Troubleshooting hard cold start on my 95, 3.9, MT. When you turn on the key, the truck will crank but not start. The Auto Shutdown and Fuel Pump relay do not pull in. I have pretty much traced every wire out, checked voltages and battery condition, re-did every ground, cleaned and resoldered the under the power center splice. As the truck sets with the ignition switch on, the Fuel Pump and Auto Shutdown relay will begin to hum. As a few minutes pass, the hum gets louder, eventually becoming chattering of the two relays. The chattering will stop after about 4-5 minutes and at that point, the relays lock in and the truck will start and run perfectly until the next time it sits long enough to trigger that issue. At this point, I cannot think of anything but an ECM issue. The humming of the relays in a DC circuit confused me for a bit until I realized the ECM logic likely runs on 5 volts and these is probably a switching power supply for that. If the power supply was failing, it may well be sending out a low grade, unfiltered voltage that is causing the buzzing that morphs into the louder noise, then the chattering, then working OK once the power supply gets fully on line. Anyone have any other thoughts on this?

 

we have a 92 that had those same symptoms . reminded me of vacum tubes warming up. in my case it was a green crusty body ground that fixed it

 

You probably have and just did not mention it but if you haven't try putting new relays in. They go bad too.

 

Yup. Already did! Thanks, though! Have positively determined the issue lies in the PCM. The problem only happens when the PCM is cold. If I open the hood for the sun to shine on it or warm it up with my heat gun the truck starts just fine and runs perfect until the next time it sits and the PCM cools back off. I have been playing with it trying to find the weak component to see if I can fix it but have not dug too far into the potting compound until I have another good PCM. Right now it's a nuisance but I can still drive the truck as is.