Ignition light trouble

[mattyguk1]

Hi all, I’m having an issue with the red ignition light on my 1970 saloon and was hoping somebody could shed some light on it ! For no apparent reason I can see, the light isn’t going out when the revs pick up ? I’ve checked behind clock and there’s no loose wires or anything unusual. Any ideas?

[myoldjalopy]

The red light not going out suggests the battery is not being charged (properly). It could be as simple as a loose fan belt, or it could be a problem in the dynamo or the voltage regulator - or if it has an alternator, a fault in the alternator itself. Or there is a fault in the wiring somewhere. First question would be does it have a dynamo or alternator?

[mattyguk1]

Thanks, it’s still using a dynamo. Battery seems fine but I’ll check fan belt tomorrow. Is there anyway to check for a wiring problem?

[oliver90owner]

There is unlikely a wiring fault, per se. Far more likely an adjustment or repair, assuming you have not had the dynamo disconnected for any reason, changed the battery polarity (or perhaps a couple other rare instances).

Checking battery voltage/dynamo output is a much more useful check. That the ignition light is operating will mean there is continuity through the dynamo field windings - unless shorted to ground.

[mattyguk1]

Thanks, I’ll search for best way to check dynamo output and report back

[simmitc]

To check actual dynamo output you will need either a voltmeter or a multimeter set to DC volts. The latter are cheap and readily available. However, a quick test is to start the engine and switch the headlamps on, then rev the engine. Do the headlamps get brighter? Yes = good output, no = bad or no output. Ultimately you will still need to know the actual, output, but the brightness test is a good start.

There could be a connection where the wire has fallen off, a loose fan belt (although I would expect you to hear a shrieking sound as the belt slips), a failed dynamo or a failed voltage regulator.

[myoldjalopy]

Agree with 'oliver90owner' that a wiring fault is not the prime suspect in a case like this. However it is possible, the wiring to the connectors which attach to the dynamo can break/come apart.
If you don't have a multimeter, another rough test for the dynamo in addition to Simon's is to see if the dynamo will run as a motor. Simply slacken off the dynamo bolts so you can remove the fan belt, then open up the control box and press down on the right hand bobbin, as you face the car, to close the contact points. Do not touch the bobbin on the left. If the dynamo is any good, it will spin briskly and smoothly. Make sure the contact points come part again after this test, they should do. If the dynamo does run briskly as a motor, and all connections at the dynamo are good, I'd suspect the control box.

[geoberni]

Chances are it's the fan belt.
But if it's not that, you can carry out the tests in the Lucas Booklet.

Item 15 on this link is Lucas Generator and Control Box tests... "intended for the small service station owner and mechanic..."
https://britishcarbreakfastclub.com/technical-library/

Only Parts 1 and 2 are relevant; parts 3 and 4 are not for the parts fitted to a Minor (they are for 3 Bobbin Control Units).

[mattyguk1]

Thanks all, just been out to check wether headlights are getting brighter with revs and I can confirm they are not! So I’ll do some more checks tomorrow with the multi meter and report back

[mattyguk1]

I’ve taken fan belt off and removed control box cover, on pushing the point’s together the motor runs at times and not at others? On the times it doesn’t run the points appear to stick together? Any ideas on next move would be appreciated

[myoldjalopy]

I'm thinking the dynamo is faulty - it should run briskly and steadily. Maybe the brushes are worn. You could pull it and part-dismantle it to have a look at the brushes. I don't suppose you have a spare dynamo to hand? The only other way is to double check the dynamo with a multi meter as per the tests in the links suggested above. The fact that the lights aren't brightening when you rev the engine shows the battery is definately not being charged.

[mattyguk1]

Just thought I’d report back as I’ve finally been able to sort the problem out, it was indeed a faulty dynamo which I’ve now replaced with an alternator and all is well. Now I’ve finally fitted the alternator I can get around to wiring in the heated rear screen I fitted some time ago in preparation for the intended alternator conversion. Thanks all