5 - With the potentiometer at the initial contact and fan speed I selected, test the voltage between contact 10 and
contact 12 of the control unit PCB. The voltmeter should read 12V.
If this is not the case, check the connections and the relative wiring between the fan control switch and the
potentiometer. Also, check the connections between the potentiometer and contact 8 of the circuit board.
6 - Verify the efficient operation of the thermal overload cutout. With the potentiometer at the initial contact and the fan
on, test the voltage between contact 9 and contact 10 of the control unit PCB. The voltmeter should read 12V in open
circuit conditions.
Detach the connector from contact 4 of the PCB and place a jumper across the two terminals of the violet leads wired
to the thermal overload cutout. If no voltage registers, with the tester connected to contacts 9 and 10 of the PCB, the
thermal overload cutout must be replaced.
7 - With the potentiometer rotated beyond the initial contact and the fan on, test the voltage between contact 1 and
contact 7 of the heating system control unit PCB, or between the two terminals DF and D- of the heater alternator.
If the voltmeter fails to produce a reading, proceed as follows:
— check the integrity of the fuse mounted to the control unit PCB
— check the wiring and connections between the alternator and the heater unit
— check the operational efficiency of the alternator
If the fault persists, the printed circuit board of the electronic control unit must be replaced.
8 - If when placed between contact 1 and contact 7 of the control unit PCB and the alternator, the voltmeter gives a
constant reading of 5V absorbed even on rotating the potentiometer right or left, this means that either the potentiometer
or the temperature sensor (thermistor) is faulty.
9 - With the engine running at 1000 rev/min and the heating system on (fan operating and potentiometer rotated beyond
the initial contact), test the voltage at terminals B+ and B-, which should register as follows:
- potentiometer at minimum setting = 1 V
- potentiometer rotated half way = 24 V
- potentiometer at maximum setting = 33 V
With the engine at full throttle, a maximum voltage of 52 V should register, although in practice the voltage will vary
normally between 40 and 45 V.
In the event that conspicuously lower readings are observed, check the alternator drive belt, and if necessary the
alternator itself.
10 - A - If the heating system starts up but fails to operate correctly, then either the resistance heater element is
not in good condition, or the wiring connections between the alternator and the heater unit have worked loose.
B - Rotate the potentiometer to the maximum setting and bring the engine to full throttle. If the alternator
output rises to 56 V and then drops suddenly to zero (due to the electronic control unit cutting in), the
system must be reactivated by returning the potentiometer back to zero and then rotating to the right
beyond the initial contact.
The excessive rise in voltage is due either to connections working loose or to a fault at the heater element.
C - If the system produces heat with the fan operating both at speed I and at speed II, but air flow is obtained
only on speed II, this means that there is a short circuit at the speed control diode. Replace the diode.
D - If the system produces heat with the fan operating at speed I but not when the selector is set at
speed II, then the speed control diode is remaining open. Replace the diode.
systems
air conditioning
86
8
355
www.maskinisten.net
www.classicmachinery.net
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