A/C Signal: Testing and Inspection
Air conditioner (A/C) compressor signal, checking
Voltage is supplied to the engine control module (ECM) shortly before switching on the air conditioner compressor. The signal ensures that the control module keeps the idle speed constant for the differing power requirements of the A/C system.
The A/C compressor is switched off for approx. 12 seconds by the ECM when accelerating from standstill and from 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 full throttle the air conditioner is switched off by the Transmission Control Module (TCM) kick-down switch.
Special tools and equipment
- VAG1551 or VAG1552 Scan Tool (ST) with VAG1551/3 adapter cable
- VAG1598/22 test box
- Multimeter US1119 (Fluke 83 or equivalent)
- VW1594 connector test kit
- Wiring diagrams
Checking conditions
- A/C functioning OK
- A/C switched off
- No DTCs stored in DTC memory.
- Vehicle at room temperature; warmer than +15°C (59°F).
Checking
- Connect VAG1551 or VAG1552 scan tool and press buttons -0- and -1- to insert "Engine Electronics" address word 01 (with engine running at idle), Refer to "Scan Tool Testing/Scan Tool Connecting and Initial Checks/Connecting VAG1551/VAG1552 Scan Tool" under "Engine", Testing and Inspection
Indicated on display
Rapid data transfer HELP
Select function XX
- Press buttons -0- and -8- to select "Read Measuring Value Block" function 08, and press -Q- button to confirm input.
Indicated on display
Read Measuring Value Block HELP
Input display group number XXX
- Press buttons -0-, -5- and -0- to input display group number 50 (050), and press -Q- button to confirm input
Indicated on display (1-4 = display fields)
Read Measuring Value Block 50 ->
1 2 3 4
- Compare display with specified values for A/C (display fields 3 and 4).
- Observe display in display field 4.
- Specified value: Compr. OFF
- Switch A/C system on.
- Select lowest temperature and highest blower speed.
- Observe display in display field 4.
- Specified value: Compr. ON
- Briefly press accelerator pedal to floor and release (short throttle burst).
When quickly pressing the accelerator pedal down the value in display field 4 must change from "Compr. ON" to "Compr. OFF" for a few seconds (compressor cut-off for vehicle acceleration).
- Press -> button.
- Press buttons -0- and -6- to select "End Output" function 06, and press -Q- button to confirm input.
- Switch ignition off.
NOTE: The air conditioner compressor will be switched off (Compr.OFF) when the engine control module recognizes full load and the air conditioning is switched on (A/C-HIGH).
Continuation
If the displayed values are NOT OK:
- Connect VAG1598/22 test box to ECM harness connector.
- Check wiring for open circuit between ECM/test box terminal 8 and A/C control head.
If the wiring is OK:
- Check function of A/C control head -E87-.
- Check readiness code. If Diagnostic Trouble Code (DTC) memory has been erased, or ECM was disconnected, generate new readiness code. Monitors, Trips, Drive Cycles and Readiness Codes