Th400 issues need troubleshooting help

Discussion in 'Got gears?' started by Jenna Nieves, Mar 10, 2021.

  1. Jenna Nieves

    Jenna Nieves Member

    Hi everyone, this past week I took my 71 skylark out for a little ride after a frustrating day. Wanted to blow off steam but my car decided she wanted to contributed to my annoying day lol so I put the car in drive to leave my garage went into drive fine. I pulled onto the main road to go for a cruise and immediately notice she wasn't shifting into second. I pulled over down the road checked the trans fluid seemed a little low not enough to cause issues checked under the car saw no leaks. Turned around to drive back still wouldn’t shift into second. Pulled over put the shifter down to first pushed it into second nothing 3rd nothing just like it was in drive the whole time. I laid into her a little felt like she pushed into second hard jerk but then popped right back out. I was able to get her back into the garage. Trans fluid was lower seemed like it was eating it but couldn’t find any leaks. A couple weeks prior to this I had her out and had no issues. Anyone have this issue before? So far on my list of checks is the vacuum line and governor. I did run the car in neutral and she did not move forward also trans fluid looks of good color and does not smell burnt or like baked pretzels. Car goes in drive fine and goes in reverse fine. Any other thoughts on this?
     
  2. BrianTrick

    BrianTrick Brian Trick

    Check the vacuum hose to the modular. That can cause that. Happened to me once. It cracked.
     
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  3. ynotbuick

    ynotbuick Platinum Level Contributor

    Also check and see if any tranny fluid is in the vac line, mod could be bad and allowing tranny fluid to be sucked through and burned off in the combustion chamber.
     
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  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The modulator line is the most common cause. If the kick down gets constant 12 volts, that will also cause the transmission not to shift, except at full throttle.
     
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  5. Quick Buick

    Quick Buick Arlington Wa

    Agreed with Larry about the kick down switch..... Long time ago TH400 trans didn't shift.... Ok no big deal got a very low mile spare. Swap trans no shift.. Oh well it's shot.. Open original trans cant find anything wrong..... Found out by blatant accident the switch was hot.... Took for a drive with wire disconnected.... Drove fine..

    66 t-bird. Got if for dirt cheap trans not shifting right.... Guy had a carb return spring hooked up to down shift linkage.... Ez fix,,, Ez money....
     
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  6. Andrew Sury

    Andrew Sury Well-Known Member

    It sounds like a simple case of the diaphram in the modulator is split and it is drinking your fluid little by little. Pull the vac hose at the trans and see if there is fluid dripping after a few minutes. It will come out of the rubber hose, not the mod itself.
     
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  7. Bens99gtp

    Bens99gtp Well-Known Member

    Is it actually in 1st gear. Or second????

    I've had the 1-2 shift valve get stuck b4......it would have reverse like normal....put it in d 2 or 1 and only have 2nd gear till you got up and going fast enough to find 3rd...

    If in 2nd gear you will be able to do 45 or 50 without it spinning up to like 5000 rpm, if in 1st gear you will only get to 30 mph b4 finding redline
     
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  8. Jenna Nieves

    Jenna Nieves Member




    It’s stuck in 1st I have to drive like a snail to get her back and that’s pushing 4000 rpm
     

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