Brake Light Switch: Testing and Inspection
The diesel injection system operates with a throttle position sensor. For safety reasons, the engine is regulated when the brakes are operated. The ECM requires signals from the brake light switch and also the brake pedal switch. This means that if the brakes are operated when the accelerator pedal is held at a constant position, the engine is immediately reduced to idling speed. Incorrectly adjusted switches may lead to unwanted regulating action.
Special tools and equipment
- V.A.G 1551/1552 Scan tool with adapter cable VAG 1551/3
- Test box VAG 1598/22 (through my 1999)
- Test box VAG 1598/31 (from my 2000)
- Digital multimeter US1119 (Fluke 83 or equivalent)
- Adapter set VAG 1594
- Wiring diagram
Checking conditions
- Fuses 13 and 43 must be OK.
- The battery voltage must be at least 11.5 V.
Test sequence
- Connect V.A.G 1551/1552 (Connecting scan tool and selecting engine electronics control module.), refer to "On Board Diagnostic II/VAG 1551/VAS 5051 Scan Tool, Connecting" under "Powertrain Management/Computers and Control Systems. Testing and Inspection
- Run engine at idle speed and press buttons -0- and -1- to select "Engine electronics" address word 01.
Indicated on display
Rapid data transfer HELP
Select function XX
- Press buttons -0- and -8- to select "Read measured value block" function 08 and confirm entry with -Q- button.
Indicated on display
Read measured value block HELP
Input display group number XXX
- Press buttons -0-, -0- and -6- to select "Display group number 6" and confirm entry with -Q- button.
- Observe display in display field 2.
Read measured value block 6 ->
0 km/h 0 0 0 000000 255
- Specification: x 0 0
- Center display = Brake pedal switch
- Right-hand display = Brake light switch
- Operate brake pedal slowly.
Both displays must change from 0 to 1 at same time.
Read measured value block 6 ->
0 km/h 0 1 1 000000 255
- Specification: x 1 1
- Release brake pedal slowly into rest position again.
Both displays must change from 1 to 0 again.
Read measured value block 6 ->
0 km/h 0 0 0 000000 255
- Specification: x 0 0
- Press -> button.
- Press buttons -0- and -6- to select "End output" function 06 and confirm entry with -Q- button.
- Turn off ignition.
If one or both displays do not change:
- Disconnect 4-pin connector from brake light switch and brake pedal switch.
- Measure resistance between terminals 1 + 4 of switch.
- Specification: Brake pedal not depressed: Infinity ohms; Brake pedal depressed: max. 1.5 ohms
- Measure resistance between terminals 2 + 3 of switch.
- Specification: Brake pedal not depressed: max. 1.5 ohms; Brake pedal depressed: Infinity ohms
If the specifications are not obtained:
- Replace Brake Light Switch -F- and Brake Pedal Switch -F47-.
- Check DTC memory and, if necessary, repair any malfunctions and then erase, refer to "Diagnostic Trouble Code (DTC) Memory, Checking and Erasing" under "Vehicle/Powertrain Management/Fuel Delivery and Air Induction/Testing and Inspection" Testing and Inspection
- Read readiness code.
If the DTC memory has been erased or the engine control module was disconnected from permanent battery positive (B+), the readiness code must be generated again.
If the specifications are obtained:
- Remove wiper arms and plenum cover.
- Connect test box VAG 1598/22 (through my 1999) or VAG 1598/31 (from my 2000) to control module wiring harness.
- Check wiring between test box and 4 pin connector for open circuit using wiring diagram:
- Wire resistance: Max. 1.5 ohms
- Check wires for short to one another, to vehicle ground and to battery positive (B+).
- Specification: Infinity ohms
If wiring and the resistance measurement values are OK:
- Replace engine control module.