Simon,
As long as you have your meter out, with the engine running at 2000 and
the lights on, measure *AC* voltage between the alternator output and
ground. Yes I said AC.
If you see more than about .4V AC you have a problem with shorted diode(s)
Rick
On Sat, Aug 31, 2013 at 6:51 PM, Barney Gaylord <barneymg@mgaguru.com>wrote:
> Simon,
>
> Do what youi should have done in the first place. Measure voltage at
> battery terminals with headlights on and engine running. It should be ar
> least 13.6 volts at idle, and something over 14.5 volts at 2000 rpm engine
> speed. Net time required, about 10 minutes.
>
> If not sufficient voltage, then remove the alternator and take it to the
> nearest auto parts store to have it tested. If it doesn't work well, get a
> replacement in the same trip. Go home and put in on the car. Net time
> required, about an hour.
>
> If it was a bad alternator, the problem is fixed that easy. If it was a
> good alternator, it didn't cost anything to figure out you were barking up
> the wrong tree, and you can get on with cleaning wire connections (like
> five days ago).
>
> Barney Gaylord
> 1958 MGA with an attitude
> http://MGAguru.com
>
>
> At 05:02 PM 8/31/2013 -0700, Simon Matthews wrote:
>
>> ....
>>
>> 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.
>> ....
>>
>
>
> On Tue, Aug 27, 2013 at 12:37 AM, PaulHunt73 <paulhunt73@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.
>> >....
>>
>
>
> > ----- 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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