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Diesel TH400's shift funny?

Discussion in 'The Garage' started by dremu, Oct 29, 2006.

  1. dremu

    dremu Officious Thread Derailer Premium Member

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    Was pulling my dead truck with my '09 CUCV today (just up the driveway, as the driveway is uphill and the truck is heavy :rolleyes: )

    The tranny in the CUCV shifts just fine, seems happy, but has always been a bit soft in reverse, like I gotta give her some accelerator to get her to back uphill ... almost like she slips a bit, or the torque converter has a crazy stall or something.

    Today, pulling the pickup, I had to WHANG on her to get her to back up... which she would eventually do.

    Is slipping in reverse a sign of the tranny going out, or is this normal in a truck with 3.08:1's and a 6.2L? ::wink1:

    -- A
     
  2. resurrected_jimmy

    resurrected_jimmy 1/2 ton status

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    sounds about normal for a military trans
     
  3. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    The original tranny for my M1009 was doing that. I figured it was a bad sign. Especially because I noticed it would do it worse when the tranny was warmed up. When cold, it would move as soon as it was shifted into R.

    I swapped in a 400 I got off ebay for $125 that was pulled out of a burb. It doesn't do it. At least not so that I notice. The tranny was supposed to have been rebuilt at some point in recent history. All indications seemed to support the claim. :dunno:

    IIRC, reverse and drive share drums, bands, and/or clutch packs. Can't remember which, but I know they share something. I add this because the other big difference I noticed was greatly improved fuel mileage on long OTR trips.
     
  4. Smitty

    Smitty 1/2 ton status

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    The TH400 in a M1009 shouldn't slip enough in any gear that would require the throttle to get it to go in the gear. There is a small amount of slipping as the clutches engage, but you shouldn't even notice it.
     
  5. diesel4me

    diesel4me 1 ton status Premium Member

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    could be...

    The modulator might have some influence on how it engauges reverse..my truck had a "delay" when shifting ino reverse,until I made some changes to the modulator controls..now it goes in quickly,with a firm "thump" to it,not "slide" in like before..

    My 82 K20 with a 6.2/TH400 has a supposedly "rebuilt" tranny..when I bought it,it would upshift only into second gear--after you reached 50+ mph,it would finally slam into drive!..I took it to a friend who does tranny rebuilds,and he said "Oh,diesel huh?--its probably that vacuum switch that delivers vacuum from the vacuum pump to the modulator..--he was right,its a plastic switch that is attached to the injector pump--it regulates the vacuum sent to the modulator,from the vacuum pump..

    In a gas motor,the modulator is hooked up to manifold vacuum--gets the most vacuum at idle,and less as the throttle is opened..none at wide open throttle..

    In a diesel,it has NO vacuum at all,since there is no "butterfles" in the intake--its at "zero" all the time--so they use a vacuum pump instead..only trouble is a vacuum pump delivers CONSTANT vacuum at ALL speeds,and they need to use that "VRV" valve to regulate it,and make it mimic a gas motors vacuum,so the modulator can shift the tranny correctly..(mine VRV valve is trash,and I have had no luck finding a new or used one so far)...

    The guy told me to run the vacuum hose direct from the pump to the modulator,and to screw IN the adjustment screw located inside the hose nipple on the modulator until it JUST bottoms out--dont tighten it!..so far,my truck now shifts as it should--maybe a bit "earlier" than I'd like,(it goes into drive at 15-20 mph now)..but it feels "softer" than I'd like it to shift...at least I don't have to wind it up to 50 or more to get third gear now,and so far,after 2 winters of abuse plowing with it,it seems to be holding up OK..

    I'd still like to find a VRV valve and make it work as intended though..I often manually shift it down, or hold it in second longer,so it wont "lug" in drive..I think it beats on the converter to let it upshift too soon,the diesels torque doesn't seem to care if its in "drive" too soon,but I can bet the converter heats up a lot when its in too a high a gear too soon..

    I want this tranny to outlive me,and have been trying not to beat it to death..but the constant shifting from reverse to drive when plowing driveways is usually a death sentence for any automatic..and yes,most all my GM automatics that were about to croak had slow reverse engaugment,then LOST reverse completely,before the forward gears showed and signs of trouble..:crazy:
     
  6. jdemaris

    jdemaris Registered Member

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    No, it is not normal. I've got several 6.2s with T400s and none of them do it - and I've got both T400 versions - the T400s that shift via the VRV vacuum vavle on the injection pump and also the T400s that use a mechanical shift/governor and no vacuum hookup at all.
     
  7. dremu

    dremu Officious Thread Derailer Premium Member

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    Well, thanks guys ... not what I wanted to hear, BUT at least I know. I have what seems to be a good tranny shop local to me, so we'll see how they do on the TH400 for the sixpack, maybe I'll make it a transmission winter :doah:

    -- A
     
  8. k204dr

    k204dr 1/2 ton status

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  9. diesel4me

    diesel4me 1 ton status Premium Member

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    Thanks!..

    Yes,I've seen those on drag cars with superchargers--they cant use a vacuum operated modulator with a blower,it messes up the vacuum signal to the modulator..the tranny shop guy suggested that solution too,but said he's done what he told me to do on many diseel's,and hasn't had any comebacks to speak of (none he could blame on the modulator mods anyway)..

    I think for 105 bucks I'll just keep mine as is--I'm hoping a boneyard will get a 6.2 thats grenaded soon,that I can rob the VRV valve off of--none wants me to "break up" a complete running one,and most have 700R4's too,so finding one has proved difficult..but I think my tranny would have failed by now,after plowing 25 storms a year ago,if the way I have it rigged now wasn't ok..hopefully it'll live!..:crazy:
     

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