First confirm the ignition warning light circuit continuity. Remove the pug
from the alternator, and with the ignition on, ground the brown/yellow ...
NOT the brown! This should make the light glow. If not check for 12v on
the white at the bulb holder with the ignition on, and with the brown/yellow
connected to the brown check for 12v on the brown/yellow at the bulb holder
(careful, the brown is unfused so don't short it to ground. By now this
should have revealed an open-circuit white or brown/yellow, a bad bulb, or
possibly a bad bulb holder.
If the light glows when grounding the brown/yellow in the first test but
doesn't when plugged back in to the alt, then either the brown/yellow is
being connected to the wrong spade (should be the normal sized one, with two
large ones for output) or the alt is faulty, but that would make three!
But if you see 12v on the brown/yellow with the ignition on and the engine
stopped that pretty-well proves it to the alternator or its connections as
the 12v you are seeing is coming from the white at the ignition switch
through the bulb.
Driving along and the engine dies and the tach suddenly drops to zero means
that the LT circuit has failed. Normally I would ask if the warning light
came on as well or not, but this seems to be iffy anyway. The coil is fed
from the relay on a 79, and the warning light from the white which operates
the relay, so with problems with both (although you could well have more
than one problem) I'd check the brown supply to the ignition switch and the
white from it, particularly where the ignition switch loom plugs into the
main loom by the steering column. Make sure the plug is pushed fully home
and none of the pins are partially pushed out the back.
To check for a drain remove the battery ground cable and connect an analogue
voltmeter on its 12v scale in its place. With an alternator you will
probably see a few volts registered, if this drops to zero when the alt is
unplugged you do not have a drain. If you see 12v registered you have a
drain, first thing to do is always unplug the alt. If it drops to zero the
alt is faulty, possibly short-circuit diodes. If not remove the purple
fuse, hazard flasher in-line fuse (brown both sides), then you will have to
start disconnecting browns from things like the ignition switch, fusebox,
starter relay, ignition relay, and finally the solenoid until it goes.
PaulH.
----- Original Message -----
From: "MonteMorris" <mmorris@nemr.net>
To: "MG list" <mgs@autox.team.net>
Sent: Tuesday, April 06, 2004 3:33 AM
Subject: battery drain
> Irritating history for last two weeks:
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