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MG MGB Technical - Help with ignition light and alternator please

As my rebuild of the 1971 roadster progresses I have reached the point of connecting the batteries and trying circuits out. One thing I immediately noticed is that the ignition light doesn't come on when I set the key to the running position, just prior to operating the starter motor.

I have used the multimeter to check the white wire on the light's bulb holder and it has 12.4 volts power. Then I unplugged the alternator and checked the brown/yellow wire in the plug and the power reached there too, so is obviously passing through the bulb OK. But the terminal on the alternator where the brown/yellow plugs in is not connecting to earth (ground).

My understanding is that until the engine starts that terminal should be earthed through the field windings. Does this mean the alternator is faulty? If so, can I fix it, or would a replacement be the best option?
Mike Howlett

Try starting the car and revving it to about 2,000 for a few seconds and see if that kicks it into action by switching off then on back on to test if light is there.

Nigel Atkins

It's not in a running condition yet. The engine is in but there are no carbs and no radiator. I was concentrating on getting the electrics right.

Anyway, further research has suggested that I unplug the alternator and connect the brown/yellow to earth. The ignition light should then work. If it does there is a problem with the alternator. Since it is possibly the original unit supplied in 1971, I think I'll get a new one, with an increased output.
Mike Howlett

As you say the ignition warning light should earth through the voltage regulator AND the field winding, as in the attached. So either the VR is faulty or there is a poor connection somewhere inside the alt. The field is on the rotor of course so check the brushes and slip-rings as well as wiring connections.

Yes earthing the brown/yellow should light the warning light, but note there can be faults inside the bulb holder that effectively short out the bulb, and will still show 12v on the brown/yellow.


paulh4

Mike,
do you need an alternator with greater output, I have one and all it really does is give the engine more work.

I also have and 'ignition' light that barely glows and I think it's the holder or the wires from it at fault as with the wiggle test I can briefly get the bulb bright but as the battery gets charged it's on the roundtoit list as it'll require very awkward working behind the dash to sort it.
Nigel Atkins

Nigel, it seems that new alternators are mostly 18ACR which should be fine on an MGB. Working behind the dash is certainly tricky. Fortunately there are no seats in the car yet so I can lie on my back on the floor with my head under the dash. I wish I had slimmer, double jointed fingers.

Things are so much simpler on my (dare I say it) Triumph Dolomite. Four screws and the entire instrument panel pulls out so you can get at the back of the gauges. As with many, many jobs on MG sports cars, I always say "How did they do it in the factory?". They can't have spent hours doing tasks like fitting the windscreen and the heater, putting the windows in the doors, connecting the instruments, and a myriad other jobs.
Mike Howlett

Totally agree, Mike, on your last paragraph. For a relatively simple car there are hassles like that.

Plus, that you can't take the gearbox out from underneath; or that you can't remove the Roadster front wings without removing the windscreen first. And you can't remove that (if you follow the book) without taking out the dash.

Or what about the built in rust trap on the top of the fuel tank, and on top of the trumpet sections under the front wings?

Other than that, they're great cars!
Peter Allen

Mike,
you should try lying on a Spridget driver's seat to get your head and shoulders into the footwell area to look up to behind the dash, wishing you'd taken the 15.5" steering wheel off to give a bit more room, as items fall to the floor under your back and you wonder if you can limbo out of the small door aperture again and be able to return to the position you were in again before said items accepted gravity.

My ignition light tails are on a very short branch of the harness that is held up high leaving very little room to even remove the bulb holder let alone cut and solder in a replacement, to get more access would require removing the speedo and tacho and other cables and connections which would give potential of disturbance issues.

If I ever find a can of CBA I might just try adding a couple of blobs of solder to the existing fitting to see if that helps but stock is always in very short supply.
Nigel Atkins

Oh I know what you mean Nigel, having rebuilt 3 midgets in the past, a 1964, a 1966 and a 1972.

Peter I don't have rust in the front wing traps any more because both the roadster and my GTV8 have had plastic wheel arch liners for years. On the roadster they have been on the car for 25 years, and when I took the wings off last year to weld new sills, the wings and trumpet sections were in excellent condition and needed no more than cleaning and repainting.
Mike Howlett

16, 17, 18ACR and many more modern alts such as the A-series all work the same, just have different maximum output levels.

The warning light provides the initial excitation current for the field which enables the alt to start charging from about 900-1000 rpm. Without that a used alt (at least) will start charging when revved to around 2k, and will then charge normally down to about 600 rpm i.e. the same as with a working warning light.
paulh4

Might be a silly question, but has the engine block got a good chassis earth?
Allan Reeling

There you go Mike even Paul agrees with me, you don't get that often, did you try the revving to 2,000 rpm then turning off and see if red light, er, lit.

