300101 Checking Clutch Booster Circuit - As Of Model Year 2010
WM 300101 Checking clutch booster circuit - as of model year 2010
Tools
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) - as of model year 2010
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 20 clutch pedal actuations until an increase in pedal pressure is clearly felt. The booster circuit is then depressurised (pressure accumulator is empty).
Pressure accumulator
Check clutch booster circuit sequence
1. Clutch: Check fluid levels (Pentosin hydraulic fluid in engine compartment and luggage compartment area).
2. Check the pressure accumulator and pressure control valve.
3. Check the pressure retaining valve in the clutch slave cylinder.
4. Check the pressure retaining valve in the pressure control valve.
Checking clutch booster circuit
1. Clutch: Checking fluid levels
Check the fluid level in the engine compartment and luggage compartment area -2-.
If the front Pentosin reservoir (in luggage compartment area) -2- has overflowed, the clutch slave cylinder is defective and must be replaced.
Front Pentosin reservoir (luggage compartment area)
2. Checking pressure accumulator and pressure control valve
Information
- This test allows you to analyse whether the pressure accumulator or the pressure control valve is not working properly.
- The analysis is difficult if neither component is working properly. In this case, the pressure accumulator and the pressure control valve (the complete hydraulic pump with pressure control valve and reservoir) must be replaced anyway.
- The test involves counting the number of clutch pedal actuations (with the pressure accumulator full) until an increase in pedal pressure is clearly felt (pressure accumulator is empty).
- In some cases, you may also have to measure the change in fluid level (fluid volume) in the reservoir (between full and empty pressure accumulator).
- Ensuring that the pressure accumulator is full (for the test) means: Cold-start the engine and leave it running at 2000 rpm for approx. 20 seconds. Then stop the engine.
1. Cold-start the engine (accumulator temperature of approx. 20 °C) and leave it running at 2000 rpm for approx. 20 seconds (pressure accumulator is full).
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.
If the system is intact, up to 20 actuations are possible before an increase in pedal pressure can be clearly felt.
In the event of other signs and symptoms, use the following overview to check whether the pressure accumulator or the pressure control valve is not working properly.
If the previous test is not sufficient for analysis purposes, you may also have to measure the change in fluid volume in the reservoir (Step 2).
Replace the pressure accumulator if:
Pressure accumulator
- More than 30 actuations are possible before an increase in pedal pressure can be clearly felt.
- The fluid volume in the reservoir (in the engine compartment), between full and empty pressure accumulator, has changed by more than 200 cm3.
- Only one or two actuations are possible before an increase in pedal pressure can be clearly felt.
The pressure control valve is defective (replace complete hydraulic pump with pressure control valve and reservoir) if:
Pressure control valve 2 (with return lines 3 + 4 and clutch pressure line 1 )
- Only a maximum of eight actuations with power assistance are possible (before an increase in pedal pressure can be clearly felt) and the change in fluid volume in the reservoir, between full and empty pressure accumulator, is less than 60 cm3.
- When you press the clutch pedal, with the engine running, there is an increase in pedal pressure and/or the pedal feels different.
2. If the previous test is not sufficient (for analysing which component is defective), repeat the test procedure and also measure the change in fluid level in the reservoir (in the engine compartment).
One of the manual metering pumps WE 1372 (which is used exclusively for Pentosin) can be used as the suction pump and measuring device (for the fluid volume).
Alternatively, a suitable suction pump and an appropriate measuring vessel can also be used.
2.1. Cold-start the engine (accumulator temperature of approx. 20 °C) and leave it running at 2000 rpm for approx. 20 seconds (pressure accumulator is full).
Measure the fluid level in the Pentosin reservoir in the engine compartment when the engine is stopped (record the current fluid level value).
2.2. 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.
2.3. Then continue to drain Pentosin until the fluid level in the reservoir is the same as it was before you pressed the clutch pedal. The amount of Pentosin removed (the fluid level) is another differentiating factor that will help to determine which component is defective (pressure accumulator or pressure control valve).
Based on the amount of Pentosin removed, replace either the pressure accumulator (if the change in volume is more than 200 cm3) or the complete hydraulic pump with pressure control valve and reservoir (if the change in volume is less than 60 cm3).
3. Checking pressure retaining valve in clutch slave cylinder
Information
- Checking the pressure retaining valve in the clutch slave cylinder (Test 3) is a direct measurement, whereas the pressure retaining valve in the pressure control valve (Test 4) cannot be checked directly.
- If there is no clutch boost after an idle period of 24 hours, at least one of the two pressure retaining valves or the pressure control valve is definitely defective. There may be no clutch boost after an extended idle period (even after one day at high temperatures) during initial actuation, depending on the system. Full clutch boost and functional capability is available immediately when you start the engine.
- 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).
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 return line -2- at the clutch slave cylinder.
To do this, press evenly on the red ring -2a- (without tilting it/never on one side) and pull the line -2- out while pressing (releasing) it.
Use disassembly tool Nr.21 to release the line.
Lines at clutch slave cylinder
3. Collect the fluid emerging from the clutch slave cylinder in a suitable measuring device (suitable vessel) and measure the volume.
To make it easier to catch the emerging fluid (restricted space in the area), a new modified return line -2- can be used as an aid. Fit (attach) this return line on the clutch slave cylinder.
4. If more than 4 cm3 of fluid is collected within a measurement period of one hour, the pressure retaining valve in the clutch slave cylinder is defective and the slave cylinder must be replaced.
-> 303019 Removing and installing clutch slave cylinder 303019 Removing and installing clutch slave cylinder.
4. Checking pressure retaining valve in pressure control valve
This pressure retaining valve (in the pressure control valve -2- ) can only be checked indirectly.
A fault exists if the clutch boost function does not last for an entire 24 hours, but the pressure retaining valve in the clutch slave cylinder (Test 3) and the pressure control valve -2- (Test 2) are functioning correctly.
Hydraulic pump (tandem pump) - as of model year 2010
If the pressure retaining valve in the clutch slave cylinder is functioning correctly (Test 3), replace the hydraulic pump -1- (tandem pump/pressure control valve assembly -2- ).
-> 489819 Removing and installing hydraulic pump - PDK and manual transmission - section on "Removing and installing" Service and Repair.