Ignition Transformer
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| Illustration 1 | g00326433 |
Components Of The Gas Engine Ignition Group (1) High energy ignition transformer. (2) Tube. (3) Extension with a spring loaded rod. (4) Spark plug. | |
The ignition transformer causes an increase of the primary voltage. The increased voltage is needed to send a spark (secondary electrical impulse) across the electrodes of the spark plugs. For good operation, the connections (terminals) must be clean and tight. The negative transformer terminals for each transformer are connected together and the terminals are connected to ground.
Timing Control System
The Caterpillar Detonation Sensitive Timing Control (DSTC) system provides detonation protection for the engine and electronic adjustment of ignition timing with a variable timing. This ignition system consists of four basic groups (Timing Control Module, Magneto Interface Box, Variable Timing Magneto, and Sensors).
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| Illustration 2 | g00481277 |
Timing Control System | |
Timing Control Module (TCM)
The TCM determines the ignition timing. The TCM communicates the ignition timing with the Magneto Interface Box (MIB). The TCM provides the system diagnostics.
Engine timing, controlled by the TCM, is based upon the desired timing signal received from the ECM. The desired timing signal from the ECM varies depending on engine speed, engine load and engine detonation.
The ignition timing is controlled by three signals that are sent from the TCM to the MIB. The MIB sends a signal that indicates that the plug is firing to the TCM. The TCM uses this signal to calculate actual engine timing.
Magneto Interface Box (MIB)
The MIB receives three signals from the TCM. These signals define how much the timing should be retarded in order to provide the desired engine timing. The MIB then closes the connection between pins "A" and "F" or "E" and "F", going to the magneto. This occurs when the magneto is sending primary voltage of 170 to 200 VDC in order to fire the spark plugs.
In order to identify the timing of the primary ignition pulses, the magneto capacitor voltage signals are also sent to the MIB. The primary ignition pulses are when the spark plugs fire. The MIB converts the odd bank capacitor voltage into a lower voltage ignition pulse signal that is sent to the TCM. When operating in Magneto Calibration Mode, the connections between "A" and "B" or "E" and "F" in the MIB are continuously made.
Variable Timing Magneto
Standard magnetos discharge the ignition capacitor to the ignition coil. This occurs as soon as the rotor enters the magnetic field of a stationary pole. For the variable timing magnetos, the ignition capacitor will discharge while the rotor is within the magnetic field of the stationary pole. The ignition timing window is defined as the timing when the rotor enters the magnetic field to the timing when the rotor exits the magnetic field. For variable timing magnetos, the capacitor will discharge only when the rotor is within the timing window and an external switch, provided by the MIB, is closed. To retard the timing, the external switch is closed farther through the timing window. To operate the variable timing magneto the same way as a standard magneto, the external switches are permanently closed.
Timing Control Sensors
The TCM uses two sensor signals for the ignition timing control. The TCM uses the detonation sensors for detonation protection. The Crank Angle Sensor (CAS) and the Speed Sensor (TCMPU) provide top center (TC) and rotational position needed to control timing. The detonation sensors provide an electrical signal of the engine's mechanical vibrations that are used in order to calculate the detonation levels.
Crank Angle Sensor (CAS)
This passive magnetic speed sensor indicates the crankshaft angle to the TCM. The crank angle sensor provides the TC signal used to control timing and calculate actual timing. The signal is generated when the TC hole (for the No. 1 piston) in the flywheel face passes the sensor.
Speed Sensor (TCMPU)
This passive magnetic speed sensor indicates engine speed to the TCM. The speed sensor produces a signal whenever a ring gear tooth on the flywheel passes the sensor. The signal is used to calculate engine speed, to monitor the crankshaft angle between TC pulses and to clock the MIB electronics.
