Tranny acting funny

2500Silv · November 15, 2006

I have 2000 Silverado 2500 (6.0L, 4l80E) that every now and then shifts extremely hard (as in chirping the tires during a 1900 RPM shift). this only happens when the trans temp is 150* or more by the trans temp guage. it doesn't do it all the time. the truck only has 66K miles and the SES light does not come on.

the only thing I can figure is that the tranny doesn't like the yoke stuck in its rear and is trying to get it out of there.

but I seriously doubt that's the issue

any help is greatly appreciated!!

2500Silv · November 25, 2006

anyone?

GM-Tech · November 25, 2006

Does this just happen once or will it shift hard every time during that drive cycle? If the latter, try pulling over and shutting the key off for 10 secs., then drive on and see if it has cleared.

There could be a code stored. Not all codes turn on the SES light and some require two consecutive failures before the light is turned on.

2500Silv · November 27, 2006

Once it starts shifting hard it continues. it seems like it only starts happenning after being on the interstate which brings the tranny temp up to 150*- 160*.

on my morning drive to work which is 11 miles of stop lights and 50mph cruising it doesn't happen and the tranny temp rarely gets higher then 140*.

the truck only has 66000 miles. I'll get it scanned and see what turns up.

Thanks!!!!

GM-Tech · November 27, 2006

You didn't anwer my question, but this sounds a lot like a P1870 code, which is a slip code. These usually set only after a certain amount of highway driving. It will never have this problem with stop and go driving or driving around town. It's not so much temp related, though that might have some effect.

What happens is, when the P1870 slip code is set, the tranny is ramped up to full line pressure. The PCM thinks it's slipping (which technically it is, but it's just the TCC) so it ups line pressure to overcome the slip. This causes neck snapping 1-2 shifts. 2-3 is usually a little hard too, but not as noticeable since you have some momentum by that time.

Shutting down clears this code from current status and line pressure goes back to normal, and shifting returns to normal. That's why I said to pull over and turn the key off and see if the problem goes away... at least temporarily.

2500Silv · November 28, 2006

I had it scanned and there's no stored codes. next time it does it I'll stop and shut it down and see if it does it anymore.

it does seem to go away after the truck's been shut off for a little while.

would the P1870 code be stored in memory?

ETA: the fluid level is fine and looks almost new (nice and pink)

is there a PCM update from GM for my truck? I vaguely remember reading about one. it didn't say is was specifically for tranny hard shifting issues but supposedly cured them. I found something about it on the net when I first starting researching this issue.

or maybe I'm just wishful thinking?

GM-Tech · November 28, 2006

Yes, the P1870 would be stored in history. I don't know if there is an update for your truck... probably... but calibration updates rarely fix problems such as yours. If it was a calibration issue, it would be a problem that has existed since the truck was new. The calibration doesn't suddenly become bad and need updating.

2500Silv · November 28, 2006

I've only had the truck a month and half. I have no clue if it's always done this or not.

I do appreciate the help!!!

WINCY99 · April 23, 2007

I've only had the truck a month and half. I have no clue if it's always done this or not.

I do appreciate the help!!!

My 2001 GMC was shifting hard after i drove 10 miles! If I turned it off,it would shift normal! Took it to local trans shop and it showed code P1870. We added a larger cooler,and a shift kit,and it did the same code!! We changed the converter and bingo! the truck shifts properly and locks up and fuel mileage went from 12-15 to 19.4 133,000 miles on truck 2wd from TEXAS BOUGHT FOR 8995.00 US 2500 HD CREW!! THANKS GUYS for your info.. e-bay truck in canada

diehrd · April 24, 2007

Check the throttle positioning sensor that will cause shifts so hard your teeth will fall out

roper202000 · May 10, 2007

wow gmtech thats excatly what my truck is doing and it has the p1870 code stored in the comp what do i need to do to fix it? i have asked around and i get all sorts of different answers. thanks for your help nick

silverado02excb · May 10, 2007

wow gmtech thats excatly what my truck is doing and it has the p1870 code stored in the comp what do i need to do to fix it? i have asked around and i get all sorts of different answers. thanks for your help nick

I had the spring for the overdrive valve in the valve body break on my old 97 pickup. Would do the same thing with no code being set. It would not go into overdrive once it got hot though. When it was cold it shifted fine including going into overdrive. But once it got hot and you stop or slowed down it would not go back into overdrive and the shifts would be very harsh from the increased line pressure.

roper202000 · May 11, 2007

wow gmtech thats excatly what my truck is doing and it has the p1870 code stored in the comp what do i need to do to fix it? i have asked around and i get all sorts of different answers. thanks for your help nick

I had the spring for the overdrive valve in the valve body break on my old 97 pickup. Would do the same thing with no code being set. It would not go into overdrive once it got hot though. When it was cold it shifted fine including going into overdrive. But once it got hot and you stop or slowed down it would not go back into overdrive and the shifts would be very harsh from the increased line pressure.

here is what mine is doing maybe it will help some one figure out what i need to do next

after i drive for 15-30 miles at interstate speeds 60-70 mph it kickes out of od and when it does this if i let off the gas pedel it will go straight to idle and it will not go back in to od \. now if i stop the truck and shut it off wait a few min and start it back up it will start all over again and i can drive in od for 15-30 miles again before it does it again. the trany is not original to the truck its out of a 95 truck excatly the same as mine just a year older. and here is one other thing the trany thats in it now has 2speed sencors in the side of it and i have been only able to find one in the wireing harrness, i got the truck a cpl of weeks ago and when i got it there was no trany with it so could there be something that i am missing? it is a 4x4 so i dont know if that makes a difference either, thank you all so much for the help in advance

nick