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Tranny questions ,modulator vacuum on diesel

Discussion in 'The Garage' started by diesel4me, Jun 25, 2004.

  1. diesel4me

    diesel4me 1 ton status Premium Member

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    I am having a problem with the TH400 in my 6.2 diesel--the tranny is shifting late and hard,because the switch is either junk or maladjusted--(the switch that controls the vacuum to he modulator)I tried running the modulator directly to the vacuum pump and it will shift normal,but very early,I know its not good to run it that way because the vacuum pump will make more vacuum at high rpms,just the opposite of a gas motor,and that will tell the modulator to reduce line pressure--I think that will lead to slipping and burnt clutches.I havent tried to get a new switch,not sure if its even available--mine seems to work only when it feels like it.My local tranny shop said to hook it direct to the vacuum pump and screw the adjustment screw all the way in on the modulator--does this sound like a good idea??
    I dont want to fry this tranny,it was rebuilt just before I got the truck. I was thinking about the cable operated modulator I've seen on drag cars with a blower(expensive,but cheaper than a tranny I guess)--anyone have a fix for this problem??. /forums/images/graemlins/1zhelp.gif /forums/images/graemlins/1zhelp.gif /forums/images/graemlins/1zhelp.gif /forums/images/graemlins/1zhelp.gif
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    I had an adjustable vacuum modulator on my TH400, and it was worth a whopping 5MPH in shift points. I think if anything that is a "fine tuning" adjustment, and nothing more.

    To be honest I never even noticed the difference between the modulator being connected or disconnected.

    I am under the impression the modulator governs how hard the tranny shifts, but shift timing is based on the governor.
     
  3. diesel4me

    diesel4me 1 ton status Premium Member

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    I know that on my gas powered vehicles what you just wrote is true--the adjusting screw didnt make much difference,like you said about 5-10 mph later and a harder shift.
    When the diesel's tranny acts up,it wants to drag out the shifts to the redline(I think I reached the govener's limit in the injection pump--engine bogs out until the rpm's come down)then it bangs into second gear,if I let off the gas pedal in second it "coasts" like its in neutral,and refuses to shift into third until about 50mph--even trying to "help" it shift manually didnt help--classic symptoms of no vacuum to the modulator.I'm gonna get busted for speeding if I have to get it up to 50 to get third gear--or pop the motor from over revving it.I even got a chirp in third when it finally banged in at 50+mph /forums/images/graemlins/eek.gif---I thought the tranny was going to spit itself on the ground!
    I'm hoping someone can tell me how to fix this problem before I really hurt something--I'd run it direct vacuum but I'm afraid that I'll burn up the clutches--and I know if it keeps slamming gears like that something will break!.
    I might have to put my van back on the road--all 3 truck now have problems,one needs a brake booster nobody has,and a knock in the motor,and the other plow truck really shouldnt be on the streets anymore--this sucks!! /forums/images/graemlins/angryfire.gif /forums/images/graemlins/doah.gif
     
  4. smprider112

    smprider112 1/2 ton status

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    My tranny does the same thing...not as bad tho...it shifts at like 50 if im hard on the gas...or it will just kinda revv at 35 mph, for a while, then FINALLY shift...mine I can make shift early tho, if I get to 35 or so, then just completly let off the gas, it will shift...so when you figure out yers..PLEASE let me know too, Im affraid with a diesel it will put more wear on an allready high milage motor.
     

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