1985 Porsche 928 AutomaticFACTORY OEM SERVICE PROCEDURE

Ignition System

  1. The ignition timing is 10° BTDC with an idle speed of 640-720 RPM. It can be made visible with a timing light or a digital tester via the TDC sensor. If a secondary image is wanted on the oscilloscope, set the cylinder selector switch to 4-cylinder scale since the ignition system is divided.
  2. Connect terminals 1 and 15 of tester on left ignition coil according to supplier's instructions. Oscillogram displays the left ignition coil. If right ignition circuit is wanted on the oscillogram, connect clip to terminal 4 or connect right ignition coil and clip to cylinder No. 1 of right distributor. Connect terminals 1 and 15.
  3. If a fault is displayed in all cylinders, it is in the primary or secondary circuit between the ignition coil and distributor rotor. If a fault is displayed in only one cylinder, it is after the distributor rotor.
  4. Primary resistance at terminal 1 and 15 should be .4-.7 ohms. Secondary resistance at terminal 1 and 4 should be 5000-8700 ohms. Voltage at terminal 15 against ground with engine running should be 10 volts.
    NOTE: DO NOT test spark gap by pulling off spark plug connectors. There is danger of destroying the ignition coil and control unit.
  5. The ignition triggering signal of the EZF control unit can be simulated for quick troubleshooting on the ignition side. This is done by disconnecting the 2-pin plug above the CEB (Green and White wire).
  6. Bridge male plug pins with a jumper wire. Turn on ignition. While touching the male terminal to a 12-volt source, a spark must jump over on the distributor end of disconnected wire terminal 4 with a gap of approximately .16" (4 mm), only if high voltage side is okay.
  7. If there's no spark jump, check the following: power for trigger box and ignition coil, ground point for trigger box (located on right front side near ignition coil), plug connections, trigger box and ignition coil components.
  8. Check control signal on both trigger boxes. Connect positive tester lead to terminal 5 and negative tester lead to terminal 2 of disconnected trigger plug. Operate starter. Oscilloscope must display a control signal on each trigger boxes plug.
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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.