Automatic Heating Troubleshooting

For all practical purposes, the heating system is separate from the A/C system. Therefore, problems should fall within one of two categories: heating faults or air conditioning faults. Each system has its own causes and corrective actions.

Troubleshooting of the heater electronic controls begins by removing the heater control module housing as described in 801 Heater. Heater control module troubleshooting is summarized in Table a. Use the photos that follow for connector and test terminals referred to in Table a.

Table a. Temperature Control Troubleshooting

Symptom

Probable cause

Corrective action (refer to photos)

Temperature control knob in position 1, control sen/o moves to maximum heating

Open circuit in passenger compartment temperature sensor

Test sensor resistance at connector B. Passenger compartment temperature sensor resistance specification: 1.9 k£2 at 20°C (68°F).

Open circuit in temperature control potentiometer

Test resistance at temperature control potentiometer terminals with wires connected and switch in position 5. Potentiometer resistance specification: 950 ± 20 Q.

Control module regulates to maximum heating in defrost switch position only

Short circuit in passenger compartment temperature sensor

Test sensor resistance at connector B. Passenger compartment temperature sensor resistance specification: 1.9 k£2 at 20°C (68°F).

Short circuit in temperature control potentiometer

Test resistance at temperature control potentiometer terminals with wires connected and switch in position 5. Potentiometer resistance specification: 950 ± 20 il

Too much heat, does not regulate correctly.

Open circuit in heat exchanger temperature sensor

Test sensor resistance at connector C. Heat exchanger temperature sensor resistance specification: 1.7 k£i at 20°C (68°F).

Temperature control potentiometer set incorrectly

Set potentiometer switch to position 5 and remove knob. Connect ohmmeter to potentiometer. Hold shaft of switch with pliers and adjust potentiometer with a screwdriver to specification: 950 ± 20 Q.

Too little heat, does not regulate correctly.

Short circuit in heat exchanger temperature sensor

Test sensor resistance at connector C. Heat exchanger temperature sensor resistance specification: 1.7 k£2 at 20°C (68°F).

Temperature control potentiometer set incorrectly

Set potentiometer switch to position 5 and remove knob. Connect ohmmeter to potentiometer. Hold shaft of switch with pliers and adjust potentiometer with a screwdriver to specification: 950 ± 20 £2.

Control module not regulating

Open circuit in power supply

Check for battery voltage at connector A terminal 15. Check for continuity to ground at connector A terminal 31.

Printed circuit board defective

Replace circuit board.

Control module defective

Replace control module.

1984 Porsche 911 Heating Control

Test connections (arrows) at temperature control potentiometer.

^ Test connections at temperature control module:

• Harness connector A (power supply).

• Harness connector B (passenger compartment temperature sensor).

• Harness connector C (heat exchanger temperature sensor).

0019349

Potentiometer Porsche 996

^ Adjust temperature sensor potentiometer:

— Set potentiometer switch to position 5 and remove knob. Connect ohmmeter to potentiometer (shown above). Hold shaft of switch with pliers and adjust potentiometer with a screwdriver to specification: 950 ± 20 £L

1984 Porsche 911 Heating Control

0019349

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