Subject: Re: [Mgs] Alternator? problems

From: "PaulHunt73" <[email-masked]>
Date: Sun, 1 Sep 2013 11:33:23 +0100

Measuring at the battery won't tell you any more than you already know, but it
won't do any harm.

If turning on the headlights made no difference to the brightness of the
warning light, or the voltages (other than them all dropping equally), then
the implication is that the alternator diodes are OK, also that the
connections between the alternator and the fusebox are OK.  However I've not
come across the test that Richard mentions, so it would be interesting to hear
what that shows.  Also compare the voltage at the alternator brown and the
alternator brown/yellow.  They should be the same, if not it does imply an
alternator problem.

Then compare the alternator brown/yellow with the fusebox white.  If they are
different then that is why the light is glowing, as it shows the potential
difference between the white and the brown/yellow.  The only way the white
should be higher is if the alternator brown/yellow is lower than the
alternator brown, which as mentioned before does indicate an alternator
problem.  If the white is lower than the brown/yellow (and consequently lower
than all the browns you have tested) then compare the brown and the white at
the ignition switch, and at its multi-plug or bullet connections (where
present), both sides.

Somewhere you must have a potential difference between the brown/yellow and
the white, at least one of the above tests should show in what area that is.

PaulH.

  ----- Original Message -----
  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.
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From: Simon Matthews <[email-masked]>
Date: Sun, 1 Sep 2013 12:31:39 -0700

14.0V at the battery with the engine over 2000rpm. Not 14.5

No AC voltage between alternator output and ground.

Simon

On Sat, Aug 31, 2013 at 6:51 PM, Barney Gaylord <[email-masked]>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 <[email-masked]>
>> **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.
>> >....
>>
>
>
>> >> 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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From: "PaulHunt73" <[email-masked]>
Date: Mon, 2 Sep 2013 08:15:36 +0100

OK, now check the voltage differences between the browns, brown/yellow and 
white.

PaulH.

----- Original Message ----- 

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