Lost overdrive

[Momsfiretruck]

Hello,

We have a 99 GMC 2500 Suburban with a automatic overdrive transmission. Last week we were driving on the highway, it shifted out of OD and did not shift back. We parked it for about 2 hours came back and it was fine driving home. It was a hot day, probably the first time we used the AC for the length of the trip, (about 2 hours). We drove home in the evening and it was cooler. I checked for error codes and there were none. We have a 94 with the OD trans and have never experienced the same problem. Has anyone had the same problem?

Thanks,

Eric

[TJ Moose]

I have a '95 Sub with the auto trans - towing last summer, after a long hill climb, lost all but 2nd - which I understand is the "limp home" mode if something goes south on the electronics. After cooling - it shifted okay again, but wouldn't lockup the OD and no cruise control - but once heating up, it again went to 2nd. No SES codes came up. Being on vacation and headed to Moab - this was inconvenient. I left the Sub in Oregon, and continued my trip down to Utah in my trail Jeep. (Now THERE'S a bit of fun driving.)
After 2 days of rest over the weekend, A/T shifted more normally, shop could not make the failure happen again, nor could they detect any codes - although still no cruise or OD. A mechanic buddy of mine suggested changing out the speed sensors on the tranny and transfer case. They're what I call the "communications interface" between the computer and tranny and if they fry, the cruise won't work, and trans shifting isn't quite what you want it to be.
So - both sensors got changed, and fluid was changed (was a little brown - but pan clean - no parts problems)and cruise came back up, OD worked, and all was right with the world.
FYI - at the time this happened - trans had 165K orig miles on it with fluid/filter changes every 30K mi or so with red line synthetic. Trans now at 180K - still running/towing fine.
From what I could see - couldn't tell much from looking at sensor what the damage might have been, but I had heated them up pretty hard the last few years towing. So I supposed, just like an FSD, it was a heat related failure. I think one of the sensors was available after market, one was only a GM deal - and together, just the parts (the 2 sensors)cost about $250.

[damork]

Check the grounds - engine compartment passenger side on head and strap to frame. Then check if you have trailer wiring connections that might be corroded. It was suggested to me and the trailer harness was a mess including the ground. Cleaned it up and overdrive came back.