94 blazer trans problems

my 94 blazer has a 4.3 with a 4 speed auto trans, and all i have is 1st and 2nd gears, no drive or overdrive. i have heard that this could be a computer problem, does anyone know if it could be a computer problem or if the trans is shot. yeah, also the first and second gears do not slip a bit. if anyone has had this problem or knows if the computer thing is possible just let me know

more information, when it started, have you done any maintanence?

[QUOTE=BlazerLT]more information, when it started, have you done any maintanence?

i have just recently bought this blazer, and drive and overdrive were already out. i bought it very cheap and am trying to get it fixed up, so i don't know what kind of maintance was ever done on it, i checked the fluid and it doesn't smell burnt or anything and it is full. one other thing that makes me think it has computer problems is that, when it is in park and you rev it up the speedometer goes up as if you were moving. i have never personally seen a trans drop drive and overdrive, and have first and second be as tight as they probably were when it rolled out of the factory. i always thought one goes they all go. well if anyone has any thoughts i would appreciate it, especially if i don't need to rebuild or get a new trans, but i think that is not to far off.

Ah, sounds like a vehicle Speed sensor is bad.

Also, jump the codes to see if the truck ever had some check engine lights come up in the past.

I have a 94 S-10 Blazer Tahoe and I ahve the same problem with it not changing to 3rd or 4th gear. The odometer is also not working. I have been told that it might be the speed sensor but i do not know where it is. Any ideas would be helpful.

Yep, speed sensor.