96 Chevy K1500 4l60e Hard Shifting

[---gyro---]

Here is what happens: When the truck first starts everything is OK for about 10 minutes of driving then the current signal going to the PCS solenoid in the 4L60E transmission drops to zero causing the pressure in the transmission to go high and cause rough shifts. When the engine stops for a minute or two this seems to reset everything and the truck is OK for another few minutes of driving before the same thing happens again. There are no codes, no check engine light and the Snap-on MT2500 scanner shows zero amps to the PCS solenoid all of the time. I have been checking the PCS solenoid current with a miniature clamp on DC ammeter connected to a Fluke multimeter. This current is .78 amps before the problem starts then goes to zero. The transmission thermistor voltage from the PCM is OK around +/- 2.5 volts and moves correctly with temperature. The resistance of the PCS solenoid is 4.8 ohms. The factory spec is 3 to 7 ohms. I checked the PCS circuit for shorts to ground and there are no shorts to ground. This truck has a Transgo shift kit that has been in for about 3 years so I don’t think that this is the problem. The folks at Transgo have said that they have found the scanner data is unreliable and to use an external DC clamp on ammeter to check the PCS solenoid current, I agree. The folks at Transgo, my transmission mechanic as well as myself are out of ideas. Replacing the PCM was the last thing we did, it did not change anything. Any helpful tips that you may have would be greatly appreciated. Thanks for taking time to read this.

 

[NickG80]

Have you checked pin figment? Possible loose terminals can cause intermittent issues, also I wouldn’t rely on that scanner either. Make sure you don’t have loose terminals also have you replaced the pcs? Check the connections in the transmission and the connections into the transmission.

 

[NickG80]

Have you checked pin figment? Possible loose terminals can cause intermittent issues, also I wouldn’t rely on that scanner either. Make sure you don’t have loose terminals also have you replaced the pcs? Check the connections in the transmission and the connections into the transmission.

 

[---gyro---]

Have you checked pin figment? Possible loose terminals can cause intermittent issues, also I wouldn’t rely on that scanner either. Make sure you don’t have loose terminals also have you replaced the pcs? Check the connections in the transmission and the connections into the transmission.

Nick: Thanks for the fast reply! No, I have not replaced the PCS solenoid. So far my trouble shooting shows that the PCS itself is OK it's the signal going to the PCS that is bad. I saw a YouTube video that shows how to test the manifold pressure switch. I have read several places the this could be my problem, so this is my next move. This will involve the plug and pins that you are referring to, so I will check those as well.

 

[---gyro---]

Well I am going to answer my own question. The problem was that the red / black wire that goes to pin "C" on the transmission was intermittently opening or shorting to ground. This wire goes to one side of the PCS solenoid. When this happens, the PCM would see this and turn off the current signal to the PCS solenoid causing the pressure to go high and making the transmission shift hard. The bizarre thing about all of this is that there was no code set. When the ignition was cycled off then on again, this would reset everything and it was good again for a while until the red / black wire shorted to ground or opened momentarily and this happened all over again. We found this after many hours of messing with this truck and replacing a lot of parts for nothing. My advice to anyone having this problem, replace this wire first. This truck has 292K miles on it but it is still a good truck but those tiny wires from the PCM to the transmission just breakdown over time. Truck is running good now.