Talk Morgan Forums Community Vehicle Talk (non-Morgan) MGB Alternator Light Issues.....

I did have that function, but lost it when I replaced the seatbelts. I'll try and find the module and disconnect/remove it.

Duh, when I typed John, I should have typed Richard of V6 fame... Sorry Richard.

Like the possible failing diode thinking Dave..

The warning light illuminating indicates a potential difference between ignition switch fed battery volts to field rotor and diode trio output to same. These diodes, separate from the main output pairs, are responsible for providing a half phase rectified DC feed to the field circuit by taking a bleed from the alternator stators raw three phase output, allowing the alternator to be self energising. One diode may have gone open circuit reducing field current but leaving enough for alternator to provide sufficient charge current hence voltage to battery under light loads.

Simplified diagram below (not showing regulator which sits in series with field rotor) does show the diode trio to right of image though.

Is it possible to test the diodes? I don't really want to buy yet another alternator and have exactly the same problem again (the wiper glowing light issue has been the same on both - the sporadic 'full on then off' is a new thing)

Yes just identify them then use multimeter on diode test or continuity, testing each of the three in turn. Each should measure the same but you may need to isolate whilst testing. Note I've added an image above which shows how they are wired.

As I type I've thought of another possibility. Poor connection between ignition switch and warning light. Here the lower than normal volts from battery put a reverse current through the warning light bulb. You could jury rig a feed to warning light bulb direct from battery to prove.

Probably not relevant to your problem but for completeness the bulb glowing brighter with engine revs issue (mentioned above) is usually down to poor battery connection. Here we have lower than normal volts to warning light from ignition switch but higher volts from the diode trio so again current flows in reverse through warning light, increasing as alternator puts out greater volts with increased revs.

Thank you!

Please excuse my basic understanding - to test the continuity of the three diodes I should connect one lead of the multimeter to one of the three spade connectors on the alternator - where do I connect the other?

I think the connection is the more likely suspect and imagine the feed goes through the dodgy fusebox. Will have a go at the weekend.

Engine revs have no effect on the light it's either the dim glow when the wiper is working, or full on.

Sorry TBM but can you confirm you have a normal self contained alternator and not the very old Lucas ACR system which has alternator plus three external components, a control unit, dedicated relay and warning light device - as fitted to Mogs 69 - 73 I believe. If the former then some disassembly is required to locate and identify the diode trio. All the diodes (nine in total) typically sit on an insulated board behind the back cover.

It is the self contained 18ACR style.

Been very weird on the way home. Light was on entire journey - about 30% illuminated on start up. Sidelights on, and it went to about 60%. When I gave the windscreen a wash and wipe, went to about 80%. However, when I got to about 3000rpm, it started pulsating/flashing between 60% and 30%. and then would stop when the revs dropped.

Charging was consistent throughout journey, and didn't fluctuate when the lamp was flashing!

Just a thought looking at Richards diagram…if you have difficulty testing the three diodes, why not disconnect the hot wire from the diode triple point to the ignition light and pop a suitable diode in series with it to see if the light goes out when reconnected in that state.

Check main earth to chassis, braided cables can get clogged with crap which affects their efficiency.