Driving at cruising speed in 3 or D, encounter a hill and the truck shudders and bucks in same gear… then will finally downshift and everything is fine. The high miles 232k may be the issue of course. But does this thing have a vacuum modulator or something on the transmission that I can replace besides the fluid and filter that will fix this reluctant down shifting problem?
You don’t have a vacuum modulator
Have you been servicing the atf and filter every 30K?
You might take a look at the Throttle Position Sensor as that is usually read by the PCM to decide when to downshift. I assume that the VSS is working correctly i.e. the speedometer is working correctly.
Researcher, hello and thanks for your comment. When I depress the throttle it tries to downshift, then shakes and shudders and finally will down shift. THE speedometer works fine. Do you still think it could be the TPS? I really don’t know what to think on this one…
Db, yes I have serviced the fluid and filter, have only put 38k on the truck since I’ve owned it. But it may be about ready for another fluid and filter service.
The TPS is one of the sensors the PCM uses to make its shifting decision. Without knowing the internal programing of the PCM it is hard to determine what other sensors are factored into the shifting program. I didn’t see a torque sensor in the drive train so that leaves the Mass Air Flow sensor, Inlet Air Temperature, turbine input speed sensor, etc. as input to the power production calculation.
The reason I suggested the TPS is that it is fairly easy to check. If you have an older VOM i.e. needle meter, use that. A DVM is a little harder to use because of the sample time of the AD conversion. Disconnect the connector to the TPS. The resistance of the two end pins on the TPS should show a constant resistance as the throttle plate is opened (engine OFF). The resistance of the center pin to one of the end pins should transition smoothly from near zero to max or vice versa. The resistance of the center pin to the other end pin should do the opposite. The ;emphasis is on “smooth” i.e. no drop outs or jumps.
The next step is take the GMC to the dealer or an independent transmission shop and have the Tech scanner attached to the OBD !! port. Have the technician monitor the scanner data as you evince your problem. He/she should be able to determine if the problem is an engine driveability problem or transmission shifting problem. What the tech should look for is a 4-3 shift command that is not happening. I am wondering if the 1-2 solenoid which is “ON” for overdrive is getting stuck. Even though the ground to the solenoid is interupted the solenoid plunger is not allowing the control oil pressure to vent. That would hold the transmission in OD even though the PCM is commanding 3rd. But, rather than throwing parts at the problem, a reasoned diagnosis could narrow down the problem.
Hope this helps.