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*UPDATED Q* TH400 wont go into 3rd gear..???

Discussion in 'The Garage' started by uglychevyZZ4, Feb 10, 2007.

  1. uglychevyZZ4

    uglychevyZZ4 3/4 ton status

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    Just got a used K20, has a th400/203. I already know the 203 growls in 4-HI.. when i got it and drove it over an hour home it shifter kinda hard (prolly because the vacume line & modulator input are plugged with mud :D) but shifted good everytime..

    I just went to go to the store and no matter what i couldnt get it to hit 3rd gear :confused: RPM's just scream in 2nd.. I dont see any wires coming off it so i dont think i have one of those lockups???
     
    Last edited: Feb 10, 2007
  2. uglychevyZZ4

    uglychevyZZ4 3/4 ton status

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  3. RingMaster4x4

    RingMaster4x4 1/2 ton status Premium Member

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    have you checked your trans fluid? for Level, color and smell?
     
  4. goldwing2000

    goldwing2000 Guest

    May seem obvious but did you try moving the shifter around? It might be misaligned.

    Other than that, check fluid level and condition, as mentioned, as well as the vacuum line to the modulator.

    Also check to see if the kickdown switch is shorted or stuck.
     
  5. 01maroonz71

    01maroonz71 1/2 ton status

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    no kickdown on a 400......mine did that once, it actually was the shifter off of the column...it was one of those...there's your sign things you know...
     
  6. uglychevyZZ4

    uglychevyZZ4 3/4 ton status

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    yes they do, most dont know its there, its a switch on the gas pedal, and if misaligned it wont kick down ...

    FOUND- mod was packed with dirt and rust, so i cleaned it out, now it shifts but has to have really high RPM's then slams in...:confused::confused::confused:
    also found the adjustable shift point screw was screwed all the way out in the mod.. andddddd it has 2 small holes rusted through..i have had them shot and make it shift hard but never take a LONGGG time to shift :confused:

    anyone else ever have one KEEP it from shifting?
     
    Last edited: Feb 10, 2007
  7. goldwing2000

    goldwing2000 Guest

    I hope you meant a switch on the gas pedal. It would suck to have to hit the brakes to hit passing gear. :D

    If your mod is that screwed up, just replace it. Even if it's not the problem, it's overdue.

    Did you check the condition and level of the fluid yet?
     
  8. uglychevyZZ4

    uglychevyZZ4 3/4 ton status

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    fluid is fine.. yes i meant gas pedal LOL
     
  9. goldwing2000

    goldwing2000 Guest

    Replace that old ' busted mod, verify it's getting full vacuum (no split or plugged lines) and go from there.
     
  10. uglychevyZZ4

    uglychevyZZ4 3/4 ton status

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    ya i am going to, i know that will make it shift hard and soon, but never seen one make them shift LATE :confused:
     
  11. goldwing2000

    goldwing2000 Guest

    If the vacuum source is bad or disconnected, sometimes they don't shift at all.

    Also, when you remove the vacuum modulator, remove the modulator shift valve that the modulator moves back and forth as this valve has a tendency to stick.
    A simple light sanding with some emry cloth makes it slide back and forth very smoothly (be sure you clean any grit off before you put it back in).

    Did you also consider removing the governer to see if possibly some of the teeth are stripped off the plastic gear and or the govenor valve is stuck?
     
  12. uglychevyZZ4

    uglychevyZZ4 3/4 ton status

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    a buddy told me that, but i can and do fix anything but never been a tranny guy, mainly because i dont wanna remove one LOL.. i dunno anything about the governor :D
     
  13. diesel4me

    diesel4me 1 ton status Premium Member

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    Its the...

    MODULATOR!!...if it has rot holes in the can,it'll never hold vacuum!..No vacuum = no upshift,especially from second to third!..put at least a "good used" modulator on,(a TH350 one will work in a pinch),and ensure the steel line and vacuum hose are not leaking or clogged,and have good vacuum at idle,I bet your shifting troubles will vanish..

    TH400's and TH350's are pretty sensitive to having the modulator work correctly, they shift sloppy,too early or late,or refuse to upshift,slip into the next gear,and feel like they "frewheel" when coasting,like you pushed the clutch in on a manual tranny,when there isn't the right vacuum to the modulator..

    I had a lot of grief with my TH400 in my diesel,it would not upshift into third until 50 mph,and the motor was winding!(and its a fresh rebuilt too)--it uses a vacuum pump because diesel engines dont create vacuum,and a switch to make the pump's constant vacuum mimic the variable vacuum a gas motor has during acceleration and coasting..that switch broke,and running direct vacuum from the pump and adjusting the modulator screw in all the way works to make it shift,but its usually too early,and a bit soft for my tastes,but better than having to go 50 before it'll shift into third (hard!)..:doah:

    The govenor gear is probably fine--it wouldn't want to come out of first OR second if the gear was stripped on the govenor,all you'd have is first and reverse if the govenor gear strips out..:crazy:
     
  14. goldwing2000

    goldwing2000 Guest

    Yeah, that's why I mentioned it last. Since the fact that I said 'modulator' multiple times wasn't good enough, I figured I'd throw something else in, too. :D

    Modulator.

    Replace that rotted out modulator.

    The modulator's probably bad.

    By the way... did I mention the dam modulator?!

    :doah:
     

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