sstrickstein

My 1984 928s shifts its auto box early into top gear. It will not back shift. Runs great. I've adjusted the Bowden cable to no effect. Could it be the modulator valve? Looking for anyone with experience who has already adjusted cable to no effect.

Kiln_Red

It could be the modulator, but make sure you have good vacuum going to it first before you make any other adjustments or changes. I have found the rubber vac line that leads to the metal line to the front of the intake to be totally missing in action on two different cars. It's no fun to replace, but in both of my experiences it restored shifting back to normal.

Jchiodi

I am no expert by any means... but the first thing to check would be the fluid level. Second condition of fluid, replace if needed. Check vacuum fitting from engine to modulator..

I have auto also, and as soon as weather warms I will be checking why I have very harsh shifts. Shifts about where it should, just too damn harsh.. others I'm sure will chime in. There is a wealth of info here if you search.

jc

Barry Chan

My 84 also shifts very smoothly and into forth at around 30 mph.
Mine will shift down when pedal down 1/2 to 3/4 way.
It will kick down to the lowest gear if floored.
Or i will roll the gear if i want gear to hold.
I kind of like the way mine is set up, i don't know if others may have done something different.
What does it mean "harsh" shifting?

Kiln_Red

Without help from the modulator, shifting is sudden and usually felt with a thud.. There can be other reasons for a hard shifting transmission, but the modulator is the first place that I go to look.

Landseer

Building on vac leak issue, modulator rubber cap sometimes cracks and leaks vac.

If diapragms in HVAC system are broken, that's a vac leak source too. The eggcrate vac accumulator reservoir buffers it a bit. The effect can operationalize as variable shifting behavior, in my experience, depending on throttle position and recovery time for the vac accumulation.