Confusing symptoms and misleading markings lead to a lengthy fault finding process for what turned out to be a simple problem.
The problem was simple: One headlight in the car was brighter than the other. My father, who is a mechanic, immediately diagnosed the problem as a poor ground. He connected a new wire from the terminal marked ground on the light, but this only made a marginal improvement. My brother then knew that it must be a faulty wire and ran a second wire from the switch to the high beam positive. However, this had no improvement. Connecting it to the low beam didn't result in any improvement. The light itself operated fine when a positive wire was run directly from the battery, eliminating it from the problem.
It was then my turn to have a go at it. I decided to measure the voltages of the good light, and compare them to that of the bad light. However, when I disconnected the good light, the bad light went out. When I disconnected the bad light, it had no influence on the brightness of the good light. I knew that the lights were wire up in parallel so the good light should have no influence on the bad light. It was as if the good light was behaving as a load in parallel, but the bad light was acting like a load in series.
I then decided to ground the ground terminal on the good light to make sure it was operating normally. However, when I did this, I blew the fuse. This should have been enough for me to work out what the fault was but, at the time, I was still at a loss.
I then decided to check the operation of the switch. I got my father to disassemble the steering wheel cover so we could access the switch. Upon inspection, it appeared that the switch was switching the negatives to the ground, rather than the positives to the light.
I now had the feeling I knew what the problem was. I spent a quarter of an hour drawing up the circuit diagram of the lights. The only way I could make it work, and replicate the fault behavior, was if the ground was actually +12V and the high and low beams were both ground.
This explained why the lights were behaving like they were. With the positive terminal (marked ground) grounded on the bad light, a small amount of electricity was flowing from the good light to the bad. This was because the path to ground was of lower resistance through the filament than through the switch. Of course, as the light brightened, the resistance would increase, sending more through the switch. This is why the lights were also dim.
The solution was simple, I just ran another wire off the +12V from the head light circuit to the “ground” terminal of the bad light.
I had mixed feelings about finding the solution to this problem. On the one hand, I did solve it, on the other, I could have solved it a lot quicker, had I realized what the symptoms meant.
Taro Deneve is a Systems Engineer with experience in Grid Connect Solar Electricity Systems, ATM repair, laser printer repair, power tool repair and an Amateur Radio Operator.