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MG MGB Technical - Bizarre Electrical Problem Solved

A couple of weeks ago I posted a message about a bizarre electrical problem where my headlights would go off when I stepped on the clutch. I finally solved this and thought I would share the cause – but first, the path to the discovery.

I traced the circuit from the battery to the switch (good) the dipper switch (good) and the wires going out to the headlamps (all good). I then installed a relay to work the headlamps taking a direct feed from the starter solenoid and having the existing circuit work the relay. Imagine my disappointment when I got the relay installed, turned on the headlights, stepped on the clutch, and out went the headlights! At this point I realized my problem was further downstream. I could hear the relay click and could measure 12 volts coming out of the relay but the headlights wouldn’t come on. I checked all the bullet connectors at the front of the car and noted that all the black wires went into one connector that the harness then brought elsewhere for grounding. I decided to add a ground right in front of the car and the problem was immediately solved. I then traced the black wires in the harness and found that they were grounded near the wiper motor above the pedals. The large bolt that held this main grounding point was loose so it appears that when I stepped on the clutch it flexed the surrounding metal ever so slightly but sufficiently to cause the ground to be lost. So it was a bad ground after all. If I had started to trace the circuit from the ground rather than the battery I would have found it sooner.
Boris

Boris,

Really, truly, these little jaunts on the trouble-shooting path IS one of the joys of ownership. Having done a total restoration,(the rear end was the only item not taken apart), a certian "mind meld" has taken place.

I find most problems are rather the simple sort. And the old wive's tale of a stich in time saves nine.

Glad to hear of your accomplishment.
glg gimbut

Boris,

I agree having rebuilt my car I now spend more time trying to get rid of the new paint to get a good earth!
T J Nicolson

I have to fix electrical repairs all the time. It is almost ALWAYS the case that after you figure out what the problem was you question yourself about your ability to find it earlier. Jobs that took me hours to sort out end up that if I looked in the right place and tested the right cmoponent it should have taken minutes to fix..MGB or other 4 wheels machines..it is all the same
Eric Willis

This thread was discussed on 19/03/2003

MG MGB Technical index

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