Thread: Autobox hard changes after Ashcroft torque converter upgrade

Originally Posted by d2dave

The old day transmissions were not electronically controlled. So there were two ways to control transmission pressures.

It was either using a cable which was often incorrectly called a kick down cable , or a vacuum modulator was used.

Through engine vacuum, the tranny could work out throttle position to adjust tranny pressure to suit.

So no, you won't find one in or on a ZF.

How do they control shift pressures on the electronic transmissions ? could it be that the torque convertor is not unlocking before changing gear ?
thus causing harsh shifting

Ian
Bittern

Originally Posted by ian4002000

How do they control shift pressures on the electronic transmissions ? could it be that the torque convertor is not unlocking before changing gear ?
thus causing harsh shifting

Ian
Bittern

Shift pressures are controlled by a governor valve which is on the out put shaft. It works using centrifugal force against a spring.

The idea of the vacuum modulator or cable is so that you have very low pressure at idle.

If the pressure is high at idle it will bang into gear when you select drive.

Then as you accelerate, vacuum decreases, and this increases pressure to clutch packs/band servos so that they don't slip with increased engine power.

Cable has the same effect. As you accelerate it pushes a spring against a valve

Governor then controls shifting.

A trick we used to do on vacuum modulators was to lengthen the rod behind it, only a small amount, about half to one mm.

This was just enough to make the changes a bit more snappy, without increasing the bang when selecting drive.