Miss-diagnosis can be costly, the parts ordered non returnable and the time taken often un chargeable, but most importantly it can cost you good customers.
We were called to look at an Audi A6, the fault was an inoperative parking brake.
On these vehicles the hand brake is actuated electronically using a motor attached to a gear on the caliper. A control unit supplies the current, which is reversed to change the direction of rotation applying or releasing the brake.
The first garage had retrieved a fault code suggesting a power supply failure. Poor understanding of the operation of the system had led to a miss-diagnosis. They checked for voltage at the caliper upon finding no voltage they diagnosed the control unit. However the control unit runs self checks and if operating current is outside an operating tolerance the output is disabled to prevent cable burning or further damage.
The cost of a new control unit was considered prohibitive, so a second hand replacement was sought.....upon fitting the situation remained the same so the car was sent to another garage who specialises in VAG vehicles.
They fitted a new caliper, but this also failed to cure the problem. They then checked continuity between the caliper and the control unit and found an open circuit. The plug at the caliper had suffered water ingress and the cables had rotted away.
Confident they had found the fault the garage repaired the cables by soldering, however the control unit now showed a new code and one much more puzzling code.
The 1st code lost communication with the transmission control module. The car is a manual but the 2nd hand control unit is from an automatic.
The clamping force code relates to the method of detecting when the brakes are fully applied. The motor runs until the control current reaches around 15 amps this is the stop signal to the control unit. If 15 amps is not achieved within around 20-30 seconds the motor stops and the code is flagged.
This suggests that the motor is turning but the caliper is not clamping the disc. A rubber drive belt is used to transmit the torque from the motor to a gear of about 50:1 ratio. Could this be snapped. Could it be a wiring fault still? Is the caliper motor of the correct resistance. How can we quickly check?
In measuring value blocks 003 the control current is displayed.
The left hand caliper is drawing 5.852 amps.
Using ohms law we can calculate the resistance of the circuit 12/5.9 = 2.05 ohms.This is within the expected range, but why does it not clamp the disc.
To check if the rubber drive band is intact apply the handbrake with the footbrake applied.
If the current increases the motor is connected to the caliper. In this case it does.
This only leaves one possibility the wiring has been crossed during repair. Causing the left hand caliper to release the brakes instead of applying them.
So this proved to be the case, swapping the wires over restored correct operation.
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