MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Ignition light/neg ground

A friend has an MGA Deluxe that is just finished a 5 year resto... It is all ready to go. He had an alternator installed (and went to neg ground)... All seems well, except the ignition light is off when the key is turned on, and glows as the rpm increases (exact opposite to what it should)... There is a yellow wire that when taken off (sorry, i wasn't there and just remember the mechanic mentioning it) the light went out, but of course didn't come on at all when it should. It was said that the alternator is working well and charging!
I know there have been a couple mentions of ignition lights doing strange things... thought someone might know what the wiring should be and i could pass it on
gblawson(gordon)

Gordon

May be worth checking his modified wiring against Barney's notes: http://mgaguru.com/mgtech/electric/ac101.htm

Steve
Steve Gyles

Circuit connections required for an alternator conversion are dependent on the type of alternator used. certain Lucas alternator conversions are well documented, but other alternators may require different wiring.

The trick is to find a wiring diagram for some car where the alternator was originally used, and duplicate that hookup when installing the same alternator in the MGA. I did that when installing a Mitsubishi alternator from a Mazda RX7 in my MGA. This one required an additional white wire from ignition switch to alternator for field excitation to produce high output at low engine speed. See here: http://mgaguru.com/mgtech/electric/ac201.htm

With this setup I had a strange issue with the warning lamp glowing after the engine was shut off. I finally disconnected all wiring from the original control box, and the problem went away. I think the problem may have had something to do with the original control box being faulty before the alternator conversion.
Barney Gaylord

Barney

That's interesting. I wired up my alternator as per your notes, retaining the control box purely as convenient terminal posts. 19 times out of 20 my system behaves totally normally. Just that once in a while the ignition light stays on after turning the ignition switch off. After about 5 seconds I then hear a click from the control box and the light goes out. On the very, very rare occasion it stays on and I have to kill the electrics using my hidden isolator switch - then all is okay again. Very puzzling, even though it is wired up exactly as you depict - bypassing the control box solenoid.

Steve
Steve Gyles

Gentlemen,
I also found that, after my alternator conversion on my 1600 roadster, the ignition light would stay on after the key was turned off. Otherwise, every thing checked correctly. I discovered that the contact side of the control box had a residual voltage that held the contact closed after switching the ingnition off. I have a new wiring harness (the wiring in this car is a mess) and didn't feel like tracking the source of the problem. A piece of electrical tape between the contacts and the problem was solved. It's been this way for a few hundred miles. The charging system is working perfectly. I did the exact same conversion on my 1500 coupe with NO problems.

Regards,
GTF
G T Foster

Ummm, for clarification, the new wiring harness is purchased but not yet installed...:)

GTF
G T Foster

This thread was discussed between 26/08/2008 and 27/08/2008

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.