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TH400 downshifts


Well-known member
May 20, 2020
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I have two vehicles with TH400s. One is a '75 Cherokee. Under medium throttle (steep hills), it will downshift from 3rd gear to 2nd. This is without the kickdown switch energizing.

The other one is a '66 Olds Toronado, with a TH425. Does not downshift the same way, but with a mild 455 the thing just chugs up any hill. I've recently installed 2.73 gears in the final drive, to make it a highway cruiser, so I'd like to get more use out of the lower gears, which the car only uses 10% of the time.

Would adjusting the vacuum modulator induce earlier downshifts, or just shift the shifts up/down RPM-wise? The governor in this thing (TH425) is crimped inside a metal can, so I don't think I can access the weights/springs.

Definitely adjust the modulator, medium increments at a time and chk for changes. A little adjustment could make a significant change.
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