Electronic Spark Timing

EST system consists of DIS assembly (coils, module, and crankshaft sensor), and ECM. These circuits perform the following functions:

^ DIS reference
Crankshaft sensor sends a signal to ignition module which generates reference pulse which is sent to ECM. ECM uses this signal to calculate crankshaft position and engine speed (also used to trigger injector).

^ Reference ground
This wire is grounded through module and makes sure ground circuit has no voltage drop between ignition module and ECM, which if open, could affect performance.

^ Bypass
At about 400 rpm, ECM applies % volts to this circuit to switch spark timing control from ignition module to ECM. An open or grounded circuit will set a Code 42 and engine will run at base timing, plus small amount of advance built into ignition module.

^ EST
ECM uses this circuit (when bypass voltage is applied), to trigger DIS module. ECM also uses crankshaft reference signal to base its calculation of amount of spark advance needed under present engine conditions.