Firstly if the warning light stayed on when the ignition was off (confirm by
there being no 12v on the white or green at the fusebox) then either the
alternator diodes are short-circuit or more likely in your case the
brown/yellow indicator wire *was* shorted to the brown from the alt to the
solenoid. When you have burned wiring due to a short it is imperative you
find the cause before reconnecting things, the alternator could have been a
symptom and not a cause, and replacing the alt without finding the original
cause could cause the new one to fail too. In the event of heat damage to the
brown to the alt you must cut the loom all the way back to the solenoid, the
damage inside the loom is likely to be worse than outside, and affect *any*
wire or wires that run beside it, not just the brown/yellow.
If you found a diode in the brown/yellow then that is just a POs cheap frig to
get round problems in the emissions plumbing or anti-runon valve circuit if
still fitted, or if all that has been removed. There was a wiring design
error in North American cars with the anti-runon valve and ignition (not
starter) relay which meant that the ignition remained live even when the key
had been turned off - if the alt is charging. However the anti-runon system
is so effective at stopping the engine instantly by cutting off the fuel
supply that no one noticed - until a problem developed with the emission
plumbing or valve, when the engine continues to run normally (not Dieseling)
for some time after the key is turned off. The correct way round this is to
disconnect from the ignition switch the white that comes from the ignition
warning light and connect it to the white/brown from the ignition relay
instead. The factory did this on UK cars with an ignition relay as those
never had the emissions plumbing or anti-runon valve so the problem was
discovered immediately in the factory. As well as the above mod they also
moved the coil feed from the ignition relay white/brown to the ignition switch
white, which was a bit belt and braces as well as going against part of the
reason for providing an ignition relay in the first place, which was to cut
down the current through the ignition switch contacts.
With the ignition off there should be a virtual ground both sides of the
warning light i.e. on the white from the ignition and the brown/yellow from
the alt. When you first turn on the ignition there is now 12v on the white
but still a ground on the brown/yellow, so the warning lamp lights. When the
engine is started and the alt is charging there is now about 14v on both the
white and the brown/yellow, so again the light goes out. Finally when the
ignition is turned off but while the engine is still spinning there is now
effectively a ground on the white but still 14v coming from the alt, so the
light glows again but flickers and dies as the engine runs down.
So if your warning light is still glowing *without* the diode being present,
you still have a fault. If it still glows when the alt is unplugged you still
have shorted wiring, if it goes out when unplugged the alt has one or more
shorted diodes.
PaulH.
----- Original Message -----
From: Aeseeyou@aol.com
To: mg-mgb@yahoogroups.com
Cc: mgs@autox.team.net
Sent: Saturday, October 25, 2003 5:11 AM
Subject: [MG-MGB] Is this normal when replacing an alternator?
This week I replaced the alternator on my 1978 MGB roadster.
Biiiig snip.
|