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Thread: THM400 reverse problems

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    THM400 reverse problems

    I have a 79 Chevy 1 ton with the thm400 and NP203 t-case. any idea on why reverse would work great at idle but when givin the peddle cuts out? (i dont think slips is the right word here, cause at idle it backs up great then you give it gas it stops and goes again at idle...) im not great with trannys but just done a trans service everything inside looked great, no metal and clean un-burnt fluid. 1st-3rd works great. just want some input before i go and spend the money on a rebuild. Could it be a vacuum problem or maybe in the t-case?

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    So it worked fine before the service and now has an issue? Assuming you pulled the pan and put a new filter in it?

    Make sure the shifter is adjusted correctly for reverse. When you give it gas the engine and trans, or the body, "torque" over some and if the shifter is right on the edge of adjustment it could pull the shift linkage slightly. Try holding the shifter with putting slight pressure up and then down and see if you can hold it in reverse. When I adjusted the linkage on mine I was surprised of how small the adjustments were.

    Can you get the shifter into 2 and 1 okay?

    Same goes with the t-case shifter, but less likely something like this could happen. Honestly wouldn't think the t-case could do this, especially without making some weird noises.

    Don't think a vacuum issue would cause anything like this????
    Brad
    '90 Blazer - 6.2 diesel, 700r4, Dana 60, 14 bolt, locked, 39.5" Super Swamper TSLs, winch, rollcage, yada, yada, yada......

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    Quote Originally Posted by Brad View Post
    So it worked fine before the service and now has an issue? Assuming you pulled the pan and put a new filter in it?

    Make sure the shifter is adjusted correctly for reverse. When you give it gas the engine and trans, or the body, "torque" over some and if the shifter is right on the edge of adjustment it could pull the shift linkage slightly. Try holding the shifter with putting slight pressure up and then down and see if you can hold it in reverse. When I adjusted the linkage on mine I was surprised of how small the adjustments were.

    Can you get the shifter into 2 and 1 okay?

    Same goes with the t-case shifter, but less likely something like this could happen. Honestly wouldn't think the t-case could do this, especially without making some weird noises.

    Don't think a vacuum issue would cause anything like this????



    the shifter adjustment could be the problem seeing how the previous owner half ass installed a B&M floor shifter. reverse was still messed up before the trans service that's why i tried that route hoping the simple fix would do it. Also was slightly concerned when i pulled the pan to find the filter incorrectly installed and half-way hanging off. The transfer case doesn't make any strange noises or bad engagements, tough the linkage gets tough here and there. thats gonna be a issue i tackle after the trans

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