Checking the APC System Knock Control
Checking the APC system knock controlTools: Pressure gauge 83 93 514
Test wiring harness with service box 83 93 548 (up to and incl. 1985 models) or 83 94 074 (as from 1986 models).
1. Run the hose of the pressure gauge from the passenger compartment to the engine compartment. Position the pressure gauge vertically to avoid erroneous readings.
2. a. Connect the test wiring harness with service box (see "maximum charging pressure").
Connect the test equipment between the wiring loom connector and the control unit.
b. As from 1986 models:
Run the test loom and box inside the cabin.
3. a. Up to and incl. 1985 models:
Set the 'knock detector' switch on the service box to ON.
b. As from 1986 models:
Set the switches on the control box as follows:
Knock detector to OFF
Solenoid valve to ON.
4. Road test at 3,000 r/min approx.
If the system is working correctly the engine charging pressure should drop by about 0.1 bar (1.5 PSI) when knocking occurs as a result of the engine being put under load at full throttle. If knocking persists the pressure will be further reduced by the system in several steps, until the engine runs without knocking.
If the engine is now maintained under full load then the charging pressure will try to increase again in steps (0.1 bar 1.5 PSI approx. in 3 seconds) towards the nominal figure until the knocking occurs again, whereupon the charging pressure will once again drop, etc.
The lower the octane rating of the fuel used in the engine, the lower will be the mean charging pressure of the engine when operating at full throttle.
Every time the knock detector is activated the reading on the charging pressure gauge will alter - the charging pressure reading will drop below the pre-set maximum figure while at the same time flashing the red LED on the test wiring harness service box.
Should the fuel in the tank be of such a high octane rating that knocking does not occur, thus obviating the need for pressure control, the operation of the APC system can be checked as follows:
4. a. If necessary, a further method of provoking engine knocking is to disconnect the full-load enrichment hose, (EU-spec. cars only).
b. Cars with USA spec:
Disconnect the electrical leads from the full-load-enrichment switch on the throttle housing.
The test procedure described below should only be used if it is not possible to make the engine knock using the fuel already in the tank.
5. Remove the vacuum hose from the vacuum control unit on the distributor. Seal the hose.
This disconnects the distributor's advance and retard mechanism with the result that the engine runs 5° advanced under full load conditions.
6. Depressing the accelerator to the floor during the road test should now cause the engine to knock and the function of the APC system can be observed.
If knocking still does not occur then fuel of a lower octane rating must be used to carry out the test.
7. Remove the test equipment.
Note: Do not forget to reconnect the vacuum hose to the distributor and the hose to the full-load enrichment device (if disconnected) on completion of the test.