(And to think I almost put 2,500 that would have spoilt it.) 😁
Nigel Atkins

Nigel - concentrate - Mike has told you why he can't rev the engine. Take 100 lines ... .
J P Hall

🤣 The shock of Paul putting the same as me overtook all my thoughts and emotions, sorry.

It's a week later so perhaps Mike has the engine in the car or can rig up the alternator to spin at engine equivalent 2,000 rpm. No, oh well.

I must do better
I must do better
I must do . . .
Nigel Atkins

The latest replacement alternator in the Midget requires 2000+ RPM before the light goes out. It stays out unless the RPM drops below about 700 RPM or so. The previous alternators have charged "light out" without meeting the high RPM threshold.

This alternator has performed satisfactorily for several years although it is a bit unnerving to have the light stay on through the backing out of the garage and starting up the street phase. Although I don't miss the maintenance that the generator required (brushes, lubrication, etc.), the faint, pulsing glow of the charge light at 1000 RPM was perfect for setting engine idle and reassuring that all was well with the world.
Glenn Mallory

Is the warning light an LED? I'm guessing not if previous alts with the same light worked as they should. Really it can only be a faulty alt in this case.
paulh4

Oddly enough having had the alt plug out and back in a number of times the ignition light is now doing what is expected of it, so it may have been a poor connection at the plug. It is a new plug and wiring on an old alternator. Once the engine can be started I'll know for sure.
Mike Howlett

The obvious but forgotten basics, that I often remind people of but forgot myself here. Just taking connectors on and off a few times can clean them up a bit, same as with turning a switch on and off a good few times can clean the switch contacts.

But I would suggest you clean the alternator male connects with some very fine glass paper and then coating them with a suitable protectant and perhaps spray the plug with electrical contact cleaner.

Personally years back I found the new plugs to be poor quality so make sure you have good connections to the wires and the female spades are a tight fit to the alternator male spades.
Nigel Atkins

Mike- Nigel
This reminds me of a new Ford truck that I had to go to the wharf and get going a while back--The freight company said it had a flat battery-
So, armed with a brand new fully charged battery, off i went , fitted the battery up thinking that was the issue---but no go, Out with the meter- battery good checked at the starter motor main cable, nothing, back up to the battery and checked the cable power, nothing, checked continuity from the battery to earth,ok, checked from battery + to cable ---nothing, but it was a brand new truck with a nice shiney terminal end on the wire-----Looking at the terminal it was really shiney and clean but decided to give it a bit of a sandpaper anyway and found that it had been coated with some sort of clearcoat, presumably to stop it corroding, it did that alright but there was enough on there (like clear paint) to prevent any contact---Anyway, cleaned up it started first go
Might be what has happened to you Mike with your new connector

Interestingly I almost mentioned earlier, had you tried earthing the Br/Y wire to earth to see if the globe fully lit--you'd mentioned what it should do but not actually testing it but the conversation changed to getting a new alternator and then i forgot to mention it -sorry

willy
William Revit

Willy, nice to know even the best can be caught out.

Decades back a mate's 18 months-old V8 LR Disco failed to start and it turned out to be just a spec of crud on the battery terminal post or clamp so that taught me to remove the clamps and check. Often just checking the clamps can find one is loose, or the battery which is said "to be good" turns out not to be.

If Mike's new plug is anything like the cheap rubbish I was sent from a well-known supplier years back then it will be worth checking. I decided I'd change it a few years later and ordered one from a different supplier but it was the same as the one I already had fitted. To be fair I've only tightened one of the female spades last year as my bulb hardly glows and I was checking all connections. On full technical testing, a wiggle test, it was the bulb holder connections at fault I discovered, after breaking the bulb trying to remove it.
Nigel Atkins

The new plug for the alternator came as part of the new wiring harness. It seems to be of perfectly adequate quality. When I dismantled the car it didn't have an alt plug, just three separate wires with spades on, one of which was in the process of breaking off. I suspect the lack of connection will be due to the male terminals on the old alternator. I'll clean them up a bit.

To answer Willy, yes the bulb (I love your term globe, but it's no stranger than our "bulb") lit when I earthed the brown/yellow. The bulb and bulb holder are new, again part of the new harness.
Mike Howlett

Fair enough. Yes clean the alternator female spades with some fine glass paper, a lot easier off the car, also clean the new plug with some electrical spray contact cleaner in case any crud got transferred to it from the alternator spades, then put some sort of protectant on the alternator spades and then you can fit and forget for many years.
Nigel Atkins

This thread was discussed between 06/05/2022 and 15/05/2022

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