Alternator not charging/ignition light not on

[Charles Farran]

Hi Jon,
The OE ACR alternators(certainly the later years)fitted to the MGB have an internal voltage regulator.
The only separate voltage regulator is used for the fuel & electric temp gauges which is sited behind the instrument panel near the steering column.
I assume the engine is running & the ignition light stays on, even though you have changed the alternator.
Have you replaced the alternator/water pump belt and/or checked whether it is slipping?
Have you used the same size pulley on the alternator as using a different size will create charging issues?
Have you checked that you haven't got any insulation issues with wiring ?
Many years ago I had a marginal charging issue when the car was being used all the year round & when it was wet, a damaged spade contact on the ignition relay on the inside wing was arcing to the chassis & hence causing a current drain - luckily I spotted it before any real damage was done, but it made the ignition light glow!
Cheers
Charles

[JontheHutt]

Hi Charles,

Yes, the engine is running and the light staying on.

I haven't changed the belt or checked if it for a slip. I think from when I put the new alternator on the pulley may well have been ever so slightly larger but that's only going on feeling by hand as it was slightly harder to put on. I used a modern upgrade and not an OEM part.
As for wiring it could well have some insulation issues, as I alluded to in my first post, even to my laymen eyes the wiring looks like a rats nest of a mess. (Electrics really aren't my strong point )
I'm planning to go and try some of the suggestions tomorrow so fingers crossed and I let you know

[Charles Farran]

Hi Jon,

If you are still using the alternator/water pump belt that was on the car when the ignition light came on & this continued with the replacement alternator when the engine was running, i would strongly recommend it is replaced with a new one as they do slip & are difficult to spot.

In theory,as i understand it, if you replace an alternator with the same charging output per rpm as the previous one , but with a larger pulley ,then you would need higher engine speeds to create the same charging level until the maximum charging capability is reached. (I also understand that the later OE ACR alternators were fitted with smaller pulleys so that the charging level at idle was higher to better meet the electrical load on the battery/system). Obviously the smaller the pulley, the greater the revolutions of the alternator for a given engine speed.

The ignition light comes on, when the engine is running if there is a miss match of charging generated compared with the electrical load.

Cheers
Charles

[JontheHutt]

Hiya,

So a little update. I went to look at the car today (It's in my parents garage, as I don't have a garage or drive at home) and started her up. I revved the engine and just stuck my hand up beside the drive column to see what I could feel. Anyway after a good Rev the light went out, I checked the battery with the multi-meter and it read 14 point whatever it was so I was happy. Incidentally I noticed after that, that the clock, which I have never seen working was ticking away.

Took it out for a spin, actually along one of the old test roots....the benefits of being an Abingdon boy ...... and along one of the rougher roads the light came back on again. That along with the clock has made me think that there must be some dodgy going's on with the wiring behind the dash, so I'm thinking the next avenue is for me to have a look into that. I've added a photo of what i pulled from the dash just for reference, and partly as I was struggling to get a continuity reading on it, so any tips there would be greatly appreciated too, as I must admit it didn't look how I thought it would.

Charles - Thanks for the tip on the new belt, I'll add it to the list.

Thanks for the help, it is genuinely much appreciated.
Jon

[Vic Butler]

I can't make out what the right hand cable has on the end that appears to go over the power supply to the light.

[Bumpa]

Is that a purple wire that's attached to the white wire? Purple wires are live all the time, so if that occasionally touches the lamp terminal it will switch the light on. I'm 99% sure it shouldn't be there and can't imagine what it is supposed to be doing.

[Charles Farran]

Jon,

I have posted up an extract of the wiring diagram again which covers the wiring for the ignition light. From your picture it isn't absolutely clear what colour the wires that go to & from the bulb holder. If the wiring is original / follows original colour arrangement, then there should only be two wires involved being a White (W) & a Brown/Yellow (NY). On your picture,it looks looks like the insulation cover to the body of the bulb holder has been pulled back - before replacing the assembly back in the dash be sure to cover the exposed tab to avoid shorting to the car body / other metal components that might provide a short also.

As "Bumpa" says any original wiring purple wire has no relevance here. Purple wiring is permanently live & supplies feeds to the cockpit light, cigarette lighter,clock & horn.

The fact that your clock started working when wiring has been disturbed suggests you may have one or more poor continuity issues in the wiring.
These cars are 40+ years old & the original wiring connections (outside the cockpit) suffer from corrosion (The ones behind the front bumper suffer the most). In due course i would recommend methodically going through all the bullet / sleeve connections & pulling them apart & cleaning & refitting them . The fuse box will probably merit attention as well.

Cheers
Charles



Cheers
Charles

[JontheHutt]

Hi Guys,

From memory the right hand cable was the Brown/Yellow one and the main one was white. I don't think there were any purple cables involved but I wouldn't be surprised, the loom looks to be in a right mess.

[Charles Farran]

Hi Jon,

I appreciate this is a move beyond the original thread but you may find this helpful.
See attached extract from the ROM (Repair Operation Manual) AKM 4070 that deals with the 1978 MGB (with supplement for 1979..).
This shows the back of the instrument panel & the respective wiring. The same harness was used for Roadsters & GTs - in the picture the plug & wiring for the HRW & it's warning light are shown disconnected .

1979/80 cars had rear fog lights fitted at the factory , although i believe the wiring was already in place ,including to a plug behind the facia so they could be added as a dealer fitted option for a period before - again i think this is possibly shown to the right of the extract.
You can see where the purple coded wires are used as i referred to in an earlier post.

Cheers

Charles

[JontheHutt]

Hi Charles,

That's amazing, thank you.

Certainly useful, I actually pulled the dash off the other day and started to clean up some of the connections behind with some contact cleaner. The little I managed to do before a) running out of the cleaner I had and b)getting dragged away by forgotten appointments seemed to make a difference so fingers crossed. When I get some more time (and more cleaner) I shall carry on and do as you suggested in trying to clean up some of the other connectors.

Thanks
Jon