The solution to the B8/8.5 Push Clutch to Start issue.

Why the Extended Slave Solves the “Push to Start” Issue:

To understand how the extended slave works, first, we must understand what causes the Push to Start issue. The clutch position sensor is a hall effect sensor at the end of the master cylinder. A hall effect sensor measures the magnitude of a magnetic field to tell when two objects are a certain distance apart. Our case measures when the master cylinder piston is sufficiently pushed in to guarantee the clutch is disengaged. Under normal circumstances, this trigger happens just before the clutch pedal reaches its mechanical end stop.

When the Push to Start issue manifests, something blocks the clutch pedal from reaching its mechanical end stop and the hall effect sensor from reaching its trigger point.

This something is the end stop of the slave cylinder. The slave cylinder is a self-adjusting hydraulic cylinder. That means that when there is no hydraulic pressure (the clutch pedal is at rest), it is free to move wherever it needs to maintain contact with the clutch fork. If you play with one while still hooked up to the car, you’ll find it has a moderate amount of outwards pressure but can be compressed by hand. The purpose of this is so that the end of the slave cannot fall out of its seat in the clutch fork.

When you go to push the clutch pedal in, the position of the slave travel becomes the starting point for the travel of the system. If that starting point is too far out in the slave travel, the slave will run into its end stop before the clutch pedal has hit the floor and the hall effect sensor is triggered. You may notice if you jam the pedal into the floor, you’ll get the car to start. This is because you’re squashing the plastic end stop on the slave. Do it too hard or too often, and it will break, leaving you stranded.

By making the slave cylinder longer, we can compensate by making it start farther back in its travel, giving more total travel from the starting point. This solves the Push to Start issue.

 Diagnosing the Push to Start Issue