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MG MGB Technical - Alternator query

Will the alternator still charge if the indicator wire is disconnected? I assume the same as running with a blown indicator bulb.
Mike
Mike Ellsmore

Hi Mike,

The lamp provides a field coil current to start the field going. This is then taken over by the alternator producing voltage at higher speed. If the lamp has blown or this feed is removed the alternator will do nothing until about 3,000 RPM and then should work as normal.

Hope this helps.

Andrew

I had to work all this out when my alternator regulator failed and I fried my battery !!! It was inputting 40 - 50 volts on the motorway for 2 hours ! Acidy mess resulted.

Andrew Walton

Thanks Andrew you have solved a puzzle for me!

I have had intermittent trouble starting my MGA fitted with 5 brg engine and alternator. Unbeknown to me the indicator wire from the alternator became disconnected during one of my sessions servicing the engine (very difficult to see as the engine has a crossflow head with twin 45 Webers). The battery was quite old (9 years) so on its last legs - I had to jump start it the other day when away from home and could not work out why my indicator lights wouldn't work initially thinking the alternator should provide power even if the battery was cactus - after gunning it up a long hill the indicator lights suddenly started working (going over the 3000 rpm you mentioned so alternator began charging)!

I have now reconnected the alternator signal light connection and replaced the battery.
Mike
Mike Ellsmore

Sometimes the warning bulb isn't enough of a resistance to kick start the alternator and revving the engine is the only way to get it to start charging. I imagine this would be the case if you used an led bulb in the warning light. On my Lotus Elan I have recently fitted a new lightweight alternator and that has a third wire connected to an ignition key controlled source whose sole job is to give the alternator that initial kick.
Mike Howlett

" isn't enough of a resistance to kick start the alternator "

Probably too much of a resistance, rather than not enough, which is why LEDs with an additional series resistance may not prime the alternator correctly. If you must use an LED for the ignition warning light then you could try a suitably low resistance in parallel with it. Funnily enough some schematics do show this parallel resistance even when standard incandescent lamps are used, but I've never know of one in practice. As for what value resistance, you could always try a standard incandescent bulb ...
Paul Hunt

When lucas alternators were first developed they never catered for a charge warning lamp as most cars so equipped came with ammeters.


The 4.2 etype was fitted with an 11Ac alternator when it was introduced with no charge warning lamp at first. But they carried on using the old 3.8 speedo with the ignition warning lamp jewel still installed!


After some complained that the charge warning lamp didn't work they fitted an oil pressure switch to the main oil gallery wired to work the charge warning lamp as as stop gap! It was described on the factory workshop wiring diagram as the "Alternator charge pressure warning light switch"



Eventually Lucas came up with the 3AW warning lamp control, which looked like a flasher unit.


Lots of cars with printed circuits on the rear of the instruments had a parallel resistor as Paul describes so the system worked if the bulb blew.






SR Smith 1

I believe incandescent bulb filaments are around 3-5 ohms.

A 4.7ohm resistor in parallel to the bulb would probably work when your bulb is blown.
Steven Rechter

This thread was discussed between 24/08/2014 and 25/08/2014

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