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MG MGB Technical - Ignition warning light stays on

It's dim but definately alight. Alternator is new so is the ignition switch. Fanbelt tight. The overdrive circuit is not used otherwise the electrics are standard. The charge is 14 volts. What do you think is going on ?

Help - always appreciated.
Roger
Roger Walker

Despite the alternator being new, I suspect a faulty diode in the rectifier pack.

Herb
H J Adler

I went through my files and found this article I wrote about checking the alternator diodes, with an oscilloscope, if you have access to one.

H J Adler

The warning light displays the potential difference between the IND terminal at the alternator (which is connected to the field circuit internally) and the white (usually) from the ignition switch.

If it's glowing, it could be because the voltage at the IND terminal is more than or less than the voltage in the rest of the system, or that the voltage on the white from the ignition switch is lower than it should be, so they should be your first voltage tests.

If the IND voltage is higher or lower than that on the output terminal it's a faulty alternator. If the glow increases with increasing electrical load then it's probably the diode pack.

If the voltage on the white is lower than that on brown wires at the ignition switch, fusebox or solenoid then bad connections are the cause - inside the switch if the brown on the switch is 'good'.
Paul Hunt

Paul,
I should have written that the ignition switch was my first concern. As the steering lock was playing up, I changed both. The ignition light is indeed brighter when the lights or fan comes on so I guess that Herb got it in one. Neverthless your post is a very useful guide for us all - thanks !
Roger
Roger Walker

Also worth checking wire continuity. I had a similar problem and found the thick wire from the alternator to the starter was dis ( GPO slang for Paul!), I mean broken! I connected a new wire and everything worked properly once more. Not saying this is the fault but worth checking. I only realised the ignition light was on when I went out at night and could then see the very faint glow. Next day I pulled the bulb holder out and could see the bulb glowing dimly.
G Webb

Had the same problem, checked Alt etc., no problems but the starter relay looked a bit dodgy, replaced it, problem solved.
Allan Reeling

"dis ( GPO slang for Paul!)"

Haven't heard that in a long time :o)
Paul Hunt

New alternator but the light stayed on - dimmer than before but just discernable. Very small voltage difference at the fusebox compared to the alternator (with the engine switched off) with obvious implications for the feed from the terminal beside the starter. Closer examination of the terminal revealed enough muck to short it to the bracket. Now cleaned - light out. Something else to add to the service schedule.
Thanks for all your help !
Roger Walker

This thread was discussed between 09/01/2016 and 19/01/2016

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