[Mgs] Alternator, battery, fan, or normal?

[Aaron Whiteman]

On Sep 21, 2012, at 1:03 AM, PaulHunt73 <paulhunt73 at virginmedia.com> wrote:

> 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.

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.

> 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.

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.