Tackling electrical gremlins

How to tackle ugly electrical faults and keep the customer on-side

Published:  23 February, 2015

By James Dillon

A common and continued theme for repair cases in the Technical Topics workshop are vehicle electrical faults. It seems that electrical faults, even those that might be straightforward, seem to put off many technicians and repair garages. With electrical faults, It may be the case that complex symptoms have simple causes (corrosion in wiring, failing electrical components, bad connections), and simple symptoms have complex causes (control units, intermittent failing components or intermittent connection faults). What's important is having the required skills, the correct information, suitable tools and a robust and repeatable process.

From a business perspective, especially in a busy service and repair orientated workshop, trying to allocate resources to what may be an open-ended labour spend is sometimes very difficult. It could be said that many repair businesses would prioritise a straightforward known profit service/repair job over a potentially complex electrical 'unknown profit' job. When each of these types of jobs compete for workshop attention the 'ugly sister' electrical fault is left waiting on the margins (or rejected altogether), whilst the more nubile service/repair job takes to the floor.

The attitude or perception of the customer may have a distinct impact on just how ugly the ugly sister is. What I mean is that the customer who doesn't 'understand' the complexity of their fault, or who has unreasonable demands over the time required to investigate, or who freaks out at the potential cost for investigation, can make a moderately easy job seem impossible. In some cases, extended customer contact, explanations and support can help to bring the customer on-side. Getting the customer on-side is moderately easy compared to keeping them here, particularly where complex and multi-layered faults are concerned.

The situation was that the hood failed to work correctly. It had to be operated with the emergency override. Even then, the rear RH hood wouldn't lock down. Someone else had taken a look previously and diagnosed a faulty RH hood actuator (it is secured by a threaded post which is clamped and released by the rotational direction of the actuator motor). The actuator was removed and the customer was using the manual/emergency release and just one actuator to retain the rear of the hood. A quick delve into Alldata with the relevant make and model details garnered the full workshop manual including wiring diagrams, so I now had the basis for a test and measurement plan.

Figure 1

The fact that the motor worked in one direction was enough to tell me that the motor was fine. Looking at the wiring, Figure 2, showed the motor uses the same wires for both open and close function, the polarity is simply reversed using a combination of relays. There were two micro switches built into the actuator to signal latch position and lock positions during the closing phase. The way the switches were wired indicates that these were pull down switches, i.e. they ground a voltage which is sent from the roof controller. We can conclude that it is a pull down switch due to the fact that the ground is permanent (switch pin 2) and the two switches (pin 1 and pin 3) make contact with ground when they are operated. The method by which the roof controller detects the position of the switch is to put a small voltage out on pins 1 and 3 which is then 'shorted' to ground. The roof controller detects this 'short' and knows the position of the switches. These are also known as limit switches as they limit the travel of the mechanical movement, to prevent over-tightening.

In simplistic terms, the relays are responsible for reversing the polarity of the motor. In the close direction, pin 1 is supply and pin 2 is ground, in the open direction pin 1 is ground and pin 2 is supply. In one direction, relay A rests while the relay B is active. In the reverse direction, relay A is active whilst relay B rests. The supply wires come from a common fuse and are then spliced several ways. I had wiring diagrams which showed the hood latch motor wiring and its polarity during opening and closing, and the relays in their various states.


Figure 2

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