Clutch Hydraulic System: Testing and Inspection

Checking clutch booster circuit

Information
Clutch actuation, for the 911 Turbo, is assisted by a hydraulic auxiliary force (clutch booster circuit). This hydraulic auxiliary force acts on the clutch slave cylinder when de-clutching. A tandem pump -1- is used instead of the hydraulic single pump for power steering (997 naturally aspirated engine) in order to generate the auxiliary force.





Hydraulic pump (tandem pump)

This tandem pump (for clutch booster circuit and power steering) -1- has a combined supply tank/reservoir -4-.

1 - Tandem pump (assembly -2 and 3-)

2 - Hydraulic pump for steering

3 - Hydraulic pump for clutch booster circuit

4 - Supply tank/reservoir (Pentosin tank in the engine compartment)





Reservoir (Pentosin tank in the engine compartment)

A pressure control valve and a pressure retaining valve are integrated into the combined supply tank/reservoir for the clutch booster circuit. These valves are located in the upper part of the tank.





Pressure accumulator

A pressure accumulator -arrow- is screwed into the clutch slave cylinder. If the engine is switched off (with the system intact), the pressure accumulator -arrow- allows up to 25 clutch pedal actuations until an increase in pedal pressure is clearly felt. The booster circuit is then depressurized (pressure accumulator is empty).

Check clutch booster circuit sequence

1. Check pressure accumulator.
2. Check pressure retaining valve in the clutch slave cylinder.
3. Check pressure retaining valve in the upper part of the reservoir.

1. Checking pressure accumulator

1. Cold start the engine (accumulator temperature approx. 20°C) and let it run for approx. 20 seconds (pressure accumulator is full).
2. With the engine switched off, press the clutch pedal repeatedly until an increase in pedal pressure can be clearly felt (pressure accumulator is empty). While doing this, determine (count) the number of times the clutch pedal must be pressed to achieve an increase in pedal pressure.
3. If the number of clutch pedal actuations (required to significantly increase the pedal pressure) is less than 8, the pressure accumulator -arrow- is faulty and must be replaced!

If the system is intact, up to 25 actuations are possible before an increase in pedal pressure can be clearly felt.





Pressure accumulator

2. Checking pressure retaining valve in clutch slave cylinder

Information
^ Checking the pressure retaining valve in the clutch slave cylinder (check 2) is a direct measurement, whereas the pressure retaining valve in the upper part of the reservoir (check 3) cannot be checked directly.
^ If there is no clutch boost after a hold time of 24 hours, at least one of the two pressure retaining valves is definitely damaged!
^ Prior to the following check, perform a visual inspection (externally!) to ensure that the system is not leaking!

1. Warm engine up to operating temperature (approx. 80°C).

Warning
Risk of material damage if too much Pentosin is used or if Pentosin comes into contact with the coolant hoses when filling or topping up. Multiple steering operations (maneuvering) and/or actuating the clutch with the engine switched off affect the fluid level in the supply tank. The fluid level rises.
^ Coolant hoses may swell. Risk of fire if escaping Pentosin comes into contact with hot components and ignites.

-> To prevent any overfilling and thus any resulting overflow, the engine should be run for approx. 20 seconds immediately after checking the fluid level.

-> If coolant hoses come into contact with Pentosin, clean them thoroughly with water IMMEDIATELY. Replace visibly swollen coolant hoses.





Return line

2. With the engine switched off and the pressure accumulator filled (before switching off the engine, the engine must have been run for approx. 20 seconds without pressing the clutch pedal), remove the plastic return line -arrow- from the reservoir. To do this, push the red thrust ring in an axial direction, without tilting it, towards the tank and remove the line.
3. Collect the fluid that runs out using suitable measuring equipment (suitable container) and read off the volume of fluid collected.
4. If more than 4 CM3 is collected after a measurement period of one hour, the pressure retaining valve in the clutch slave cylinder is faulty and the slave cylinder must be replaced.

3. Checking pressure retaining valve in upper part of reservoir

This pressure retaining valve can only be checked indirectly.

A fault exists if the clutch boost function does not last for an entire 24 hours, but where the pressure retaining valve in the clutch slave cylinder is functioning correctly (check 2).

If the pressure retaining valve in the clutch slave cylinder is functioning correctly (check 2), the reservoir must be replaced.





Reservoir