown. When I replaced it, I noticed the check engine light was no longer flashing. Turned the key to the Start position and it cranked! Baby steps. However, it would NOT start. Bummer. Checked the fuse. Blown.

I found that as soon as I move the key to the On position, the fuse blows.

Been researching that now for 2-3 hours. Lots of mentions about it happening to people, but no one reports the fix.

Not sure if that is 100% a bad ECM, or there is something (sensor?) external being grounded causing it to blow the fuse. Wiring diagram doesn't show much else connected except for the ECM. But I don't know...

More clues, hope someone can help!

[chevy pilot]

Is it possible that a rodent has taken up residence under the hood ?

[102SS]

Looks like you didn't clean the battery ground.

I would start there first.

[silversleeper]

Originally Posted by Gunn

Been researching that now for 2-3 hours. Lots of mentions about it happening to people, but no one reports the fix.

Thats the problem. Some people get tired of suggesting when the OP doesn't even have the courtesy of saying what it was afterwards. So all you're left with half a doz guesses which might not be the best answer.
I have a few ideas. Might even cause damage but I don't think so.
First start from the ECM back follow the wiring harness and look hard for chafing leading to shorts. Where ever it touches something is a hassle but that's where you want to look. My memory isn't great so I can't rememeber if there was a spot on the firewall behind engine that some people have a short, hard to see.
My second idea which might be a crap idea. Is unplug everything from ecm and see if it still blows fuse. In my mind that would be a short somewhere for sure and maybe rule out the ecm, free and easy to try too.

[Gunn]

Originally Posted by chevy pilot

Is it possible that a rodent has taken up residence under the hood ?

Nowhere I could find. And it's driven pretty much all the time, so low chance of that happening.

Originally Posted by 102SS

Looks like you didn't clean the battery ground.

I would start there first.

I hope this was a joke that I just didn't get. That was the very first thing we checked. Even changed the battery. The battery terminals are like brand-new, no corrosion at all.

Originally Posted by silversleeper

I have a few ideas. Might even cause damage but I don't think so.
First start from the ECM back follow the wiring harness and look hard for chafing leading to shorts. Where ever it touches something is a hassle but that's where you want to look. My memory isn't great so I can't rememeber if there was a spot on the firewall behind engine that some people have a short, hard to see.
My second idea which might be a crap idea. Is unplug everything from ecm and see if it still blows fuse. In my mind that would be a short somewhere for sure and maybe rule out the ecm, free and easy to try too.

I spent the day today doing just that. Even unwrapped the engine harness in a few spots and peeled it back. Nothing. Everything looks good.

I will unplug the two large terminals from the ECM tomorrow and see what happens to that fuse, good idea.

Thanks,

[102SS]

Sorry I only read the bullet points in post 13.

Ignore my early morning stupidity

[Gunn]

Originally Posted by 102SS

Sorry I only read the bullet points in post 13.

Ignore my early morning stupidity

No worries!

Originally Posted by silversleeper

My second idea which might be a crap idea. Is unplug everything from ecm and see if it still blows fuse. In my mind that would be a short somewhere for sure and maybe rule out the ecm, free and easy to try too.

Unplugged both connectors. Fuse did not blow in any key position. Reconnected x1, the 98-pin connector. Fuse did not blow in any key position.

Connector x2 has 58 pins. In the first 6, there are 3 ground pins and 3 pins labeled IGN VOLT. I don't have the diagram with me at the moment, but I think pin 1, 2 are ground, 3 is IGN VOLT, 4 is ground and 5, 6 are IGN VOLT.

The 3 ground pins in the connector show a good ground.

I tested continuity on all 6 pins on the ECM and they ALL show that they have continuity, no matter which two I check. Does that mean anything, I'm not sure?

Not sure where to go now?

Again, wiring harness still looks good, I checked more today. Arghhh.

[silversleeper]

Originally Posted by Gunn

No worries!




Unplugged both connectors. Fuse did not blow in any key position. Reconnected x1, the 98-pin connector. Fuse did not blow in any key position.

