Cruise Control

Cruise Control, turbo


Fault symptoms
- Basic boost pressure constantly, even when the Cruise Control is disengaged. M90 and earlier: battery positive (B+) on pin 35. M91 and later: pin 31 connected to ground.
- The car does not go down to basic boost pressure when the Cruise Control is engaged. M90 and earlier: 0 V on pin 35. M91 and later: about 11 V is applied to pin 31. It is felt that the engine does not pull cleanly.


Cause of fault
No Cruise Control signal or signal incorrect (not applicable to LH 2.4.2 and ETS, which do not have basic boost pressure with the Cruise Control engaged).


Diagnostic procedure




1 Check the Cruise Control system's vacuum pump
- Ignition switch in the ON position.
- Activate the Cruise Control.
- Select command code 301 (-M90) or 201 (M91-) while the car is being driven to check whether the Cruise Control's vacuum pump has closed the vacuum switch.
- Alternatively, with the engine switched off, disconnect the hose from the Cruise Control's vacuum pump and build up a vacuum using a separate vacuum pump.

Important: On M91 and later, the result shown on the ISAT scan tool display is the opposite of what it was on earlier cars.




The following readings should be obtained for -M90:
- 8B100 = closed vacuum switch / about 11 V, pin 35.
- 8B000 = open vacuum switch / 0 V, pin 35.
The following readings should be obtained for M91-:
- 8B100 = closed vacuum switch / 0 V, pin 31.
- 8B000 = open vacuum switch / about 11 V, pin 31.

Are all readings OK?
YES Check that the vacuum hoses and vacuum switch are in perfect working order and then continue with point 2.
NO Check and if necessary repair or replace the lead between DI/APC control module pin 35 (-M90) or pin 31 (M91-) and the vacuum switch. Also check the lead between the switch and ground and the lead connected via the selector and transmission range switch to the +54 circuit.

2 Final check
- Implementation of driving cycle:
Drive the car at varying engine loads and rpm for 5 minutes.
- Evaluation of driving cycle:

Does the fault recur?
YES Continue with "Before Control Module Replacement". Service and Repair
NO The steps taken to rectify the fault were correct.