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

Some more mud for the water....

if you do decide to wire in the temp ign light circuit as described.... Might be worth giving the original ign light wire now removed from it`s connection to the alternator, a definite good earth anywhere that is handy, and watching how the original ign light performs while you are also monitoring your temporary ign light set-up with the engine running...? Of course the original ign light should then be constantly lit with the ign switched on whether the engine is running or not , and if seemingly intermittent in terms of it`s brilliance then perhaps indicates the fault is within it`s interconnections with the ign switched side of the circuitry and not in the alternator...?

Worth a try... ?

Sorry George but if I've understood your suggestion correctly you do NOT want to be shorting any part of the ignition to warning light circuit to earth/chassis. The warning light works on potential difference across its contacts not at all on any direct ground connection. To work as TBM describes it can only be receiving a faulty reverse current.

Hi TBM
First of all I know nothing about MGB'S so what I am about to suggest might not be practical. What I would do is first is remove the distributor cap so that the engine does not start. Turn the ignition key to start, does the engine turn over in a sprightly manner?. If so you can be fairly sure that the battery terminals are secure and the earth strap to the engine is good. refit the distributor cap. From the battery to the starter solenoid will be a thick cable to carry the starter current and I am guessing and it is a guess that from there will be a thinner cable to the fuse board to supply the fundamental power feeds via fuses to the various items, wipers, ignition, headlights etc.

In this problem there are two sources of power one is from the alternator supplying one side of the alternator bulb and the other from the battery supplying the other side of the bulb. So I would try and investigate the issue by eliminating one of the power sources i.e. lets first not start the car. hence only one power source, the battery and do some tests. If it is possible to get at the fuse panel where the main supply cable to the fuse box arrives, I would find a good earth for the Common or Negative of your test meter. Switch on the ignition and measure the voltage at the battery terminal and then with the negative of the meter still at the same earth point measure the voltage arriving at the fuse panel, or if you can get at it arriving at the live side of the ignition light. Are they the same? They should be within a few millivolts. Then with the ignition still turned on, turn on the wipers, are the two voltages still equal ? (they should still be) then turn on the headlights, are the two voltages still equal, they should be. If you find that as you increase the load i.e. turn on more and more items the voltages are getting gradually further apart, then you can be pretty sure that you have a bad connection somewhere between the battery + and your fuse panel. this would match the description you give of the ignition light getting brighter when you turn on more load, i.e. wipers and sidelights from your previous post. It would also kind of point to the fact that you mention, the fuse panel needs a bit of work. Bear in mind that I have never owned or been anywhere near an MGB GT, but if someone bought the car to me for some help, that is what I would be doing as a start to look at the problem.

Hope that this is of some help.

Regards ZACH

Massive thank you to all the help and advice. It is very much appreciated.

Status Quo - car started fine this morning (-3degrees) so happy battery is OK. Charged well all journey to work. Lamp was on full for the majority of the journey (it went off for about 5 minutes).

I think I may have conflated two issues -


Next step: This seems likely to be a result of the manky wiring/fusebox and I'll tackle this during the easter break


I think is down to a dodgy diode. This secondary problem started shortly after my last complete battery drain, so possibly the big recharge process has upset it.

Next step: I'm going to put the old Alternator back on at the weekend If the lamp goes off, it's a diode issue. If it stays on, then it's definitely down to the wiring.

When I first went to view the MGB, the owner said that the alternator warning light was on. I took my old Morgan alternator, swapped it and the light went out (we didn't check charging voltage at that point). I then later replaced that alternator with a new one as I thought the 'wiper glow' light was down to the alternator.

I'm just wondering if the wiring issues may be causing a failure in the warning light diode part?

Unlikely. The supply from diode trio to field circuit is internal to alternator. There is no diode in the warning light itself or associated wiring.

Just to recap when all working correctly the diode trio and main alternator outputs should be at the same voltage (or very nearly so). This voltage to a degree is controlled by the battery and how much current it demands which in turn is controlled by the regulator adjusting the field current to control the output voltage. When the battery and loads combined drag the alternator output voltage down below its regulated limit around 14 volts (ideally 14.4v) the regulator will cause the alternator to run flat out commensurate with engine revs.

Thank you!

Richard, no need whatsoever to apologise relative to advising where I might get things wrong...Always pleased to learn, thanks.

I think I understand the illumination of the ign lamp depends upon potential difference, and that there should be none once the alternator is generating voltage..

My thinking on the alternator end of the warning light circuit was once it had been disconnected from the alternator was to then to ground it, was more as an "extra" check o confirm that it`s switched live side of the ign light circuit was not picking up some sort of grounding via the many paths available to it along the lines that Dave suggested i.e. through a failing seat belt diode..

Always greatly appreciate your input Richard.

If you can get a multimeter on say the 10 volt DC range place the negative probe on the ignition switch side of alternator warning light and positive probe on battery positive or as close as you can get to it, even the ignition switch itself. With engine running the reading will show the volt drop you are getting which is driving the reverse current through the warning light. If my suspicions, based on your findings above, are correct you will find this volt drop due to resistivity somewhere in feed to lamp, increases with extra loads. The pulsating you got is likely due to alternator regulator action.

On a voltage range accepting up to 15 volts you could also measure the voltage between alternator side of warning light and ground. This should be very close to battery volts accepting it will rise with engine revs above idle. If so the fault is definitely due to a significant resistance in wiring between ignition switch and warning light. Please let us know how your investigations progress.

BTW the seat belt warning diode appears to be a red herring. It's there to stop back feed from shared buzzer triggering seat belt warning light when door is open as per below.

Thank you - I've actually bought a new alternator (a smidge over £50 from the MGOC so seemed a no brainer). I'll fit that tomorrow (in between all the forecast showers) to see if it cures the 'second' problem above.

Fingers crossed I'm just left with the first problem (the wiper/washer glow).

Testing is a litte difficult as I've got to remove a fair bit of dash and dials to get to the back of the alternatior lamp and associated wiring, and the battery is under the rear seat. I'll see what I can rig up.

I'm embarrassed on behalf of the previous owner to post a picture of the current fusebox, It's so bad I've been put off from trying to fix it as it's currently working but I think that it lies at the heart of the problem!

It's hard now to think back but the early B's had two fuses as I recall, which later increased to four, perhaps on the offside inner wing. It always amused me that some of the wires going in were a totally different colout to those coming out. It just made no sense.

The black spade covers are likely to be the originals. That was the BL style in period. That is a masterpiece of "making do".........................