Subject: Re: [Mgs] Alternator? problems

From: "P. Chast" <[email-masked]>
Date: Mon, 26 Aug 2013 23:48:41 -0400

Bad Belts?

On Mon, 26 Aug 2013 23:42:58 -0400, Simon Matthews  
<[email-masked]> wrote:

> 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.
>
> Is there anything other than the alternator that I should be looking at?
> The battery seems to be holding its charge (and I just put in a new  
> battery
> due to a misunderstanding).
>
> The alternator appears to have 5 terminals, but only 3 connected. It  
> looks
> like the picture in this pdf file:
> http://www.mossmotors.com/graphics/products/PDF/130-100.pdf
>
> The Moss replacement describes connecting only 2 terminals (see this  
> note:
> http://www.mossmotors.com/graphics/products/PDF/540-280.pdf) , so I am a
> little confused about what I have. It looks like 2 of the terminals are
> both connected with heavy gauge brown wire, so it would not be  
> unreasonable
> to assume a PO has made a redundant connection.
>
> If it is the alternator, should I just replace the whole thing or is  
> there
> a likelihood that the diode pack or brushes have failed? As I mentioned,  
> it
> does appear that we are getting some current out of it -- unless there is
> some other mechanism that could cause the ignition light to be on?
>
> Simon
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From: "PaulHunt73" <[email-masked]>
Date: Tue, 27 Aug 2013 08:37:16 +0100

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

[email-masked]

---

From: "Hans Duinhoven" <[email-masked]>
Date: Tue, 27 Aug 2013 19:49:47 +0200

I had the same issue some years ago.
It started with very dim battery warning light.
After that the battery was empty when the car was not ran for a week or so.
I got fed up with this and took an amp meter and measured, if there was any
current running from the batteries when the ignition was turned off.
And yes - there was some 10 milli amps running all time.
The cause?
A diode in the alternator was leaky.

As the alternator still was the original one I took it to the local car
electrics shop.
They installed a new diode pack, new brushes and brush ring, which was worn
out as well.
They also can verify, if new bearings are needed as well.
The unit is apart anyway, so replacing these won't cost much extra.

Mine never failed again and reliability always is important for such car
parts I think.

Cheers,
Hans
71 BGT

-----Oorspronkelijk bericht-----
Van: [email-masked] [mailto:[email-masked]] Namens
PaulHunt73
Verzonden: dinsdag 27 augustus 2013 9:37
Aan: Simon Matthews; MGS
Onderwerp: Re: [Mgs] Alternator? problems

PaulH.

----- Original Message ----- 
> (red) light on the dashboard only goes out (well, it goes very dim) 
> when cruising on the freeway.

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---

From: Simon Matthews <[email-masked]>
Date: Sat, 31 Aug 2013 17:02:25 -0700

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 <[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.
>
> 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

[email-masked]

---

From: Barney Gaylord <[email-masked]> mail.com>
Date: Sat, 31 Aug 2013 20:51:59 -0500

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:
>....
>....

> >....

[email-masked]

---

From: Richard Ewald <[email-masked]>
Date: Sat, 31 Aug 2013 18:55:47 -0700

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 <[email-masked]>wrote:

> 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
>  On Tue, Aug 27, 2013 at 12:37 AM, PaulHunt73 <[email-masked]>
>> **wrote:
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