Hello all. New to this forum. Just bought a 97 GMC Jimmy 4.3 Vortec - 173,000 miles. Got a great deal on it 'cause it wouldn't start. Got an AllDataDIY subscription for it and followed the diagnostic procedure for "cranks but doesn't start." Previous owner had replaced plugs, wires, coil and ICM, and gave up. There is no spark at the coil. The procedure I followed asked to test for 1-4 volts AC coming from the ECM on the IC control line to the ICM (pin 9 on the white connector) - while the engine is cranking. I got .6 v once and 0 volts the next several times I tested it. No open or short between the ECM and ICM. Procedure said I needed a new ECM. (Can you see where this is going?) Got the new ECM and had it flashed installed it - no start.

Any ideas?

 

Have you tested all of the grounds and power supplies to the ECM?

 

No. I'll see if I can find them in AllData. Thanks for the reply!

 

I found the page in alldata showing the power and grounds to the ECM. One in particular seemed promising - this was the pink wire ignition circuit 439 that is supposed to be hot in run and start. I spliced into the pink wire leading to pin 18 C4 (black connector) to the ECM and connected a 12V test light to ground. The lamp lit up in "run" and went out in "start". This makes me think maybe I have a bad ignition switch. I rigged up a 20amp inline fuse and connected the pink wire to the + battery post. Aha! I heard the fuel pump pressurize the fuel lines for the first time. But, it still will not start. Just to make sure that I had a signal from the ECM to the ICM, I checked the "B" terminal on the ICM connector with the key turned to "start" with my fused jumper attached. Sure enough, it read ~4 volts AC. So, progress, but not home yet.

Are there other connections made in the ignition switch start position that are needed in addition to the 439 circuit, or maybe a diode that prevents me from energizing it from the lead to the ECM connector? Is there an anti-theft feature that is preventing start at this point? I think I will next check to see if I have spark at the coil in "start".

Any help is greatly appreciated.

 

New ignition switch now provides voltage to ICM and spark. Thanks to WolfPack for excellent instructions for changing out the ignition switch. I'm not sure how this forum works, but even though the truck still won't start, the topic title has been resolved. I do now have voltage to the ICM. So, I'll start a new thread about the "no start."

 

If you think it is out of time, a compression check will tell you if it is atleast close. If it is close (decent compression), you can confirm that it is out of time by removing the distributor hold down bolt and rotating the distributor to try to bring the spark back into the appropriate timing.

You could also bring the engine to TDC #1 and see if the rotor is pointing at the indicator in the distributor cap mounting flange.

 

Thanks for the reply. I'll get on it tomorrow and see what I've got. Should I start a new thread for the "no start?"

 

You can rename this one. I think it is beneficial to keep it all together to see the history of what has been done.

 

Started this morning by pulling the plugs. Searched "remove spark plugs" here and found the trick to removing #3 - Put the spark plug socket on the plug, then use a 3/4" offset box wrench on the hex on the end of the socket to crank it out and in. Worked slick! The AC platinum plugs looked fouled with gasoline. #5 actually was full of gasoline; it dripped out when up-ended. I hoped that might be why it was backfiring and not wanting to start. With the plugs out, I checked the compression, but my old gauge won't hold the max reading and I either have to place the gauge where I can see it while cranking or run out really quickly to guess what it was. Results: #1: 180 #3: 195 #5: 150 #2 200 #4 150+ #6 150
I realize the high/low difference is out of spec., but the cylinders were cold and probably all the oil was washed off the crosshatch from the gasoline, and the engine had not been run for at least a couple of weeks so maybe the valves were not as tight as they might be on a warm check. Anyway, I did not find a bad piston, broken spring, or bad valve. Marked the distributor edge in line with the #1 cap contact. Removed the distributor cap - cleaned the cap contacts (green) and the rotor contacts. Watching the rotor, had the wife spin the engine with my finger in #1 plug hole. At top dead center (pfishhh) the rotor was just passing the position for #1 contact, so I figured it was not out of time. Could find NO mark for TDC on the harmonic balancer. Is there one? I installed new NGK v-power copper core plugs gapped to .045" because I didn't feel like shelling out $40 for the AC plats just to see if it was gonna' go. Double checked the spark at the coil (good). Reinstalled the original computer cause I figured it knew the engine better than the new one. Cleaned the MAF wires (dirty). Double checked the plug wire routing, applied silicone to the boots and buttoned everything up. It started after just a couple of spins and idled pretty smoothly. (Yay!) But then....as I opened the throttle a bit, it started to run really roughly. Missing in an uneven way, not like a single cylinder miss. Return to idle and it is pretty smooth. Unplugged the MAF, still rough on acceleration. Replugged MAF. This set P102 but no other codes were set. Could not find anything obvious unplugged, don't hear a vacuum leak.

Oh yeah, I checked fuel pressure before starting 62 psi then 58 and hold...After starting it up at idle: 55 psi, quick throttle blip: 62 then back down to 55, slowly open throttle: 60 psi then down to 58 and steady.

Any ideas on the rough run?

 

Hey Bill!
Umm, you said you gave it a little gas and it ran rough. Am I thinking TPS?