[Mgs] Alternator? problems

[Simon Matthews]

Paul,

thanks for your suggestions.

At the fusebox and at the alternator, I measured about 14.2V with the
engine running and revved up. With headlights on, this drops to about
13.8V. So, clearly, the alternator is working and the connection to the
fusebox is not too bad (I measured 0.2 ohms between the alternator and the
fusebox, which did seem rather high).  Turning on the headlights on had no
effect on the brightness of the ignition light on the dashboard.

What should I look at next?

Simon


On Tue, Aug 27, 2013 at 12:37 AM, PaulHunt73 <paulhunt73 at virginmedia.com>wrote:

> 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.