Subject: Re: [Mgs] DPO butchering of Alternator replacement?

From: Simon Matthews <[email-masked]>
Date: Sat, 25 Oct 2014 16:33:14 -0700

A little more on this. I see 3 brown/yellow wires:
1. Connecting to the small pin (of the group of 3) on the alternator.
2. Attached to the old 3-pin connector (which is hanging loose, so not
connected to anything)
3. Hanging loose.

I assume that the two brown yellow wires that are not connected to
anything are the loop described in the conversion instructions.
Furthermore, after consulting the wiring diagram, I assume that the
medium brown wire is the one that connects to the solenoid (along with
the thick brown wire). So, perhaps this could be dangerous: if the
thick brown wire became broken or disconnected, all the current out of
the alternator would flow through a wire which is not capable of
carrying the current without overheating.

Simon

On Sat, Oct 25, 2014 at 4:03 PM, Simon Matthews
<[email-masked]> wrote:
> I have a '71 MGB-GT on which I am replacing the alternator.
>
> According to Moss Motors, the replacement only uses 2 pins of the
> 3-pin connector (the instructions  are badly written, but this is what
> I understand from them).
>
> Looking at the existing installation, I see an alternator that has the
> new 3-pin setup. The wiring on this 3-pin setup has a thick brown wire
> (presumably the Positive wire) connected to one terminal, a medium
> brown wire (presumably the "A" wire) connected to another terminal and
> a thin brown/yellow wire (presumably the Indicator wire: "D")
> connected to the smallest terminal on the Alternator.
>
> Also hanging from the wiring harness is the old 3-pin connector, which
> has one wire still attached -- a thin brown/yellow wire.
>
> Was there ever an alternator in which the connection of these 3 wires
> would have been necessary? Is the connection of the medium brown wire
> just redundant, or could it be harmful?
>
> Any suggestions?
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From: Charles Hill <[email-masked]>
Date: Sat, 25 Oct 2014 19:17:02 -0500

Simon,
Did you get my original message?  It went to just you.  I can resend it 
if not.  It should answer most of your questions.

Charles Hill

  On 10/25/2014 6:33 PM, Simon Matthews wrote:
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From: "PaulHunt73" <[email-masked]>
Date: Sun, 26 Oct 2014 15:22:15 -0000

The first internally regulated alternators (16ACR 69-71) used two plugs to 
connect to four terminals - the main charging terminal, a battery voltage 
sensing terminal used to control the regulator - both brown that went to the 
battery cable on the solenoid, and two terminals that the ignition warning 
light - brown/yellow, was connected to.  This last went to one terminal in 
each plug and I suspect was a way of protecting the alternator should one of 
the plugs fall out.

On the face of it both brown wires would be at the same voltage as they went 
to the same place, so in theory voltage regulation could be done by a simple 
internal connection inside the alternator.  But under high current 
conditions there will always be a volt-drop in the main charging wire from 
the alternator to the solenoid, which will result in a lower voltage at the 
solenoid than is being output by the alternator.  Battery sensing uses the 
voltage at the solenoid to control the regulator, which ensures that the 
required voltage exists at the solenoid at least, even though that resulted 
in a higher voltage at the alternator output terminal.

The next alternators (also 16ACR 1972) used a three-pin plug with one large 
spade for the charging wire which went to the solenoid, a standard-sized 
terminal for the brown/yellow indicator wire, and a third terminal of a 
different shape or size for the battery sensing wire which also went to the 
solenoid.

Subsequently this alternator was modified to have the regulator sensing wire 
connected internally to the output wire, and these are machine sensing 
instead of battery sensing.  The same 3-pin plug was used, I don't know what 
the third terminal was like.

The next variant was the 17ACR (73-76) which used the same three pin plug 
but seems to have reverted to battery sensing again, possibly because of low 
voltage at the solenoid.

The final variant was the 18ACR which reverted yet again to machine sensing, 
using the same 3-pin plug but now had two large output terminals plus the 
indicator terminal, and two thick brown output cables from the alternator to 
the solenoid, and up to a splitter by the firewall.  Two output cables 
halved the volt-drop under high current conditions.

73-76 i.e. battery sensing wiring is directly compatible with the later 
machine sensing alternators with the two large output terminals, the thinner 
brown wire just acts as an additional output wire, which reduces volt-drop a 
little.

The same can be said of the earlier four-pin, two plug alternators if at 
least the thick brown and the correct brown/yellow are connected to the 
plug.  The thinner brown can be connected to the spare output terminal for 
additional current carrying capacity, if not it must be carefully insulated 
and not allowed to come into contact with anything, unless it is detached 
from the solenoid and securely taped back.  The brown/yellow that comes from 
the warning light should be identified and the remaining two taped back out 
of the way.  If the two brown/yellows are left connected together then that 
should be safely insulated and the single brown/yellow used for the plug, or 
vive-versa.

PaulH.

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