Alternator not charging/ignition light not on

[GeoffK]

Hi
I have read most of the threads on the above subject going back to 2017 and I think that the comments by Ian F on 04/12/18 are close to the solution where he says that the ignition light should be the alternator light. My problem is that the ignition light stays off when I turn the key to the first position when it should be on When I try to start the engine the light flickers as the engine starts and then goes off when the engine is running.
The connection to the alternator seems okay but the brown/yellow wire at sometime has broken and been soldered together but seems quite solid. I have cleaned the mucky connection block and the alternator connectors.
The fact that the ignition light flickers indicates that there is a connection and that the bulb is okay. Fan belt could be tighter.
Any thoughts?
Geoff

[Bumpa]

Have you got a multimeter? I consider one essential and they are really cheap these days. Have a read of Paul Hunt's article on the warning light here http://mgb-stuff.org.uk/alternator.htm#theory

Scroll down to find this section "Ignition warning light doesn't glow when it should:"

[Vic Butler]

Simple test to check charge is to connect a multimeter across the battery terminals, set to volts and run the engine at 2,000 rpm. The meter reading should be between 13.5/14 volts.
Also check the connections at the alternator plug and the ignition light. Check for continuity between the ignition light terminal on the alternator plug and the ignition light .
The ignition light being on excites the alternator and starts it charging. When the voltage is equal both sides of the ignition light it goes out to prove the alternator is charging.

[Michael Barclay]

Hi Geoff
From what you have described i think the basic Alternator charging circuits are working. /How many miles has the alternator done? Have a look at the Brushes.They may need replacing as the outer one wears a lot quicker than the inner.You could also try swapping them over but as New brushes are cheap its worth replacing them. Its an easy job that can be done withe alternator in situ, just remove the large black plastic cover.
Mike Barclay

[GeoffK]

Progress so far. Waiting for mate to bring his multimeter so no continuity checks done so far. Noted list of checks to do with multimeter arrives but have noted that they are checks only and no actions.
Brown/yellow cable to alternator seems ok
White and brown/yellow cables to ignition light seems okay.
Alternator terminals and block cleaned. Cables tight.
Warning bulb tested outside the car and is ok
Phase 1 Warning bulb does not light up when ignition is turned on.
Phase 2 Bulb lights up strongly when engine is being turned over.
Phase 3 Bulb goes out when engine running.
Still confused about what could be happening during Phase 1 and 2.
Happy with Phase 3 if I get 13.5 - 14.0 volts at 2000 rpm when multimeter arrives.
Cheers Geoff

[Vic Butler]

Firstly what year is the car?
It's possible that the feed to the ignition light has been connected to the start side of the ignition switch instead of the first position.

[Not_Anumber]

I would recommend buying a multimeter rather than relying on borrowing one. They are not at all expensive and there are loads on Ebay.
You will only need a very basic one for most car uses as 99% of the time all you will need is a low DC voltage range to check the presence of battery voltage and a basic resistance range to check continuity.

[GeoffK]

Referring to Vic’s comments. Firstly the car is a 1973 GT (chrome bumper).
I have been doing a lot of thinking about this problem and I came to the conclusion that power was not coming through the starter switch when it was in the first position therefore no light. It was when the starter was engaged moving to position 2, which maybe correctly wired, causing the light to come on. This would explain why the light was not on in the first position but comes on in the second. In both instances there would be no voltage on the brown/yellow coming from the alternator. But when the ignition switch goes back to the first position when the engine is running this would suggest that there was no voltage from the alternator, but also no voltage from the ignition switch, therefore the light would not illuminate and there could possibly a lack of output from the alternator.
So if I changed the alternator the warning light would always be on when the engine was running unless I found the potential fault in the starter switch or its wiring.
Multimeter coming tomorrow.
Cheers Geoff
Ps Very difficult looking up at the starter switch and trying to identify all the wires when wearing variofocals!

[Vic Butler]

The warning light will go out when voltage either side is 12v. I don't have my earlier workshop manual as it's out on loan otherwise I could see to which terminal on the ignition switch the warning light is connected.
Is there an auxiliary position on the switch ie when it's turned to the first position the ignition isn't on but accessories like radios are turned on without the ignition being on.

[Bumpa]

I don't know if this will help. It is the back of the switch on my 1971 B.