Subject: Re: [Mgs] Alternator, battery, fan, or normal?

From: "PaulHunt73" <[email-masked]>
Date: Fri, 21 Sep 2012 09:03:58 +0100

The light will glow if there is a potential difference between the white 
side of the warning light, which comes from the ignition switch, and the 
brown/yellow side which comes from the alternator indicator terminal.  In a 
good system it shouldn't glow.

It usually indicates the alternator is failing, possibly one or more of the 
diodes, but it will also happen if you have bad connections between the 
alternator output terminal, via the solenoid connection and ignition switch 
to the white circuit.  In both these cases it will glow brighter with 
increasing load.

Measure the voltage between the output (brown) and the indicator 
(brown/yellow) terminals on the alternator - wiring connected, engine 
running, electrical loads switched on, such that you have the glow.  If 
there is a potential difference between those two (other than perhaps a 
couple of tenths of a volt), then the alternator itself is the culprit.  If 
either of them are over 14.7v then again the alternator is the culprit.  If 
they are much the same but there is a potential difference between the 
output terminal and the ignition switch white such that the white is lower 
than the output terminal, then it is a bad connection between the two.

The heater fan and the overdrive come off the white circuit so the problem 
could either be in the brown or the ignition switch.  If the headlamps make 
no difference it will be the ignition switch circuit, if the headlamps make 
a difference then it will be in the brown circuit.  Exactly where it could 
be in either case depends on the year as wiring varies.

PaulH.

----- Original Message ----- 
>I noticed tonight that the red battery light has just a hint of red as I 
>was
> driving home.
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From: Aaron Whiteman <[email-masked]>
Date: Fri, 21 Sep 2012 06:50:07 -0700

The white side, eh?  Can't believe I forgot that.  When I say glow, I mean "a
hint of glow."  I'd never see it in daylight.

diodes, but it will also happen if you have bad connections between the
alternator output terminal, via the solenoid connection and ignition switch to
the white circuit.  In both these cases it will glow brighter with increasing
load.

I didn't write this last night, but it was "brightest" at steady speeds around
60mph with the OD on.  Turning it off made the glow go away, but I did that
not to reduce load, but to increase engine speed.  The glow also appeared to
go away as I slowed coming into town (but again, ambient light could have been
the problem here).

> Measure the voltage between the output (brown) and the indicator
(brown/yellow) terminals on the alternator - wiring connected, engine running,
electrical loads switched on, such that you have the glow.

Voltmeter in voltage mode, one probe on each terminal, right?  (I hate playing
with devices that produce rich chunky unfused amps, so I want to be sure)

> The heater fan and the overdrive come off the white circuit so the problem
could either be in the brown or the ignition switch.  If the headlamps make no
difference it will be the ignition switch circuit, if the headlamps make a
difference then it will be in the brown circuit.  Exactly where it could be in
either case depends on the year as wiring varies.

It's a '75, but the wiring isn't all '75.  The alternator has two large output
terminals instead of 1, so I run a second parallel brown circuit between there
and the solenoid terminal to the battery.  I've spliced in feeds to the
headlights and added relays to reduce the load on the original brown circuit.
I'll make a run out tomorrow night and see what happens.
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From: "PaulHunt73" <[email-masked]>
Date: Fri, 21 Sep 2012 16:04:15 +0100

You can measure each terminal to earth, but it's easier to measure between 
the two at the alternator, maybe less so when comparing the brown at the alt 
with the white at the ignition switch.  Meter on volts, all you have to 
watch out for is that each probe doesn't touch earth as well as the terminal 
it is supposed to be on.  If one is on a terminal, the other can be on an 
earth quite safely.

Both my single-speed heater fans take about 3 amps, which is quite 
surprising given the relatively feeble performance.  The motor out of a 
2-speed heater may take a little more, although the two speeds were obtained 
by inserting a resistor to get the lower speed, rather than any mod to the 
windings themselves, although the two do have different part numbers.   If 
the windings are shorting out or there is any mechanical drag on the fan 
slowing it down it will take more.

OD should take less than an amp, fuel pump will only draw current in short 
occasional pulses - the original SU type anyway.

PaulH.

----- Original Message ----- 
Voltmeter in voltage mode, one probe on each terminal, right?  (I hate 
playing with devices that produce rich chunky unfused amps, so I want to be 
sure)

But the fact that the OD and heater are both on the same white, that's 
interesting to me.  Fuel pump too, IIRC.  If the heater fan is going bad, it 
would draw more current, right?  Could that too be the problem?  What are 
they supposed to draw, because testing *that* is really easy :)= 
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