Connector x2 has 58 pins. In the first 6, there are 3 ground pins and 3 pins labeled IGN VOLT. I don't have the diagram with me at the moment, but I think pin 1, 2 are ground, 3 is IGN VOLT, 4 is ground and 5, 6 are IGN VOLT.

The 3 ground pins in the connector show a good ground.

I tested continuity on all 6 pins on the ECM and they ALL show that they have continuity, no matter which two I check. Does that mean anything, I'm not sure?

Not sure where to go now?

Again, wiring harness still looks good, I checked more today. Arghhh.

Just a word on "continuity".
If something is assumed turned "on" (aka always on) inside like ecm there would be some "continuity" if you checked it as a simple yes no tester, sort of false reading.
Ohms resistance might help slightly more as if it said zero resistance could indicate shorted inside. A circuit will have some resistance. If it's not shorted. or burned up to short. I have no idea what "normal" resistance is on the pins and not about to risk breaking and bending pins on mine to try to find out for you and beside my ecm is different than yours. Sorry.
Again you could test a junkyard ecm before spending much to see if it matches yours. Programmed or not the ohms resistance should be near identical.
On a related side note IDK what pins drive things such as Body Control Module and Fuel Pump Control Module so those harness or unit could be the actual problem or associated relays so that's why testing on complicated problems is expensive and time consuming. Again I'm too lazy to look up pins and diagrams...sorry. Maybe some ambitious member will help.

[Gunn]

Thanks for the good info on continuity. Totally get it, and I recall learning that very thing long ago. I will check for resistance instead of just a simple continuity test. There should be some resistance between ground and battery voltage terminals, due to the circuit.

I broke off of this thread into my own, as I was getting off track of the original topic of this thread, since mine (unfortunately) does not seem to be a bad ground problem, but instead a short to ground.

See my specific thread here: https://www.camaro5.com/forums/showthread.php?t=620398

Thanks everyone.

[silversleeper]

Originally Posted by Gunn

Thanks for the good info on continuity. Totally get it, and I recall learning that very thing long ago. I will check for resistance instead of just a simple continuity test. There should be some resistance between ground and battery voltage terminals, due to the circuit.

I broke off of this thread into my own, as I was getting off track of the original topic of this thread, since mine (unfortunately) does not seem to be a bad ground problem, but instead a short to ground.

See my specific thread here: https://www.camaro5.com/forums/showthread.php?t=620398

Thanks everyone.

Yeah big fat wires will show no resistance. Big fat starter winding wires will show some resistance because they are so long wound up.
A THING is there exist a few delicate items. Like cpu's that operate nowdays like 1 volt. A typical ohm meter has internal batteries maybe 6 or 9v. If you apply that to delicate things it actually burns them out just testing them. One example is car's oxygen sensors. They are made to PRODUCE a low voltage like under 2v so pushing high volts TO them burns them out. Some things that seem harmless can actually cause damage.
A continuity testor that eg goes beeeep does that too, puts voltage across the probes. You could take a second volt testor and see what this voltage is. IIRC some newer testors made for cars intentionally step down the probe volts to make them "o2 sensor safe" as mechanics without some electrical background use them to touch anything and everything.

[Gunn]

Originally Posted by silversleeper

Yeah big fat wires will show no resistance. Big fat starter winding wires will show some resistance because they are so long wound up.
A THING is there exist a few delicate items. Like cpu's that operate nowdays like 1 volt. A typical ohm meter has internal batteries maybe 6 or 9v. If you apply that to delicate things it actually burns them out just testing them. One example is car's oxygen sensors. They are made to PRODUCE a low voltage like under 2v so pushing high volts TO them burns them out. Some things that seem harmless can actually cause damage.
A continuity testor that eg goes beeeep does that too, puts voltage across the probes. You could take a second volt testor and see what this voltage is. IIRC some newer testors made for cars intentionally step down the probe volts to make them "o2 sensor safe" as mechanics without some electrical background use them to touch anything and everything.

Thanks that's great to know!