1984 6.9 with c6 won't stay in 3rd gear, keeps down shifting no matter the load.
When I start in drive I have to manually put in 2nd for it to shift.
Then I put in D but it won't shift into 3rd until I drop down into 2
nand force it to shift, it shifts for a moment but qucikly
downshifts back to second no matter the load. It was shifting
hard now not at all. Please help!!
Sound,s like tranny may be going out,check fluid,if full get ready for rebiuld or replace.
Check the trans kick down valve on the injector pump, it could be disconnected from the vacuum supply or out of adjustment or just went bad. Make sure all of your vacuum lines are connected and don't leak. Of course I am assuming that your vacuum pump is operating properly.
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