1987 Saab 900 SPGFACTORY OEM SERVICE PROCEDURE

Knock Control

  1. Connect Test Wiring Harness (83 94 074) with service box. Set knock detector switch on service box to "OFF" position and solenoid valve switch to "ON" position. Road test at 3000 RPM. If system is working correctly engine boost pressure should drop by about .1 Bar when knocking occurs as a result of engine being put under load at full acceleration.
  2. If knocking persists, pressure will be further reduced by system in several steps, until engine runs without knocking. If engine is maintained under full load, boost pressure will try to increase in steps (.1 Bar in 3 seconds) toward nominal figure until knocking occurs again. Boost will once again drop.
  3. The lower the octane rating of fuel used in engine, the lower mean figure of boost pressure that engine works with when operating at full acceleration. Each time knock detector is activated, reading on boost pressure gauge will alter boost reading and will drop below preset maximum figure while at the same time flashing red light on test wiring harness service box.
  4. Should fuel in tank be of such high octane that knocking does not occur, pressure control is not required. This test procedure should only be used if it is not possible to make engine knock using fuel already in tank.
  5. Remove vacuum hose from vacuum control unit on distributor. Seal by connecting hose to drain tap on thermostat housing. This disconnects distributor advance and retard mechanism with the result that engine runs 5° advanced under full load conditions.
  6. If necessary, a further method of provoking engine knocking is to disconnect full load enrichment by disconnecting insulated cable terminal at throttle microswitch. Depressing accelerator to floor during road test will now normally cause engine to knock and function of APC system can be observed.
  7. If knocking still does not occur, fuel of a lower octane rating must be used in order to carry out this test. Remove test equipment. Reconnect vacuum hose to distributor and disconnected cable to throttle switch on completion of test.
Factory Service Tools

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

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