All that follows assumes that the wiring is standard and hasn't been mucked
about with.
The only way the warning light can, well, light if it has a voltage across
it. Under normal circumstances this is if there is 12v on the white from
the ignition switch and a ground from the alternator - i,e, the ignition is
switched on but the engine is either not running, or not running fast enough
to have started the alternator charging. The other circumstance is when the
alternator is putting out 12v and the ignition is off so there is a
near-ground on the white coming from the coil, fuel pump, things like that.
This usually happens when then ignition is switched off but before the
engine has stopped spinning. This circumstance can also happen if one of
the diodes in the alternator is short and is feeding battery voltage out on
the warning light wire (brown/yellow). The light goes out when the engine
is running because it has 14v or so on both sides of it, and is out when the
ignition is off and the engine is stopped because it has a near ground on
both sides of it. If the alternator is unplugged it will stop the light
lighting under any circumstances.
The conclusive test is to measure the voltage on each of the white and the
brown/yellow with respect to ground, with the ignition off and the
alternator plugged in. If there is 12v or so on the brown/yellow and a near
ground on the white when your light is lit then your auto shop do not know
what they are talking about. If however the 12v is on the white and the
near ground is on the brown/yellow, then your ignition switch, or wiring, is
suspect. This would normally prevent you from turning off the engine, but
if you have the full emissions plumbing and anti-runon valve it could
explain why you can turn off the engine even though the ignition is still
switched on.
PaulH.
----- Original Message -----
From: Tom Bott <tcbump@leading.net>
To: MG NET <mgs@autox.team.net>
Sent: Tuesday, July 24, 2001 12:50 AM
Subject: Alternator OK, any suggestions?
> Thank you to those who responded to my problem with the 79 mgb ignition
dash
> light being on even with the keys out of the ignition. Consensus was the
> alternator so I took it to a auto electric shop where it was tested and
> scoped. I'm told it is ok. Any suggestions where to start looking next?
///
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