A/C Compressor Signal, Checking
A/C compressor signal:
Voltage is supplied to the Engine Control Module (ECM) shortly before switching ON the A/C compressor. This signal ensures that the ECM maintains a constant idle speed even when differing A/C system power requirements are present.
A/C cut-off:
The A/C compressor is switched OFF for approx. 12 seconds by the ECM when accelerating from a standstill and low speeds to ensure that the full engine power is available when accelerating. If the accelerator is released earlier, the time is shortened to a minimum of 3 seconds. At Wide Open Throttle the A/C is switched OFF by the Transmission Control Module (Kick-down switch).
Special tools, testers, measuring instruments and auxiliary items required:
- Scan Tool VAG 1551 with cable VAG 1551/3
- VAG 1598/22 Test Box
- Fluke 83 Multimeter
- VW 1594 Adaptor kit
- Wiring diagram
Test conditions:
- A/C functioning OK.
- A/C must be switched OFF
- No DTC's stored in DTC memory
Check DTC memory
- Vehicle at + 15 °C minimum
Test sequence:
- Connect VAG 1551/1552 Scan Tool
- Start engine and let idle
- Press 0 and 1 buttons to select "Address word" 01
"Engine electronics"
Display will appear as shown
Rapid data transfer HELP
Select function XX
- Press 0 and 8 buttons to select Function 08:
"Read measuring value block"
- Press Q button to enter input
Display will appear as shown
Read measuring value block HELP
Input display group number XXX
- Press 0, 5 and 0 buttons to select "Display group 50"
Display will appear as shown (1 to 4 = Display zones)
Read measuring value block 50 ->
1 2 3 4
- Check A/C specifications (Display zones 3 and 4)
- Observe Display in Display zone 4.
- It must show: "Compr. OFF"
- Switch ON A/C system.
- Select lowest temperature and highest blower speed.
- Display in zone 4 must change to: Compr. ON.
- Floor accelerator pedal quickly and release again
(short burst of throttle).
When quickly pressing the accelerator pedal down the Display in Display zone 4 must change from "ON" to "OFF" for a few seconds (compressor cut-off under vehicle acceleration).
- Press -> button.
- Press 0 and 6 buttons to select Function 06: "End data transfer"
- Press Q button to enter input
- Switch OFF ignition.
NOTE: A/C compressor is switched OFF by the Engine Control Module when accelerating from standstill and from low speeds. At Wide Open Throttle the A/C compressor is switched OFF by the Transmission Control Module (TCM) kick-down switch.
If display does not appear as described:
- Connect VAG 1598/22 Test Box to Engine Control Module harness connector.
- Check wiring for open circuit between Test Box sockets 8 and A/C using Wiring diagram.
- Wire resistance: Max. 1.5 Ohms
If wiring OK
- Check function of A/C Control Module.
NOTE: If DTC memory has been erased or the Engine Control Module was disconnected from its Voltage supply the Readiness Code must be created again. Refer to "Readiness Code - Reading/Creating at On Board Diagnostic (OBD II)".