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Electrical Problems on '74 'B

To:, mgs@Autox.Team.Net
Subject: Electrical Problems on '74 'B
Date: Sat, 8 Aug 1998 15:01:16 EDT
Thu, 6 Aug 1998 18:26:40 -0400 "Mike Lishego" <>wrote:

But the big problem is this - when I shut off the key, my ignition warning
light stays on, no matter what I do.  Anyone have a good idea of what I can
do to fix this little problem?  I'd rather not disconnect my battery every
time I shut off the car, but...

Any leads would be appreciated.


First, check out the alternator article on the VTR web site for general info
on the workings of the warning light. The URL is:

Basically, one side of the light is connected to a set of diodes in the
alternator, which are hot only when the alternator is running, and the other
side is connected to the battery through the key when the key is on. 

When the key is on and the alternator is not running, voltage is applied to
the ignition side of the lamp, and the other side is grounded internally to
the alternator.

When the key is on and the alternator is running, 12 volts is applied to both
sides of the bulb, and it will not be lit.

When the key is off and the alternator is not running, no voltage should be
applied to either side of the lamp.

Since the lamp is lit with the key off and the alternator not running, you
must have a short to a 12 volt source somewhere in the lamp circuit. 

If the short were on the ignition key side of the lamp, all the other loads
fed from the key would also have 12 volts on them, such as the wipers, guages,
etc. Since these other loads are not on, the short is not on the key side.

If the short were on the alternator side, the other side of the lamp would
find a path to ground through all the other loads mentioned above, and the
lamp would light.

It appears, then, that you have a short to 12 volts on the alternator side of
the lamp. To see if the short is internal or external to the alternator,
remove the plug from the alternator. If the light stays lit, the short is
external. If the light goes out, the short is internal. 

If the short is internal, it most likely is one or more of the diodes.

My guess is this is your problem.

Dan Masters,
Alcoa, TN

'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74

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