Detonation Sensors
The detonation sensor is a powered device that outputs a filtered electrical signal and a amplified electrical signal of the engine's mechanical vibrations. When increased levels of vibration are occurring, the ECM calculates the engine detonation. If necessary, the ECM will adjust the ignition timing in order to control detonation. This is done by sending a desired timing signal that is retarded as much as six crank degrees to the TCM. When the level of vibration has returned to normal, the ECM will adjust the desired timing signal in order to gradually allow the ignition timing to return to operation. This adjustment is based on the desired timing map that is part of the personality module.
Timing Control And Magneto Interface Box
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| Illustration 3 | g00481303 |
G3600 Ignition Timing System | |
The Timing Control provides three signals to the Magneto Interface Box (MIB) in order to communicate the desired ignition timing. These signals are the Ignition Interface Clock, the Reset Pulse signal, and the Manual Override signal. The MIB returns the Ignition Pulses to the Timing Control. The Timing Control calculates the Actual Engine Timing. The Timing Control performs some ignition diagnostics from this signal.
Ignition Interface Clock
The Ignition Interface Clock signal is a square wave version of the speed sensor signal. This signal provides a timing clock for the MIB.
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| Illustration 4 | g00329509 |
Relationship Between Speed Sensor And Clock Signals | |
Sent from Timing Control (pin-G) to MIB (pin-E, 10 pin Connector).
The waveform is a square wave version of the speed sensor signal, with peak voltage of 2.5 V and minimum voltage of 1 V. The positive-going edge of the clock signal should align with the negative-going zero-crossing of the speed sensor signal.
Reset Pulse
The Reset Pulse signal indicates to the MIB the ignition timing desired by the Timing Control. The pulse is sent once from TC to TC.
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| Illustration 5 | g00329510 |
Interface Reset Pulse Signal Relative To Crank Angle TC Signal | |
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| Illustration 6 | g00329512 |
Close up Of Interface Reset Pulse Signal Relative To Crank Angle TC Signal | |
Sent from Timing Control (pin-H) to MIB (pin-G, 10 pin Connector).
The Interface Reset Pulse signal is normally below 1 V. The Reset Pulse goes high to about 2.5 V. This signal should go high once from Top Center (TC) to TC.
Manual Override ("Mag Cal" Mode As Seen In DDT)
The Manual Override signal tells the MIB to control fully advanced ignition timing.
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| Illustration 7 | g00329513 |
Manual Override Signal, Timing Control In Electronic Timing Mode | |
Sent from Timing Control (pin-E) to MIB (pin-C, 10 pin Connector).
The manual override signal should remain below 1 V when the system is in Electronic Timing Control mode. A 5 V signal on this line will tell the MIB to run the ignition at fully advanced timing.
Ignition Pulses
The Ignition Pulse signal is the odd number bank's capacitor charge. The signals waveform indicates the discharge of the MIB and firing of cylinders. One pulse is shown for each number cylinder. This signal is used by the TCM to calculate ignition timing and some ignition diagnostics.
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| Illustration 8 | g00479636 |
Ignition Pulses Relative To Crank Angle TC Signal (Six Cylinder Engine) | |
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| Illustration 9 | g00479637 |
Close up Of Ignition Pulses Relative To Crank Angle TC Signal (Six Cylinder Engine) | |
Sent from MIB (pin-A, 10 pin Connector) to Timing Control (pin-C).
From TC to TC, this waveform should show one pulse for each number cylinder. The pulse is normally at 5 V and goes below 2 V when the MIB detects the ignition firing.
Interaction Of The Interface Signals
The manual override signal is held below one volt, the MIB is placed in "Mag Cal" Mode. The TCM generates the Clock signal by squaring the Speed Sensor (TCMPU) signal. This clock signal is used by the MIB electronics in order to keep track of the rotational position. When the the Reset pulse is received from the TCM, the MIB counts nine Clock signal edges. The MIB will then signal to fire Cylinder Number One. The MIB continues to monitor the Clock. The MIB signals to fire the remaining cylinders through the rotation. When the MIB discharges to fire the cylinder, an ignition pulse is generated. The Ignition Pulse signal is a reduced voltage signal of the odd number bank's capacitor voltage. Ignition Timing is calculated by comparing the timing offset between TC from the Crank Angle Sensor and the Ignition Pulse for Cylinder Number One.
