The ignition system is designed so the ICM operates independently of the ECM until the ECM is ready to take control of spark timing. The ECM signals it is ready by applying 5V on the tan and black "Bypass" wire connected to the module. Some of the vehicles equipped with '7747 ecm's had instructions to set ignition timing by jumpering two pins in a connector under the dash. Others used instructions that said the Bypass wire should be disconnected under the hood. Problems can occur if you use the wrong set of instructions as the ignition timing can be off significantly.

If the ignition timing is correct it's possible to force the ICM to ignore the ECM. This can be an important step in diagnosing a spark problem. If the ECM is disconnected and the coil stops producing spark it's obvious the ECM is not the culprit. Locate the tan/black Bypass wire to the ecm and disconnect the wire to keep the ECM out of the picture.

Even though the engine is not equipped with a knock sensor the ECM still uses knock counts. The value in the knock counter will be different each time the engine is started but should not change during the time the engine is running. As long as they do not change while the engine is running the ECM will think there is no knock.