Timing Control Module
The Timing Control Module (TCM) helps to provide detonation protection for the engine. The TCM also provides the electronic adjustment of the ignition timing with a variable timing magneto. This ignition system consists of four basic components:
- Timing Control Module (TCM)
- Magneto Interface Box (MIB)
- Variable timing magneto
- Various sensors
The TCM determines the ignition timing. The TCM communicates the ignition timing with the MIB. The TCM also provides the system diagnostics.
The engine timing is based on the desired timing and the engine's operating conditions. The desired timing is programmed by the operator. The TCM automatically adjusts the engine timing according to the engine speed and the engine detonation. The ignition timing is controlled by three signals. The signals are sent from the TCM to the MIB. The MIB sends a signal that indicates the firing of the spark plug to the TCM. The TCM uses this signal to calculate the actual engine timing.
The TCM uses sensors and internal circuitry to monitor the system components for problems. The TCM will accomplish the following tasks if a component failure or a harness failure occurs:
- Sensing the problem
- Illumination of an LED indicator
- Annunciation of a diagnostic code
Diagnostic codes can be viewed with the Digital Diagnostic Tool (DDT).
Magneto Interface Box
The Magneto Interface Box (MIB) receives three signals from the TCM. The signals define the desired engine timing. The MIB closes the connection at the magneto between pin A and pin B or the MIB closes the connection at the magneto between pin E and pin F when the magneto causes the spark plug to fire. The voltage signals for the magneto capacitor are sent to the MIB. The MIB converts the capacitor's voltage for the odd numbered bank into an equivalent low voltage signal. The MIB sends the signal to the TCM. Pin A is continuously connected to pin B and pin E is continuously connected to pin F when the engine is operating in the manual override mode.
Variable Timing Magneto
Fixed timing magnetos discharge the ignition capacitor to the ignition transformer when the rotor enters the magnetic field of a stationary pole. For the variable timing magneto, the ignition capacitor can be discharged at any time while the rotor is within the magnetic field of the stationary pole. The ignition timing window is the engine timing from the entrance of the rotor into the magnetic field to the engine timing when the rotor exits the magnetic field. For example, the ignition timing window can be from 32 to 18 degrees.
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| Illustration 1 | g00661360 |
Variable timing magneto | |
The capacitor will only discharge when the rotor is within the timing window and an external switch is closed. The external switch is located within the MIB. This external switch is closed further through the timing window in order to retard the timing. The external switches are always closed when the variable timing magneto is operated as a fixed timing magneto. Place the magneto in the manual override mode with the DDT in the "MAG CAL" mode in order to operate the variable timing magneto in this manner.
Engine Sensors
The Timing Control Module (TCM) uses two sensor signals for the control of the ignition timing. The TCM also uses detonation sensors for detonation protection. The Crank Angle Sensor (CAS) and the speed sensor provide the information for the Top Center (TC) and the rotational position that is needed to control the timing. The detonation sensors provide electrical signals of the mechanical engine vibrations that are used to calculate the detonation levels.
The Crank Angle Sensor (CAS) is a passive magnetic pickup that is used to indicate the crankshaft angle to the TCM. The CAS indicates the Top Center (TC) of the number one cylinder. The CAS is used for the control of the timing, the calculation of the actual timing, and the calculation of the engine speed. The signal is generated when the TC hole in the flywheel passes by the magnetic pickup.
The speed sensor is a passive magnetic pickup that is used to indicate engine speed to the TCM. The speed sensor provides fine resolution for the crankshaft angle. The speed sensor produces a signal whenever a ring gear tooth on the flywheel passes by the sensor. The signal is used for the following functions:
- Calculation of the engine speed
- Monitoring of the crankshaft angle between TC pulses
- Provide a clock signal for the MIB.
The detonation sensors monitor the engine for excessive detonation. The sensor generates a signal to the TCM. This signal represents the engine's mechanical vibrations. The sensors are mounted in the center of each cylinder bank in order to monitor the engine's vibrations. The vibration signals are processed by the TCM in order to determine the engine's detonation level. The TCM adjusts the ignition timing due to the detonation level. The ignition timing is adjusted in order to control detonation while optimum performance is maintained.
Programmable Parameters
The TCM has two programmable parameters that affect the performance of the ignition system. These two parameters are the desired timing and the speed timing map. The DDT is used to change the desired timing and the speed timing map in order to meet the needs of the application.
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| Illustration 2 | g00660757 |
Diagram of the timing control ignition logic | |