2000 GMC, no start cold, brain twister.......

OK this one really has me scratching my bald head. Son in Law brought his Jimmy over with "no start when temperature is cold" problem. Here's what I did:

Cold this morning and didn't start sooooo

1. Checked fuel pressure....OK
2. Easter egged crank sensor for $61.00, no change, still wouldn't start.
3. Installed spark tester on coil stack.....no spark.
4. Took ignition module to O'Reillys and checked out OK.
5. Checked coil...OK
6. Noticed that when you let the key off of crank would get one quick spark.
7. Checked voltage on red wire feeding ignition module, had 12V when ignition was on but it dropped to zero when cranking.
8. Battery OK
9. Used jumper wire from ignition module to battery and got good spark when cranking and car would start.

Question: What supplies 12v to ignition module when cranking? Haynes/Chilton wiring diagrams worthless for this.

I noticed that with key off when I jumped 12V to ignition module the fuel pump would run for the 2 seconds.

Quick fix as they had to return home was to install a switch between 12v and ignition module red wire that you could flip on temporarily to get car started. Once car started and the day was warm...no problems. Evidently he has had this problem only during cold weather for the past 2 years. During warm weather months it is not showing up and car starts perfectly.

Ignition switch?? Cold grease in it??

Thanks

Lynn D. Wagoner
Yukon, OK

It could be the ingition switch or a bad connection. below is a copy of the wiring system if it is a 4.3
Enhanced Ignition System Diagnosis


http://service.gm.com/engif/000/000/623/623367.gif
http://service.gm.com/tif.gif (http://service.gm.com/servlets/RetrieveTif?pic=623367)

Thank you sir...looks like voltage on crank doesn't come from computer, thank goodness. I have a buddy with access to GM service manuals. I'll let you know.

Lynn

Final resolution....for all of you electrical genuis folks.

After going over the drawings I've finally determined that the voltage for the coil, and ignition module during cranking phase comes from the VCM, or ECM, or computer...whatever you want to call it. The yellow wire from the ignition switch tells the computer that the engine is in the crank phase. The computer supplies 12V to the coil and ignition module for start. AFTER the engine starts, the computer monitors several sensors, i.e. crank sensor, oil pressure sensor, and several others to determine that the engine is running thus supplying power for fuel pump, etc.

I theorize that there is a bad solder joint or PC board run in the computer that is contracting due to the cold weather and causing the loss of voltage during cold weather. I installed a switch between battery (fused) and the pink wire on the ignition module to supply crank voltage during those rare times it is cold enough to cause problem. I guess another alternative would be to use a relay driven from crank voltage to supply the 12v to the pink wire.

Thanks for the help and I hope you never encounter this one......

Lynn Wagoner
Yukon, OK