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MG MGB GT V8 Factory Originals Technical - Flashing Ignition light
|I have a factory BV8 which has an ignition light which doesn't come on before starting the engine, but flashes rhythmically when the engine starts. It flashes slowly on tickover but faster on increasing the revs. This stopped happening after the engine was warm, until yesterday when I drove my usual 10 mile trip home at the end of the day and it didn't stop flashing at all! Has anybody any ideas as to the problem?|
It sounds like the diode(s) which control the ignition light. Separate from the charging circuit so the likelyhood is that you're charging OK. You'll find that the intensity of the light will vary according to what's switched on - the more you turn on, the brighter it gets.
|This is the diode bridge in your alternator. Have it fixed asap. After the diode bridge is replaced, please check if battery is loading at right voltage. This should be between 12 and 14 V. The company that repairs your alternator could supply this info also, they have a test machine.|
|Frank de Groot|
|I agree that it is most likely to be the diodes which supply the reference voltage to the ig. light, though I feel that they would all have to go open circuit to prevent the Ig. light coming on before the engine is started. If all 3 diodes are gone the light would never come on - or have I missed something? Also this reference voltage sets the charging voltage of the whole alternator so I would doubt if it is charging correctly. Repair is easy if it is the original AC Delco unit, even being a DIY job if you can get the diode block. The only tricky bit is getting the brushes back in place - until you spot the hole in the back plate for the retainer wire!|
I must admit to little knowledge of the workings of the alternator, but I was always led to believe that the warning lamp circuit was separate to the charging circuit - this was with reference to Lucas units.
I ran a car for a few weeks in the 70's with a pulsating warning lamp without ill effect. Alternators were relatively expensive then, so some black tape over the lamp was an effective stop gap!
|With the ignition on unplug the cables from the alt and ground the thin brown/yellow (NOT the thicker brown). This should light the ignition warning light. If it does then the problem is inside the alt. Whilst three of the diodes in the diode pack (of nine altogether) *are* connected to the warning light these supply the field winding and turn the warning light *off* once charging has started. The light is also connected to the field winding and regulator so failure to come on initially may be more of a problem with these, or field bruishes, than the diode pack.|
|With the older B alternators the warning lamp was separate. The voltage regulator/diode bridge was in a little black case. With the Delco or more modern alternators this is incorporated in the alternator.|
Perform the check Paul describes and you'll know exactly what is wrong. You could also take your alternator to a local car-electrician. Remember it has to be fixed as soon as possible.
|Frank de Groot|
|The diodes were always inside the alternator, it is the voltage regulator that is separate on the 16AC type. On these alts the IND terminal has the brown/yellow wire as normal, which goes to the regulator '+' terminal before going on to the warning light on a brown/black.|
|Thank you gentlemen, I shall have to sort out the alternator when i lay her up for the winter, which will now be sooner than expected.|
This thread was discussed between 19/12/2002 and 22/12/2002
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