An MGB, I seem to remember. Side-marker lamps only have two wires - 12v
supply and earth. Originally they came on with the headlamps and there is
an additional blue wire in the harness by the front headlights, but during
1972 this changed to come on with the parking lights from a red wire in the
harness by the headlights. The earth is wired to black wires by the
headlights, which goes back to a body earth by the starter relay. However
that earth also supplies the headlights, so if they are working it's not the
body earth that is the problem. Nor any of the connections by the
right-hand headlight, but it could be the one bullet on the black wire from
the side-marker by the left-hand headlight. Locate the red and black wires
from that side-marker, and test for voltage with a meter inside the
bullets - don't dismantle them. If you have no voltage on the red the
problem could be either side of the red bullet by the left-hand headlight,
or one bullet by the right-hand headlight. If you have voltage on the black
then the earth at that bullet is the problem. If you have 12v on the red
and nothing on the black then the problem lies between those two points i.e.
either inside the light or the wires leading to it.
The only light unit on an MGB that has three wires including a black earth
wire is the headlight unit. All MGB rear light clusters have three wires
but none are black, the light unit earths through its physical mounting.
Chrome bumper front parking lights only have two wires and also earth
through their physical mountings. Rubber bumper front indicators do have a
black earth wire but only two wires as they only have one single-filament
bulb.
PaulH.
----- Original Message -----
> The problem is that the left side front sidemarker light still does not
> light
> up. It could be a bad bulb, but I want to clean its ground connection.
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