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MB engine MAC12D Onboard Diagnostics

• Ignition off
  
  
• Fuse in holder on top of fuel pump relay
• Turn the ignition on but do NOT operate the starter
• After four seconds:
  • Fuel pump relay J17 activates
  • Fuel pump runs - if not, see 28-103
  • Idle stabiliser valve buzzes
  • Remove the fuse and the check engine light on dashboard flashes 4433
• Replace the fuse. After four seconds:
  • Fuel frequency valve N7 (behind metering head - green connector) buzzes - if not, see 28-105
  • Remove the fuse and the check engine light on dashboard flashes 4441
• Replace the fuse. After four seconds:
  • Wastegate frequency valve N75 operates - can be heard in a quiet garage - best detected by touch between finger and thumb. If not, see 28-107
  • Remove the fuse and the check engine light on dashboard flashes 4442
• Replace the fuse. After four seconds:
  • Cold start valve N17 (blue connector - side of inlet manifold) clicks five times - if not, see 28-109
  • Remove the fuse and the check engine light on dashboard flashes 4443

If one component fails to operate, the problem is most likely in that component. If all fail to operate, the problem is most likely in their common power supply to rail F2 via S28 - the rearmost of the longitudinal fuses in the fusebox. Check that every socket with contacts in down the side of the fusebox has a fuse in it, and that the fuse has a red clip-on cap marked "Moteur" - Audi part # 893 941 819.

If performing these diagnostics repeatedly, turn the engine on the starter every five times or so to prevent the cold start valve test flooding a cylinder and causing hydrolock. Audi recommend removing fuse 13 from the main fusebox to stop the fuel pump running. If this is not done, it is advisable to spin the engine on the starter betwen tests

MB 1B MC Diagnostic leads

From the left: ISV tester, ISV current measuring lead, WoT switch simulator, wastegate frequency valve LED light, cold start valve monitor

1. The ISV tester is a 9v battery connected to the terminals of the idle stabilization valve valve and energised via the pushbutton. It should be impossible to blow through the valve when de-energized and easy with the switch pressed.
2. The ISV current measuring lead places an ammeter in circuit with the idle stabilization valve. This measurement requires care - if the circuit is broken at any time the ISV controller will stop operating - inserting the lead or even switching range on the meter can break the circuit. Ideally the reading should be 430ma - a reading of exactly 479ma is a distress signal from the ISV controller.
3. The WoT switch simulator is placed between the WoT switch on the side of the throttle body and the connector - the button shorts pins 2 and 3, simulating WoT activation.
4. The wastegate frequency valve LED lead is connected between the WGFV and its connector to check for WGFV operation at WoT.
5. The cold start monitor lead is used to time cold start valve operation and detect its use during acceleration.

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