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

KenTaz · December 31, 2019

I originally thought that it might have been pinned differently also. My thoughts on the repining of the ECM: I'm using another ECM with the same service number. No matter which wires would have been changed I should see the same functions with either one of the ECM's. ( Unless the ECM's are programmed differently or the one from the suburban is also bad).

Yes, I've seen those diagrams on the Autozone website also.

I haven't noticed any other odd things happening with the truck in second.

I'll get back here after I pull the tranny fuse and go for a ride.

KenTaz · December 31, 2019

Ok, I pulled the tranny fuse and it ran fine in every gear that I manually shifted to. So it could still be a problem with the tranny manifold pressure switch circuit or the shift solenoid circuit or both. And it could be in the ecm also. Correct me if I'm wrong

riverbanks · December 31, 2019

Can't correct you, you've been right all along,darn it,still don't see the correlation between the two,would you go so far as disconnecting the cluster?it's just further isolation,v.s.s. will now be out of the loop

KenTaz · December 31, 2019

Lol. I have a few different diagrams showing the manifold pressure switch wiring. Some are showing the switches being internally grounded with a 12v signal coming from the ecm. Other ones, including the Haynes manual, showing the switches along with the solenoids sharing a 12v supply coming from the tranny fuse, which would mean the wires going to the ecm would be the grounds. If there would be a short in the ecm involving the grounding circuit, that could affect almost anything else inside the ecm as well I suppose.

KenTaz · January 12, 2020

On 12/31/2019 at 3:46 PM, riverbanks said:

still don't see the correlation between the two,would you go so far as disconnecting the cluster?it's just further isolation,v.s.s. will now be out of the loop

Ok, let me see if I can try to explain what I think the correlation between shifting the tranny manually and the tranny shifting to second on it's own.

When it is shifted manually into 2nd, the manifold pressure switches are activated in such a way to not let any of the range circuits, A, B, or C, go to ground and complete the circuit in the ECM. When the tranny shifts to 2nd on it's own, the same switches are activated, creating the same scenario to the ECM.

As far as disconnecting the cluster harness to take the vss out of play, there is a separate wire coming from the vss buffer to the speedometer and another from the buffer to the ECM.

riverbanks · January 12, 2020

Remembered this site,download it,wow,4L60E_80E_CODE_BOOK.pdf

KenTaz · January 12, 2020

Wow is right! Thanks. After I downloaded this I realized that I already have it. Lol

Edited January 12, 2020 by KenTaz

KenTaz · May 9, 2020

Hey riverbanks! Sorry I didn't get back sooner. I took a little break from working on this issue until a few weeks ago. I decided to start fresh with the basic diagnostics, after replacing many parts and testing many others. I found that I actually tested the ground voltage drop incorrectly. I did not have the ignition turned on to test the grounds. Who knew that this would make all the difference in the world. I found a .03v drop on the two ground wires going to the ECM. They are grounded to the thermostat housing. So, the ground from the battery is hooked to the RH intake manifold stud. Other grounds are hooked to the LH intake manifold stud. The thermostat housing is naturally hooked to the intake manifold. Not such a great place for a good ground GM! I cleaned the studs and nuts and put new wire ends on the wires. I was confident at this point that I had finally found the problem. It ran great for a few weeks until one dreary rainy day it started running bad again. I immediately checked the ground wiring. Turns out, the stud in the thermostat housing was registering a .025v drop. I made a jumper wire and ran it to the LH intake stud to solve the issue. The wires weren't long enough to run directly to the stud, hence the jumper wire. I didn't want to run the jumper directly to the main ground in case I would lose the ground to the intake and fry the ECM. All is taken care of now. The only issue I am having is the idle is at 1500 rpm when cold, but goes down to 800 when it completely warms up. I think it may be the IAC valve is reacting too slowly. Thanks for all your help!