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MG MGB Technical - Power loss when Lights on

Have a '71B, noticed that when the lights are on, my alternator charge drops from 14 to 12.4. Removed the fused for the side lights and there was no change in level (stayed at 12.4). Suggestions as to where to look?

Bart
bart

Check out the connections/connectors for the lighting circuits. The ones most likely to have corrosion and produce high resistance are those in the cluster of connections for the headlights under hood latch panel up front. They get the constant barrage of dirty, spray, etc and often are pretty badly corroded. Also, because these connections utilize the infamous Lucas barrel connectors to connect bullet type harness terminations, you may find that they have broken (they are only spring brass, after all) and the electrical connections are tenuous at best.

New barrel connectors, cleaned up bullet terminations, and the use of a little vaseline or dielectric grease on both, should go a long ways towards minimizing the voltage drop you see when you turn the lights on and protecting these connections from the slings and arrows of outrageous road trash.
Bob Muenchausen

Bart. Have the alternator tested under load. While Bob is correct that bad connections will result in lower voltage, that loss is seen the the component, not in the system. If you battery connections are clean and good, the starter connection clean and good and the alternator plug fully inserted, a system voltage drop indicates a bad alternator as the most likely suspect. If, however, you are showing 13.5-14.5 volts at the battery clamps, with the car running, but only 12.4 at the lights, the connectors and wires would be the place to start. Les
Les Bengtson

Bart,

The first thing to do is check the voltage at the battery with the engine at a high idle and the lights on. If you have the correct voltage, then your problem is in the wiring. If not, then you probably have an alternator problem.

I had a similar problem in my TR6; when I turned the heater fan on, the voltmeter reading would drop about 2 volts. Knowing good and well that the heater fan didn't draw enough current to drop the alternator voltage, I determined that the problem was at the connector supplying power to the heater. I had added the voltmeter to an ammeter equipped car, and had tied the voltmeter input lead to a point on the heater side of a connection. When the heater was on, it drew enough current through the bad connector feeding the heater that the voltmeter read low. The alternator was just fine, as was every thing else in the car. Just that one connector was dropping the volts to the meter.

Since your MG didn't come with a voltmeter (I don't believe), check the connections from your VM to the power supply.
Dan Masters

This thread was discussed on 22/12/2003

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