Question about 1999 GMC Sierra

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Transmission has developed occasional hard shifting and will not go into 4th (overdrive) sometimes. Sometimes it behaves perfectly and shifts normally. What may be the cause and is there a diagnostic test that can find the problem?

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First check the oil level in the gear box if its low fill it up,,it could be a blocked filter in the box itself or the velve block pistons sticking it may need a set of rubber "o" rings fitted to the pistons to sort out this problem

Posted on Jul 13, 2010

Testimonial: "Thanks for the ideas. Maybe the o-rings."

  • Michael Thorpe
    Michael Thorpe Jul 13, 2010

    hi its mike756 again

    in the past when ever iv had an auto box thats playing up the first thing i do is drop the sump and clean out any filters i find or the oil strainer/pick up pipe,,,i look for bits of rubber in the sump if there is any i know its from the valve block and the piston valves need a re-seal done to them,,,if its clean i refill the the box with clean fresh oil,,,if that dont sort it out there is an oil addertive you can put in the gearbox to smooth out the gear changes,,it makes the rubber seals swell a bit too and this helps smooth the gear changes as well but check the box it adjusted right first

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