Okay, See If the TechMasters Can Solve This

TxDoc · February 5, 2002

1997 GMC, 350, 4x4, InstaTrac.

Shift into 4WD. Start to drive (on snow covered road). Hit first shift point and get a sudden very bad "dieseling", jerking, studding and shuddering. Lights flicker and after it finally shifts, maybe 4-6 seconds, smooths out. Does it at the next shift point, too. Threw a code. Light went off two days later. Dealer said they cannot find anything. They can't get it to do it, but I can. When I demonstrated, they just said, "that is strange". Something electrical, electronic and/or something that occurs when the tranny wants to shift.

I wish I could award a prize to anyone who solves it. I kinda need my 4WD to work. We have about 7 inches of snow and now sleet on the ground. ;) Not good in Texas.

CMNTMXR81 · February 6, 2002

Could it have something to do with the "torque management" system that backs off power between shifts gone awry?

2001 HD · February 6, 2002

Tried resetting your PCM?

TxDoc · February 6, 2002

It has done this for three years, now. The tranny was rebulit about 6-7 months ago and it feels the same as before. Is the torque management thing a module or something that can just be replaced? If it is that, why would it do it only when in 4WD?

On the PCM. How do you reset it?

CMNTMXR81 · February 7, 2002

The "torque management" is part of the PCM's programming. There is nothing you can do externally to it. It'll need to be "re-flashed" at the dealer. There could be a TSB out for something like this.

In order to reset your PCM, go to www.autotap.com and they give a thurough procedure for doing this as their are actually a couple of different way for various GM vehicles to reset it.

As to the transmission, it would most likely be in the torque converter if anything. I can't think of anything in the valve body that would cause this symptom. It may be worth having them hook the TechII up and looking at the Torque Converter Clutch slip speed (TCC slip speed) on it. It could well be trying to lockup way sooner than it need be, anotherwards "tight".