A/C Compressor ON Signal

Checking A/C compressor ON signal

The A/C compressor ON signal increases the basic value forthe Idle Air Control (IAC) valve, ensuring that idle speed remains in the center of the control map range.

- Using VAG 1551 scan tool, initiate "Read Measuring Value Block" function 08, display group 004. Refer to On Board Diagnostics under Computers and Control Systems. Connecting and Operating VAG 1551 Scan Tool

- Display will appear as shown:

Read Measuring Block Value 4
1 2 3 4

- Observe first digit of 4-digit display (switch inputs) in display field 4.
^ Specified value: OXX X

NOTE:
Switch inputs shown by digits 2 to 4 are ignored for this test.

- Switch A/C on (manual A/C) or, with automatic A/C, press buttons "AUTO", temperature "LO" and blower speed "HI."
^ Specified value: 1XX X (zero changes to one)

- Press "arrrow" button.
- Press buttons -0- and -6-to select "End Output" function 06.
- Press -Q- button to confirm input.

If the 4-digit display does not change:

- Switch ignition off.
- Connect VAG 1598 test box with 1598/11 adapter harness to ECM and wiring harness. Connecting and Testing VAG 1598 Pin Out Box
- Connect LED tester to test box sockets 11 and 45 (Ground).
- Switch ignition on.
- Switch A/C on as described above.
^ LED must light up

If the LED does not light up:

- Disconnect ECM harness connector C.

If LED now lights up:

- Replace ECM .

If LED does not light up:

- Check continuity of wiring and/or short circuit using wiring diagram as follows:

For manual A/C:

- Check wiring harness between ECM connector C terminal 11 and A/C switch -E30-.

For automatic A/C:

- Check wiring harness between ECM connector C terminal 11 andA/C control head -E87-.

If wiring harness is OK:

- Check A/C system.