Questions and Answers

GMC Sierra 2500 Shifts Hard Hot Q. Vince, I have a 2000 GMC Sierra 2500 4WD, 52,000. It has the electronic 480LE. The transmission when warm (120°F) , transmission temperature gauge standard on vehicle, shifts hard into fourth gear. It doesn't do it all the time however it is happening more often as of late. There is also another problem.

When the transmission is hot (pulling a trailer or load) 150°F and higher the transmission will shift hard into all gears. I would pull over to side of road and turn the vehicle off, turn it back on and it will shift normally. I had it diagnosed by a dealership as the torque converter needs to be replaced, however I find this hard to believe simply because of the later problem. The problem seems to be more electrical and not physical. What do you think?

Best Regards,
Joe

A. I'm wondering if there was any DTC's stored in the PCM when the dealer looked at it to make them condemn the convertor. It may have had a slip code which will make it default to a hard shift to help save the transmission in case of a loss of pressure. I would talk to the service manager or better yet the technician and see if any new software updates are available for your truck and possibly road testing with him to see if he can look at a scanner while you make the concern happen, he can take a snapshot of it and show you what's going on. These are good transmissions with little problems unless overworked and overloaded.

Added 3/5/03:

Vince, I told you I would get back to you with the final fix... Here it is.

After talking to GM TAC (technical assistance) automotive, the GM service rep. was 99.99% sure it was the Converter. This weekend I changed the torque converter and sure enough it was the problem. Apparently the torque converter had some internal design flaw that would not allow the torque converter to lock up properly (some sort of bad material internal to the torque converter).

The transmission when warm would then slip going into overdrive and send an P1870 code to the PCM. The PCM would then change the transmission characteristics to shift hard into all gears in order to save the transmission. (some sort of safe mode) So this was it. Thanks again for answering all of my questions!!!!!!!!!!!!

Best Regards,
Joe