56
MPU
The magnetic pickup unit usually counts teeth on the flywheel and is mounted in the bell housing. It is
important to ensure the gap between the MPU face and the teeth is 1mm to 2mm, as this distance greatly
affects the output of the MPU. The unit requires a minimum of 1.2V RMS at low speed to ensure proper
crank disconnect. Setup requires the calculating the number pulses per 10 revolutions. The calculated
number is entered into the menu system “Cal Value”.
Calculate : Number of Teeth x 10 = Speed Pulses Per 10 Revolutions.
Eg: 125 teeth x 10 = 1250.
Enter the number 1250.
Battery Charging Alternator
A low cost speed source can be derived from the battery-charging alternator if fitted. It unfortunately is also
the most troublesome, and not recommended for reliable and repeatable long-term service. The difficulties
arise from the in-exact relationship of alternator output frequency and engine RPM. As a result of slippage
and variable mechanical coupling both calibration and long-term stability are compromised.
The ratio of crank pulley to alternator pulley is indeterminate, as it depends very significantly on where the
coupling belt rides in the V grove. Belt tension plays a very significant role, as does belt wear and
instantaneous belt loading.
Immediately after starting an engine the alternator is required to re-charge the partially discharged battery.
The loading on the alternator is very high and belt slippage is common as is belt set low in the V groove.
The output frequency may be lower than expected. If speed calibration is performed at this time, the speed
representation will be too low and subsequently as the load reduces on the alternator, the engine speed
will appear to erroneously increase.
For setup the number of pulses per 10 revolutions must be calculated and entered into the menu system
“Cal Value”. The battery charging alternator output frequency and current RPM must be known. This is
achieved by independently running the engine, and measuring the battery charging alternator frequency on
the W (sometimes D+) terminal. Excitation must also be provided to ensure self-excitation and an adequate
output signal. Some small variation in reported speed may result but this can be calibrated out later during
engine commissioning.
Calculate Speed Calibration value as follows:
Cal Value = (Freq x 600) / RPM.
Eg: (257Hz x 600) / 1500 = 102.8.
Enter the number 103.
4.11.10 Battery Charging Alternator Excitation
The battery charging alternator excitation system is implemented using a burst mode pulse controlled
system. This ensures reliable self-excitation while managing current consumption during standby, heat
dissipation during fault conditions, and flashing the battery-charging alternator prior to cranking for improved
speed signal output.
During standby and READY, the alternator excitation is pulsed once per minute to maintain some level of
residual magnetism in the alternator but still maintaining minimum power consumption. On receiving a
start signal, the unit pulses the alternator excitation input with a burst of 200mA pulses. The pulse width
is dependent on the battery voltage. This pulse burst establishes a definite magnetic field in the battery-
charging alternator prior to cranking. This ensures a significant speed-sensing signal is generated for
crank disconnect sensing. The alternator excitation is turned off and the engine is cranked without the
usual alternator burden loading the cranking process. This ensures easier starting. When the engine has
fired and is running, more 200mA pulse bursts are applied, which will ensure that the alternator is excited.
Given the alternator is rotating at more than the 3000-RPM the alternator will achieve self-excitation.
If self-excitation is not achieved this process will repeat for a few seconds before the excitation failed
warning is given.
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