A/C System Signal, Checking

A/C System Signal, Checking

Special Tools and Equipment







- VAG1526A
- VAG1527B
- VAG1594A
- VAG1598/31
- VAS5051 with VAG505/1
- or
- VAG1551 with VAG155/3B

The A/C system signal notifies the Engine Control Module (ECM), that the A/C system requires a higher idle speed to achieve the desired interior temperature (regardless of whether heating or cooling is required).

The increase in idle speed based on presence of the A/C system signal is control module dependent and occurs only in vehicles with All Wheel Drive (AWD) and manual transmission.

Test sequence
- Connect VAS5051 tester or VAG1551 scan tool and select the control module for engine electronics using "address word" 01. [1][2]Scan Tool Connecting and Initial Checks
Engine must run at idle for this.

When indicated on display


Rapid data transfer HELP
Select function XX


- Press buttons -0- and -8- select "Read Measureing Value Block" and press -Q- button to confirm input.

If specified values are not obtained:
- Connect VAG 1598/31 test box at wiring harness to ECM, do not connect ECM.

When indicated on display:


Read Measuring Value Block Q
Input display group number XXX


- Press buttons -0-,-5- and -6- to select "display group number 056" and press -Q- button to confirm input.
- Switch on A/C system, press "Auto" button and select the lowest temperature.
- Check indications in display field 3 and 4.

Display Group 56:

- Check the following wiring connection for open circuit and short circuit to Ground (GND) and B+:

Test box VAG 1598/31 Socket A/C control head -E87- Terminal
40 -

- Repair open circuit or short circuit if necessary.
- If there are no malfunctions in the wiring, check function of A/C system.