CODE 12 (No Distributor Reference Pulse to ECM)

1. Check connections at tachometer filter assembly. See Fig 1 . If okay, go to step 2).
2. With ignition on and engine stopped, check voltage from ECM side of tachometer filter to ground. If voltage is less than 4 volts, go to step 3). If voltage is greater than 4 volts, check for open in wire from filter to ECM terminal No. 19. If wire is okay, ECM or connection is faulty.
3. Disconnect wire from tachometer filter to ECM. Recheck voltage at filter side of connector. If voltage is less than 4 volts, go to next step. If voltage is greater than 4 volts, check for grounded wire from tachometer filter to ECM terminal No. 19. If okay, replace ECM.
4. Check voltage at tachometer lead from ignition coil with lead disconnected from tachometer filter. If voltage is less than 4 volts, go to step 5). If greater than 4 volts, replace tachometer filter.
5. Repair open in wire from filter to tachometer terminal of ignition coil.

Fig 1: Wiring Diagrams for CODE 12

Courtesy of GENERAL MOTORS CORP.