When the Manual Override signal goes above one volt, the MIB operates in Manual (Standard) Mode. The MIB will no longer control ignition firing. The MIB will generate an ignition pulse at the most advanced ignition timing. The Ignition Timing is calculated in the same manner as in Electronic Timing Mode.
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| Illustration 10 | g00329517 |
Interaction Of Reset, Clock, Ignition Pulse And TC Signal | |
When the MIB receives the Reset Pulse, the MIB generates a ignition pulse after 9 Clock Signal edges (both rising and falling edges). The MIB generates the Ignition Pulse for Cylinder Number One. This should occur before the TC signal of the engine.
Ignition Pulse Firings
From TC to TC, this waveform should show one pulse for each cylinder. The pulses should go from 190 V to ground when the cylinder is signaled to fire.
Magneto Wiring Diagram
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| Illustration 11 | g00481308 |
G3606 Engine Magneto Wiring Firing Order 1-5-3-6-2-4 Pin Order A-B-C-D-E-F | |
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| Illustration 12 | g00481311 |
G3608 Engine Magneto Wiring Firing Order 1-6-2-5-8-3-7-4 Pin Order A-B-C-D-E-F-H-I | |
Magneto
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| Illustration 13 | g00481312 |
Variable Timing Magneto (1) Magneto interface box. (2) Alternator section of magneto. (3) Electronic firing section of magneto. (4) Magneto input connector. (5) Magneto output connector. | |
The magneto is made of two sections.
- Permanent magnet alternator section
- Breakerless electronic firing section
There are no brushes or distributor contacts.
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| Illustration 14 | g00481314 |
Cross Section Of Variable Timing Magneto (2) Alternator section of magneto. (4) Magneto input connector. (5) Magneto output connector. (6) Vent. (7) Speed reduction gears. (8) Pickup coil. (9) Drive tang. (10) Energy storage capacitor. (11) Rotating timer arm. (12) SCR solid state switch. | |
The engine turns the drive tang (9) . The drive tang turns the alternator section of the magneto (2) , the speed reduction gears (7) and the rotating timer arm (11) . As the alternator section is turned, the section provides power to charge the energy storage capacitor (10) . There are separate pickup coils (8) and SCR (silicon controlled rectifier) solid state switches (12) for each cylinder. The timer arm passes over the pickup coils (8) in sequence. The pickup coils send signals to the magneto interface box (1) which then turns on solid state switches (12) which release the energy stored in the capacitor (10) . This energy leaves the magneto through the output connector (5) . The energy travels through the wiring harness to the ignition coils. The energy is transformed to the high voltage needed to fire the spark plugs.
Standard magnetos discharge the ignition capacitor to the ignition coils as soon as the rotor enters the magnetic field of a stationary pole. For variable magnetos, the ignition capacitor will discharge while the rotor is within the magnetic field of the stationary pole. The ignition timing window is defined as the timing when the rotor enters the magnetic field to the timing when the rotor exits the magnetic field. For variable timing magnetos, the capacitor will discharge only when the rotor is within the timing window and an external switch, provided by the MIB, is closed. To retard timing, the external switch is closed farther through the timing window. To operate the variable timing magneto in the same manner as a standard magneto, the external switches "A" and "B" or "E" and "F" are permanently closed.
Engine Start-up
At engine start-up, the Timing Control performs some system checks not done once the engine is running. The Manual Override signal places the MIB in Manual Mode until the engine speed is above 500 rpm. The Timing Control checks the Ignition Pulses signal for Cylinder No. 1 firing while the engine speed is between 120 and 300 rpm. If this ignition pulse is not present, the Timing Control will display the "No Magneto Interface Signal" fault. Once the engine speed increases between 300 and 500 rpm, the Timing Control will compare the timing of Cylinder No. 1 firing to the "Mag Cal" Timing stored in internal memory. If the two timing values do not match, the Timing Control will display the "Magneto Out Of Calibration" fault.