Boost Pressure Gauge Gives A Low Reading

Symptom Of Fault.
Weak engine.
The turbo/APC gauge gives a low reading.

Conditions.
No diagnostic trouble codes.

Diagnostic Help.

NOTE: Cars with a manual gearbox have basic boost pressure in 1st gear.

The APC valve can be activated with the ISAT scan tool.
- Connect an ISAT scan tool.
- Select "ACTIVATE".
- Select "BOOST PRESS. VALVE".





Diagnostic Procedure.
1. Investigate the cause of basic boost pressure. If the control module initiates basic boost pressure for any reason, there may be many reasons for this.
- Select "READ FUNCTIONS".
- Select "BASIC CHARGE PRESS.
When the car is driven at wide open throttle the ISAT scan tool will show the cause of the fault.

Does the ISAT scan tool shown "BOOST PRESSURE"?
YES:
^ Continue with point 2.
NO:
^ The ISAT scan tool shows "KNOCK CONTROL" as the cause of the fault. The trouble may be due to an inferior grade of petrol or a defective ignition discharge module. See "Fault diagnosis/Fault Symptoms, Poor performance".





2. Investigate the cause of the fault
In the following it will be determined whether the fault is a control fault or a basic engine/turbo fault.
- Disconnect the air hose marked "W" from the boost pressure control valve, checking at the same time that it is connected to the wastegate.
- Drive the car.
When driving at wide open throttle from low engine rpm, the pressure switch should open before 3000 rpm is attained.

Does the switch open?
YES:
^ Continue with point 3.
NO:
^ Check and, if necessary, rectify the following:
^ Leakage in the intake system between turbo unit and throttle body (e.g. in the bypass valve).
^ Clogged air cleaner.
^ Turbo unit fault.
^ Incorrectly adjusted basic boost pressure.
^ Basic engine fault.
^ Clogged exhaust system.




3. Check fuse 5

Is the fuse OK?
YES:
^ continue with point 4.
NO:
^ Continue with point 6.

4. Check the air hoses
Inspect the valve's three control hoses, making sure that they are in good condition and correctly connected.

Are the air hoses OK?
YES:
^ Continue with point 5.
NO:
^ Rectify any fault discovered..

5. Check the power supply
- Ignition switch in ON position.
- Check that the valve is supplied with power (+54 circuit) by connecting a test lamp to pin 2 of the valve's connector and a good grounding point.

Is the test lamp on?
YES:
^ Continue with point 6.
NO:
^ Repair or replace the lead between pin 2 of the connector and fuse 5.




6. Check the resistance of the valve Take resistance readings in the valve's connector across:
- Pins 1 and 2
- Pins 2 and 3
Nominal resistance is 3 ± 1 ohm in both cases.

Are the readings OK?
YES:
^ Continue with point 7.
NO:
^ Change the boost pressure control valve.

IMPORTANT:
If there is a short circuit in any winding the control module will probably also be damaged. If a new valve is then fitted, it will inevitably be rendered unserviceable. In the event of a short circuit, both valve and control module must therefore be changed at the
same time.
If necessary, change fuse 5 afterwards.

7. Check the lead connected to pin 3 of the valve
- Ignition switch in ON position.
- Plug in the valve's connector. A weak buzzing sound should be heard.

Can a sound be heard?
YES:
^ continue with point 8.
NO
^ Check the lead between pin 3 of the valve's connector and pin 2 of the control module. Repair or replace the lead if necessary.





8. Check the lead connected to pin 1 of the valve
- Ignition switch in ON position.
- Plug in the valve's connector.
- Select "ACTIVATE".
- Select "BOOST PRESS. VALVE".
A loud buzzing sound should be heard.

Can a sound be heard?
YES:
^ Change the boost pressure control valve.
NO:
^ Check the lead between pin 1 of the valve's connector and pin 26 of the control module. Repair or replace the lead if necessary.





9. Check functionality
- Drive the car on test and check whether the fault symptoms persist.

Do the fault symptoms persist?
YES:
^ Proceed at "Before Replacing a Control Module". Component Tests and General Diagnostics
NO:
^ The remedial measure taken was correct.