I have a 1997 2500 and am having this problem with the tranny not wanting to shift into OD. I thought it was only happening when it got hot but maybe I am wrong. It will work again after shutting down and going again, and then it drops off again. I get no codes at all and I am being told that it needs a total rebuild but that just does not make sense to me. It works great other wise and only when I hit the highway would you notice the problem. Were should I start?

No help on this?

First thing I would start with is getting it hooked up to a scanner and watch the data as you drive. Make sure the computer is actually commanding 4th to come on.

Will give that a try.

I have a 1998 GMC2500 that I just got. I do not think I feel it shift into overdrive. At 60 mph I am at 2000 rpm and at 70 mph I am at 2500 rpm. I am new to this automobile, so I do not know what it should be. Can this be computed from the gear ratios? If so, does anyone know what they are for this car?

Just to give you an idea, my 93 Chevy Dually with 4.10 gears and 4L80 runs at around 2800 at 75MPH.

Rockman are you sure you have 4:10's, that's to many rpm's for 4:10's. I have run gear ratio calculations and Devildog is right on the money for 4:10's.

Larry the best site to check gear ratio's is here: http://www.angelfire.com/fl/procrastination/rear.html The 4l80e OD is .75 ratioand 3rd gear is 1 to 1.


Check here to find out what gear ratio you have, best place to start. http://dieselplace.com/forum/showthread.php?t=9676

That's what the glovebox says. But with this truck, who the heck knows! :rolleyes: It's kind of a Frankenstein:frankenst from what I have found. But everything seems to be working fine. If I tap the brake pedal it will jump the R's up some. Could be that the speedometer is off also and I am actually doing 80? Seems like I do pass people like they are standing still, but I have seen many troopers and never been stopped.

So honestly, I have no idea! :D

My 6.5 with 4.10's reads around 2800 @75 as well. None of these factory tachs are very accurate.

plug the scanner in & check for codes & watch the speed sensors (input & output) to see if they are reading correctly, We have had multiple loss of overdrive complaints that lead back to speed sensor issues.