When the engine is cranked, an alternating current signal is generated by a four-tooth magnetic pickup in the distributor. This alternating current signal is sent directly to the igniter where it is conditioned into a square wave by a waveform shaping circuit.
While cranking, this square wave signal is sent to the ECU on the Ne wire and to the igniter power transistor. The ignition system delivers spark at initial timing under this condition.
When the engine starts and exceeds a predetermined rpm, the ECU begins sending the lGt signal to the igniter. The igniter switches to computed timing mode and uses the IGt signal to operate the power transistor. Timing of IGt is based on information from various engine sensors.
Because the VAST system triggers the igniter directly from the magnetic pickup while cranking, the engine will start even if the IGt circuit to the igniter is open. If IGt signals are not received by the igniter once the engine has started, it will continue to run, defaulted at initial timing, using signals from the magnetic pickup.
The VAST system is only used on the 2S-E, 22R-E, 22R-TE, 4Y-E, and 4A-GE engines.
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