A/C Compressor Not Operating

Created: 02. 06. 2003
Updated: --. --. ----

Model: 405 STI 1993
Engine: XU10J2
System: Regulated heating ventilation with A/C. Control panel type with horizontal sliding control. (Not dials)

Air not cold. Compressor clutch does not engage.

A dry solder in the A/C control unit did not allow to pass any current to the compressor's clutch.
See pictures below.

Circuit board of A/C control unit The A/C control unit's cicuit board with the faulty solder in the top left corner. The problem here is in the output stage where the compressor relay is mounted to the board.
It is very common for this type of defect to occur in electrical circuits with high current flow. The cause for it all is probably a poor solder (air trapped) right from the day of manufacturing of the component.
It would therefore not comme as a surprise if this car had a history of intermittent compressor problems.
Dry Solder Enlarged view of the defectiv solder joint. Also note the burnt appearance of the crack, an indication of prolonged arching. It is a superb example for demonstration.

Explanation of the compressor circuit.
The A/C control unit receives the request to activate the the compressor from the 'A/C' button on the control panel. The request is processed in the logic circuit and if all conditions are met the A/C relay is activated. The relay is integarated in the control unit. Current is now sent from the ECU to the A/C pressostat. If the system is filled with suficient gas the pressostat switch should be closed and the current will pass via pin 1 and 2. Next the current will continue to the airconditioning cut-off relay. (located in the plenum chamber near the engine ECU) This relay is normaly at rest and will pass current in the rest state straight to the compressor, through the clutch coil and back to ground at the earth point M040. The earth point is located on top of the gearbox.

Circuit Diagram

The diagram below shows the main components involving the A/C compressor operation.
Click on the various components to get more information.

Circuit Diagram

fusebox compressor cut-off relay pressostat supply unit control panel control unit

List of common compressor operation problems

  1. Pressure switch faulty
  2. Switch in control panel faulty
  3. Evaporator temperature sensor giving wrong reading    -More- 
  4. Fuel injection trottle switch stuck on full
  5. Fault in cooling fan system causing the cut-off relay to activate when the engine is not overheated.
  6. Faulty engine ECU (Rare but possible)
  7. Burnt contacts in cut-off relay
  8. Refrigerant level low.
  9. Defectiv compressor clutch

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