When the engine is cranked, an alternating current signal is generated by the pickup coil. This signal is shaped in the igniter and then relayed through a control circuit to the base of the primary circuit power transistor.
When the voltage at the base of this transistor goes high, current begins to flow through the coil primary windings. When this signal goes low, coil primary current stops flowing, and a high voltage is induced into the secondary winding. At cranking speed, spark plugs fire at initial timing, a function of distributor position in the engine.
When the engine is running, spark timing is determined by the relative positions of the pickup reluctor (signal rotor) and the pickup coil winding to each other. This relative position is controlled by the centrifugal advance weights and vacuum advance diaphragm positions.
As engine speed increases, the reluctor advances in the same direction as distributor shaft rotation. This is a result of the centrifugal advance operation.
As manifold vacuum applied to the vacuum controller is increased, the pickup coil winding is moved opposite to distributor shaft rotation.
Both of these conditions cause the signal from the pick-up coil to occur sooner, advancing timing.
3 © Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.
TCCS Ignition Spark Management, Electronic Spark Advance (ESA), and Variable Advance Spark Timing (VAST)
The advent of ECU spark management systems provides more precise control of ignition spark timing. The centrifugal and vacuum advances are eliminated; in their place are the engine sensors which monitor engine load (Vs or PIM) and speed (Ne). Additionally, coolant temperature, detonation, and throttle position are monitored to provide better spark accuracy as these conditions change.
To provide for optimum spark advance under a wide variety of engine operating conditions, a spark advance map is developed and stored in a look up table in the ECU. This map provides for accurate spark timing during any combination of engine speed, load, coolant temperature, and throttle position while using feedback from a knock sensor to adjust for variations in fuel octane.
TCCS engines use two versions of ECU controlled spark management, Electronic Spark Advance (ESA) and Variable Spark Timing (VAST).
To monitor engine rpm, the TCCS system uses the signal from a magnetic pickup called the Ne pickup. The Ne pickup is very similar to the magnetic pickup coil used with Conventional EFI. It has either four or 24 reluctor teeth, depending on engine application.
Engines equipped with the ESA system (and the 4A-GE engine with VAST) use a second pickup in the distributor called the G sensor. The G sensor supplies the ECU with crankshaft position information which is used as a reference for ignition and fuel injector timing. Some engines use two G sensors, identified as G1 and G2.
Was this article helpful?