Diagnosing faults in wiring looms is an area I am considered to be an expert in. I am often able to trace the broken wires, and repair them without having to fit a costly replacement loom. Costly, because the wiring loom might run from the front of the car to the back, and will need to be stripped out, and replaced as a whole. When wires break due to corrosion, which is often the case, the wire has to be cut back by at least 15 cm each side of the break to clear all the corroded wire, otherwise in a short time the wire will break again. Sometimes a broken wire is easy to identify but if the wires have corroded without breaking the outer casing, a very slow and patient investigation is required to find the damaged wire.
Here is an example of the complexities of repairing wiring looms: A Jaguar X Type was recovered to my workshop as a non-runner. A diagnostic check was performed, and an open circuit fault on the fuel pressure control solenoid was stored in the car’s ECU. The solenoid was tested, and found to be okay. The electrical plug was replaced, but the fault remained. Following further tests it became apparent there was a broken wire in the wiring loom, and the only solution was to strip the wiring loom casing to expose the wires and trace the particular wires in question until the break is found. The section of corroded wire was cut out and replaced with a new piece of wire, and reconnected using heat shrink connectors to seal the joins, and the loom was rebuilt. The fault codes were cleared from the ECU, and the engine fired up at the first turn of the key. The first photo shows the wiring loom. The position of the broken wire only looks obvious now because I have already cut away the casing of the wire. The second photo shows a section of the engine bay wiring loom with its many plugs, which I’m often faced with when looking for a broken wire, and gives an indication of just how long the job can sometimes take.