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Loss of engine braking- TRANNY OR ENGINE ISSUE?

Discussion in '1982-Present GM Diesel' started by Can Can, Apr 5, 2003.

  1. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    A couple of years ago I replaced the injector pump on my 6.2. The truck has always shifted a little wierd but I've owned the truck for 7 years so I'm used to it. The diesel shop tried to "fix" my shifts(it's a 700r4), but I made them re-adjust it back. I DO know that they played around with the TV cable as well as the TPS(Throttle Position Switch).

    Anyhow, it never seemed quite right. It feels like the tranny totally disengages if I let off the throttle, eg. going down a hill. It's almost like I have a standard and I'm pushing in the clutch- when I get to the bottom of the hill and re-apply throttle, the tranny seems to re-engage.

    This became very evident last weekend when I took a load of lumber across the mountains. If I let off the throttle the truck would actually pick up speed going downhill, and the wierd part is that I could slow the truck down by pressing the gas pedal........ /forums/images/graemlins/confused.gif

    So here's my question-

    Is this problem diesel related or 700r4 related? And how can I fix it?

    I'm gonna post this in the 73-91 forum in case it's a tranny problem as opposed to a diesel issue.
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I'd say it's totally tranny related...there's no way a 6.2 would freewheel like that, there's way too much compression.

    Rene
     
  3. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    Yeah, I was kinda thinkin that myself. The only part of the equation I don't understand is how the TPS affects shifting- I have a feeling that that's where my problem lies.

    FWIW, I used to have great engine braking before the diesel shop monkeyed around with the shift points.
     
  4. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I'd consider taking it back to the shop that messed with it and complaining.

    Or swap a SM465 into it...then you'll have more compression braking than a Pete with a jake /forums/images/graemlins/thumb.gif

    rene
     
  5. Tybee

    Tybee 1/2 ton status

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    [ QUOTE ]
    This became very evident last weekend when I took a load of lumber across the mountains. If I let off the throttle the truck would actually pick up speed going downhill, and the wierd part is that I could slow the truck down by pressing the gas pedal........


    [/ QUOTE ] /forums/images/graemlins/eek.gif That had to be scary!!!
     
  6. arveetek

    arveetek 1/2 ton status

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    It's your tranny. And it's acting like it's supposed to, I'm afraid.

    Here's what's going on: the 700R4 has a lock-up torque convertor. The TPS on the side of the injection pump controls the lock-up function, nothing else. The TPS unlocks the TC when your foot is off the throttle, locks it again at partial throttle, and then unlocks again at full throttle. What feels like the transmission shifting is actually just the TC locking and unlocking. Your TPS is adjusted so that when you let up off the throttle, the TC unlocks. This in effect, is like pushing in on the clutch of a manual-equipped truck. When you depress the throttle just a litle bit, the switches close inside the TPS, locking up the TC again. Your K5 acts exactly like my '86 Suburban did. I hated it.

    My '81 doesn't use a TPS. I have it wired up so that the TC locks up in 4th gear only, and it stays locked unless the brake pedal is depressed. One of these days I'm going to add a switch so that I can manually lock it in 2nd and 3rd.

    I'm afraid I can't help you with the TPS. There are two screws holding it on with slots, so it is adjustable. I've never adjusted one, so I don't know how to tell you to fix it. I do know that you could figure out which wire goes to the tranny to control lock-up, and that you should be able to install a switch to keep the TC locked when you want to.

    Hope this helps!

    Casey
     
  7. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    Hmmmm..........I'm willing to bet that the diesel shop either disabled or "turned down" the TPS. Like I said, before they got in and monkeyed around with the shift points, my truck would never freewheel downhill. I was looking at my emissions sticker under the hood and it says the TPS needs to be set at a certain point- I believe it needs to be set up with a hand vacuum pump.

    Does anybody have any info on this? /forums/images/graemlins/1zhelp.gif
     
  8. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    I'm not sure what controls the TC lockup on your rig (I wired mine fully manual), but it sounds like it used to lock going down hill, and now it doesn't. It could be that it wasn't working properly before (to your advantage), and now it is. I know the 700's don't like to be "over-run", (i.e. the vehicle pushing the tranny, such as would happen going downhill and compression braking), this was from a tranny builder who "seems" to know his stuff on 700's. I'm not saying they (mechanic's) didn't F something up, but it could be that they actually fixed it. I will see if I can verify an earlier post as to when your converter should and shouldn't unlock and that may shed more light on your situation.
     
  9. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    Thanks for your input, brother. /forums/images/graemlins/thumb.gif It's starting to sound like I'm gonna figure this out after all. Let me know if you dig up anything else that's relevant.......
     
  10. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    Any luck yet?
    I'm still checking....hoping to see my tranny guy this weekend.
     

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