This could be one of the most difficult faults to diagnose on modern vehicles.
A Golf MK V has a side light/brake light failure, however the local garage and the main agent have come to an expensive conclusion. It must be the body system interface. This computer controls many of the body and lighting functions on the vehicle. Replacement is costly not just the parts but the time plugged into the dealer diagnostic interface "coding" the unit.
This vehicle has few relays and even fewer fuses. The circuit at fault has no apparent circuit protection. If a fault is detected by the body interface it simply turns off the supply.
This in turn puts a fault light and message on the multi function display on the dash.
This vehicle had no such message or warning light. No fault codes were found in the body system interface. The lamp had been checked and passed both visual and resistance checks.
Yet when the side lights were switched on, or the brake pedal depressed the lamp failed to light.
The previous repair shops had checked for voltage at the lamp, and the output of the BSI.
No voltage means no output from the BSI, with no codes the garages had assumed this meant an ecu fault.
Is this diagnosis correct?
The main agent was unsure and requested we checked the system. The BSI checks for faults two ways termed hot and cold monitoring.
Cold monitors run every time the ign is switched on, hot monitors run while the circuit is in use.
A quick check with the scope showed the cold monitor was indeed running. So why was there no output, and if the was a circuit fault why no code? Or warning light/message?
You will often hear me banging my drum about Voltage Drop testing, however this can only be carried out with the circuit in use. ( Dynamic Testing) in this case this is not possible, so we tested using resistance checks, first continuity to ground (short to earth) then short to live with no fault error message we expected these simple checks to be fruitless. So it proved.
However the resistance between the BSi and the lamp showed 57 Ohms. Bingo this test was then repeated along the loom until the section at fault was identified. A loom connection proved to be the culprit showing signs of corrosion, and once cleaned and protected from further water Ingres normal operation was restored. It appears the fault is outside the fault code parameters but within the fault limit. This results in no output and no warning light/message on the multi function display. This has to be the worse case for the technician.
