Brake Light Switch: Testing and Inspection
NOTE: For safety reasons, the instruction from the accelerator pedal sensor (potentiometer) to the ECM to open the throttle valve is suppressed when brake is being operated. This requires the brake light switch signal and also the brake pedal switch signal in the control module. If brake pedal is operated with accelerator pedal constantly depressed, engine is immediately regulated down to idle speed. A malfunctioning switch can lead to undesired regulation.
Test sequence
- Connect VAS5051 tester or VAG 1551 scan tool and select control module for engine electronics using "address word" 01.
- Ignition must remain switched on for this.
When indicated on display:
Rapid data transfer HELP
Select function XX
- Press buttons -0- and -8- to select "Read Measuring Value Block" and press -Q- button to confirm input.
When indicated on display:
Read Measuring Value Block Q
Input display group number XXX
- Press buttons -0-, -6- and -6- to select "display group number 066" and press -Q- button to confirm input.
Indicated on display:
Read Measuring Value Block 66 ->
1 2 3 4
- Check brake light/brake pedal switch in display field 2.
Display Group 66:
If indications do not resemble description:
Check switch
- Remove driver - side storage compartment.
- Disconnect harness connector at brake light/brake pedal switch.
- Connect multimeter between terminals 1 and 4 for resistance measurement.
- Specification: Infinity ohm (no continuity)
- Depress accelerator pedal.
- Specified value: approx. 0 ohm
- Connect multimeter between terminals 2 and 3 for resistance measurement.
- Specified value: approx. 0 ohm
- Depress accelerator pedal.
- Specification: Infinity ohm (no continuity)
If specified values are not obtained:
- Replace brake light/brake pedal switch.
If specified values are obtained:
Checking voltage supply
- Connect VAG 1527B voltage tester as follows:
Harness connector terminal Measure to
1 Chassis Ground
- LED must light.
Harness connector terminal Measure to
2 Chassis Ground
- Switch ignition on.
- LED must light
If LED does not light:
- Check wire connection from terminal 1 or 2 of connector for open circuit and short circuit to Ground:
- Repair open circuit or short circuit if necessary.
If LED lights:
Checking activation
- Connect VAG 1598/31 test box at wiring harness to ECM, do not connect ECM.
- Check the following wire connections for open circuit and short circuit to Ground and B+:
Harness connector terminal VAG 1598/31 test box socket
3 55
4 56
- Repair open circuit or short circuit if necessary.