Question about 1988 Chevrolet C1500

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1988 TH400 won't shift correctly

Turbo 400 will start out in 1st gear usually then won't shift till about 40MPH. Will hold 2nd gear till I guess around 60 or 70. Any load will kick it back to 2nd. If at a stop light or something will start in 2nd gear unless i put in 1st or acclerate hard. Then starts all over with 1st gear problem. This is in a 1988 C1500 with a 1989 350 engine. Did swap from a 305 to a 350 a month ago but problem just started. Has had a problem with truck going into reverse since moter install. seems to take to long to drop into reverse but holds strong after its in. No slippage noted with the tranny. Unknow history prior to moter swap. Tranny fluid is clean and smells good. Modulator looks new and gasket on tranny is new also so I am guessing that recent service to tranny prior to me getting it. Fluid levels good with out any leaks noted. Vaccume found at modulator valve and according to data scan 12mm/ug vaccume at idle and about 8 at 2500RPM. HELP??

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  • fyremedic115 May 30, 2009

    This is what i did not want to hear. What do you mean about short clean and rebuild? I know little to nothing about the internals of tranny. Don't have the money to have rebuilt. Looks like i might be learning

  • Thomas Neubauer
    Thomas Neubauer May 11, 2010

    Sounds like a sticking shift servo which is why it stays 2nd gear unless forced. I am guessing it had these issues before your motor swap which is why it appears to have a flurry of new parts thrown at it.

    I would tear it down and do a short clean and rebuild. Look for any sticking components in the band servo circuits as these can cause delayed and erratic shifting issues as well as missing gears altogether. Plan on o-rings for servos and maybe get a clutch pack for it if you can find it at a reasonable cost.



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You will need a good manual that is focused on the transmission and possibly someone to lean over your shoulder to help you. Clean workspace is imperative. These are rather easy as you are for the most part stacking parts back into the housing when reassembling it. I would say look for a yard trans to put in it meanwhile and then you can learn at your leisure.

Posted on May 30, 2009


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