1995 GMC Sierra K1500 5.7 engine misfiring after transmission shifts into second gear.

KenTaz · December 15, 2019

Hello, Ken here. I have posted about this issue in another topic which has received only 1 persons reply.

My 95 Sierra 5.7 has been misfiring, backfiring, and almost stalling out, after the automatic transmission shifts into second gear. If I manually shift it back down to 1st the engine then runs fine. I have tested the map sensor, purge valve, and EGR solenoid, which are all ok. I have replaced the EGR valve, TPS, distributor cap and rotor, spark plugs, spark plug wires, ignition control module, and all of the vacuum lines. I have rebuilt the throttle body, including the fuel pressure regulator. I cleaned and tested the injectors which are spraying nicely. I've also checked the timing with the electronic advance wire disconnected and it is fine. The truck is NOT getting a "Check Engine" light and there are no codes stored. Could this be an ECM issue or a sensor that is throwing the wrong info to the ECM once the transmission has shifted into second? Again, the truck runs great in first gear and at idle. Very frustrated after months of trying to figure this out. Thanks for your help!

riverbanks · December 15, 2019

?,what about testing fuses,both sides for a voltage drop, important ones, ignition?or at coil ,do those tests when it's misbehaving

KenTaz · December 15, 2019

I can't test those while it's misbehaving. It's only acting up while driving and after it shifts into second gear. It shifts fine by the way.

riverbanks · December 15, 2019

Yes,takes some wiring,I was just thinking,power is being pulled down,worth pulling electric connection off of tranny for a inspection?

KenTaz · December 28, 2019

Ok. I pulled the ECM and back probed the connector's battery+ and ignition+ pins. When the truck started to act up both voltages stayed exactly the same. I also did a voltage drop test on both battery and ignition pins. I got a voltage drop of .001v on both pins. I also did a voltage drop test on all of the ECM grounds including the sensor grounds. I got the same .001v drop. So I am not losing power to the ECM and have excellent grounds.

The truck runs great until after it shifts into second gear. Then the engine starts to hesitate, misfire, and sometimes backfire. If I manually shift it back into first the engine runs great again. I manually put the transmission into the second gear position at a dead stop and I couldn't even pull out without the engine acting up like crazy. I had to shift it into D or 1 for the engine to run right again.

riverbanks · December 28, 2019

Putting it in 2nd gear alone,starts the bad behavior? did you inspect tranny connection?

KenTaz · December 28, 2019

9 hours ago, riverbanks said:

Putting it in 2nd gear alone,starts the bad behavior? did you inspect tranny connection?

Sorry, I forgot to mention that I removed and inspected the transmission plug. It was clean with no corrosion. It looked as if it were brand new. The 4l60e has an external neutral safety/backup switch with a 7 pin and a 4 pin plug. The 4 pin plug is for the transmission range switches A, B, C, and P, as seen in the attached picture of the plugs on the right. I cannot find a wiring diagram of the engine harness, which includes the transmission, fuse box, engine components, and sensors leading to the ECM. I need to know where these 4 wires go and how they interact with the transmission solenoids and the ECM.

riverbanks · December 28, 2019

The round plug,is to pcm,that neutral safety switch only does 3 things,p,n start, reverse lights and gear selection on instrument cluster

KenTaz · December 28, 2019

Oh ok, I see. That makes sense. So what sends info to the ECM from the transmission. Just the transmission range sensor inside the trans? A, B, and C.

riverbanks · December 28, 2019

Google,4l60e in limp mode,many wiring diagrams show up,my only thought with the problem shifting into 2nd gear,sitting or driving is ground control to solenoids from pcm to tranny,so the 12v feed in connection is back feeding a ground to pcm?there is not much else happening at that event,so bad harness or fluid filled connection?

KenTaz · December 29, 2019

19 hours ago, riverbanks said:

Google,4l60e in limp mode,many wiring diagrams show up,my only thought with the problem shifting into 2nd gear,sitting or driving is ground control to solenoids from pcm to tranny,so the 12v feed in connection is back feeding a ground to pcm?there is not much else happening at that event,so bad harness or fluid filled connection?

I'm going to check the voltages coming from the ecm to the pressure switch manifold in the tranny. There is no problem with the tranny shifting, so if there was a back feed in the ground circuit of the solenoid(s) the tranny would either not shift or be very sporadic when it tries to shift, correct? I'm also going to try another ecm from my son in laws 95 suburban if it has the same service number as mine. 16197427. Thanks for all your time and suggestions. I really do appreciate it.

riverbanks · December 29, 2019

What about pulling fuse for tranny,put it in limp mode,then start it,shift to 2nd and if motor doesn't misbehave, circuit problem?

KenTaz · December 31, 2019

I will give that a try today. I got an ECM from a 95 suburban with the same service number. 16197427. I swapped the prom out and gave it a try. Things were acting goofy when I turned the ignition on. Fuel pump didn't run, all the warning lights that usually come on with just the ignition switch in the on position weren't on, and I heard a clicking noise in the engine compartment around the fuse box. It sounded like a relay. The truck would not even attempt to start. Are these ECM's programmed to each vehicle or is it just the proms that are programmed differently? Shouldn't I be able to buy a new ECM with the same service number and just use the prom from the trucks original ecm?

riverbanks · December 31, 2019

I would say you are correct,but,being the last year of obd1,? maybe v.i.n. is involved also,after fuse pull for tranny test,what else could be isolated?only thing that comes to mind is unplug harness from instrument cluster,what else happens in 2nd gear,that doesn't happen in the others?

riverbanks · December 31, 2019

Did someone in the past,repin ecm for some reason,a fault,still,just thinking out loud,a/c relay was energized, instead of fuel pump?I did go on AutoZone site, halfway decent schematics for your truck