Dad has a 04ish Chevy Express Cargo Van 2500, 4.8 with what should be a 4L80E, 2wd, and about 130k miles on it. No reverse, no overdrive. Started on the highway, it just shifted back to third but works completely fine 1-3. Here's where it gets tricky, no check engine light at all, not even when you turn the key, and my OBDII code reader really freaks the dash out when I plug it in (gauges freeze for a minute then zero out, abs and security lights come on, headlights turn on, ect...) so I can't read codes. Only light on in the cluster is the little wrench and "genie lamp" thing. I think the genie needs changing. Or the oil. Something. Transfluid is full and smells good.
Could the computer have crapped itself and would that cause overdrive and reverse not to work?
Not a Chevy, but my E250 had weird problems as well. Only 1st gear was acting up. Ended up being the wiring harness between the computer and the tranny had shorted out. Of course I didn't find that out until after having the tranny rebuilt. 
Good luck!
Will
Reader
9/1/09 4:38 p.m.
I'm not a Chevy expert but Fords have a sensor called the TRMLP (Transmission Manual Lever Position) that tells the computer where the driver has actually placed the shift lever. They go bad on Fords regularly and when they do the computer has no idea where the driver has positioned the shifter and starts randomly shifting gears.
I'll ask tomorrow morning in class. I'm currently taking an introductory powertrains class and my instructor is the wife of the owner of one of the two best tranny shops in the area. She's also worked for the shop for 21 years.
Ok, I asked. 3rd and reverse together would make sense. The overdrive her guess is maybe an electrical issue. As for the dash freaking out guess would be a ground problem. In other words they don't know.
JThw8
SuperDork
9/2/09 7:38 a.m.
Do you have 1, 2 and 3 or is it doing a 1-3 jump?
When the 4L60E in my wifes SUV went she dropped reverse, 2nd (and I think OD)
It jumped 1 - 3 but wasnt quite as noticeable.
The sunshell inside the trans had destroyed itself (common on this trans)
But with your other symptoms I'd look at electrical first. Doesn't AAMCO or one of those places offer free diagnostics? Might be easier than beating your head over this.
- Dash freaking out
- Trans position switch not working
So, in my unofficial opine you have a low voltage condition causing a "flicker" in the state of the dash. Check the battery first - there is a voltage point beyond which solid state devices will turn off and if you are right near it some will and some won't. When you plug in the meter you can pull it down a little.
As said before, check your grounds. Look in the area of the trans switch in question... maybe a partial short in the harness but I wouldn't be surprised if they are two separate issues altogether so check the switch function with a meter.
I will bet $1.00 that the battery is starting to push the ground out of the case.
It shifts 1-2-3 just fine. Even shifts fine WOT. Also, revving it and dropping into reverse produces a rearward clunk and a tiny bit of motion.
Guess I'll get to checking harnesses and grounds.
.....
No reverse sounds more mechanical than electrical.
Reverse does sound mechanical, that is why my instructor was asking about third, because 3rd and R are on the same gearset on that tranny. OD and the dash souned like an eletrical issue. If the tranny control module dectects a problem it will disable OD first.
Eh, I just had him take it to the dealer. If it needs a tranny or something easy we'll get it back and do it ourselves. If it's electrical they can deal with it lolz.
