Does it glow more brightly with the headlights on? That is an indication of
bad alternator diodes or bad connections.
The first thing you should do is to measure the system voltage on the brown
wire at the fusebox, with respect to earth. Ignition off should normally be
about 12.8v (unless the engine has only just been turned off when it will
probably be higher). Ignition on will drop a couple of tenths. Engine
started and running at about 2k rpm should be about 14.5v. As you start
turning things on it will drop, but with headlights, heater fan, stop lights
etc. all switched on it should still be above 12.8v.
If below that measure the voltage on the output wire of the alternator, a
thick brown, under the same conditions. If that is also below then the
alternator is faulty. If clearly above what you measured on the fusebox
brown then there are one or more bad connections between those two points.
The connections between the two brown wires and the battery cable on the
solenoid stud is a candidate for that, remove the battery earth strap for
safety before tackling these. Also measure the voltage on the brown and
white wires at the ignition switch and its multi-plug connections. Good
voltage on a brown but lower voltage on a white points to the ignition
switch.
The first link shows a late model 3-terminal alternator, although the two
large spades are linked together and are both output spades. The
standard-sized terminal is the warning light. The other two terminals were
never used on MGBs. MGBs *did* have five (69 to 71) and four (68) terminal
alternators with two plugs, and the second link, left-hand image shows the
two-plug, five terminal arrangement that a 71 would have had originally. It
depends on what you have now as to what you have to do, if you have the
three spades in a row, two large and one small like the second link, image
on the right, then the mod has already been done.
PaulH.
----- Original Message -----
> We have a '71 MGB which is having charging problems, I think. The ignition
> (red) light on the dashboard only goes out (well, it goes very dim) when
> cruising on the freeway.
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