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1993 Chevy Silverado K2500 6.5 L Turbo Diesel 4L60E Transmission has no overdrive

Discussion in 'General Chevy & GM Tech Questions' started by darthtater, Jul 1, 2012.

  1. darthtater

    darthtater New Member

    I have a 1993 Chevy Silverado K2500 6.5 L Turbo Diesel with a four speed automatic 4L60E. The transmission has 1st, 2nd, and 3rd gears which shift one to the next flawlessly...that is, until it gets to 4th. When the problem first started, the overdrive (4th gear) was spotty. Sometimes it would slip back down to 3rd and you could put it in neutral and restart the truck and it would work fine. Then one of the heads on the motor cracked and I rebuilt it. After the rebuild the transmission would shift into O/D and then downshift to 3rd immedetly after and would not shift back into O/D until you completley stopped, put it in park, restarted it, then took off again, even then it would do the exact same thing over and over again. Then one day I took it for a drive and it didn't even try to shift in to O/D and it hasn't since. Then the motor was making a terrible knocking sound, so we rebuilt it again, and the same thing, no O/D. I let a technical school rebuild it (for the experience) and they messed the shift linkage up, so when it says its in neutral (N), then its acutally in O/D. Now heres the kicker, when you downshift to just D from O/D, the engine brakes come on and RPM's go up by about 500 the truck starts to slow down. And when you upshift back to O/D, the engine brakes shut off. But still no O/D. This is a heavy duty hauling truck, therefore it has crossover gears (gears in between gears so it can downshift when going up a hill and such without actually fully going back to 3rd gear). This tells me that its getting stuck on the crossover gear from 3rd to 4th. What could cause this? I know the trannys not goin out because it's been driven like this for 20,000 miles and the first 3 gears work flawlessly still. And I replaced the brake switch and cleared all the codes so I know its not those two things causing it. And I smelt the transmission fluid and it doesn't smell burnt at all so I know it's not a burned up band. All help is welcome and greatly apprechiated. Thanks :D.
  2. Chris Miller

    Chris Miller Rockstar 100 Posts

    First thing I would try is readjusting the linkage. There's an adjuster right next to the steering shaft. Just take the bolt loose, move it a bit, and tighten the bolt again. If nothing else, you can get the selector to work correctly.
  3. darthtater

    darthtater New Member

    I tried to and the plate keeps getting stuck on the bolt that goes into the tranny. I tried moving them sepertley but they won't, even with taking the nut compltley loose. And I know I was trying to adjust the right thing. I got the codes from it today and came up with 5. Code 22: TPS Volt Low ; Code 59: Fluid Temp Low ; Code 68: A/C Compressor relay circuit failure ; Code 85: PROM Error ; and Code 87: EePROM Error. I think the TPS is bad but it's $200 for this diesel o_O . So if you could point me in the direction of a cheaper one it would be much apprechiated :D.
  4. Chris Miller

    Chris Miller Rockstar 100 Posts

    If you've got 85 and 87, that would explain a whole lot. Your transmission is electronically controlled, and if the computer's going bad, all sorts of things go with it.
    First, I'd get a new computer and chip. It's probably not able to control the transmission solenoids. Eventually you're going to lose second and third. Chances are, the codes 22, 59 and 68 are caused by that bad chip, so change that out first. Even if they aren't caused by it, you need the computer anyway.
  5. darthtater

    darthtater New Member

    Hey thanks :). Could you maybe point me in the direction of one of those chips and maybe so info on how to change it out :)? If so I would apprechiate it. Thanks! :D
  6. darthtater

    darthtater New Member

    And when you say computer and chip do you mean the ECM or what? Thanks!
  7. Chris Miller

    Chris Miller Rockstar 100 Posts

    The computer I was referring to is the ECM, yes. The chip is the PROM that plugs into it. A computer you'd buy at the auto parts doesn't come with a PROM.
    You might get lucky getting the chip from the dealer, but it's probably a good idea to replace the computer too. Otherwise, find the same year and model in the junkyard and yank its whole computer.
  8. darthtater

    darthtater New Member

  9. Chris Miller

    Chris Miller Rockstar 100 Posts

    That's not the original ECM. Probably best check O'Reilly or something.
    As for the PROM, the dealer would be ideal, but hit up some junkyards too.
  10. bcountry

    bcountry New Member

    My 1990 Silverado has been doing the exact same behavior. Did you guys ever come to a resolution on this? Did the ECM work? New PROM?

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