MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Headlamp

Hi Folks:

A fellow MG Car Club-Long Island Center member has a problem with one halogen headlamp that is only glowing on a Mark II MGB. Checked the lamp grounds and still has same problem. Any suggestions from past experience?

Thanks: Rich Boris 67 B roadster
Rich Boris

Both high and low beam wires connect to bullet connectors in the front of the car. Trace the wires from the headlight to the bullet connector and take apart and clean. Mine are under the slam panel in the middle of the front. Bad connection there could effect only one light or both.
Jim Lema

If both main beam and dipped beam are affected, in fact there seems little or no difference between the two, then the ground is bad. If only one beam is affected it will be the feed to that beam that is bad. In both cases it could either be in the connectors that Jim describes (although in all the cars I have worked on the connectors are at the ends of the slam panel and not in the middle) or in the bulb holder. In the case of the bad ground it could be a break in the circuit anywhere between the casing of the bulb, the holder, or the ground wire in the pig-tail for that headlamp. I have had a case where the insulation for one of the wires cracked in this pig-tail and the copper conductors eventually corroded through and broke the circuit. That was quite tricky to locate.
Paul Hunt

Reading grounds with a standard VOM will show a good connection, even if only one strand is still in existence. Best way to check grounds is to use a jumper wire that completely bypasses the suspect connection. Another thing that can happen is that if the ground is completely open the current can flow back through the other filament, back through the other bulb and to it's ground. You'd never notice the other filament glowing slightly, and the two filaments in the dim bulb would both be slightly hot.
R. L Carleen

If is is a single-strand ground that is causing a dim headlamp then although you will see a full 12v on the supply side of the lamp you will *not* see 0v on the ground side as you should but something above it. This is as long as the bulb is plugged in and glowing, which is what you must do if voltage readings are to mean anything. If you take voltage readings with the load disconnected then, unless there is a complete open-circuit, with most voltmeters you will always see 12v on the supply side and 0v on the ground side no matter how bad the connection is.

If the ground connection is open circuit then both filamants will be in series, at half the voltage each, and hence both glowing dimly. Both filaments on half voltage gives nowhere near the brightness of a single filament with a full 12v. Both filaments glow in exactly the same way regardless of whether main or dipped beam is selected, and that is one of the diagnostic indications of an open-circuit ground - a dim glow and no change between main and dip.
Paul Hunt

This thread was discussed between 01/12/2004 and 04/12/2004

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.