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MG MGB Technical - Charging problem

Hi all,
Trying to sort out a charging problem on my 78B roadster (UK build). When I turn on the ignition and the idiot light glows as it should. After starting the car, at idle it still glows faintly. As I increase the throttle it gets dimmer until it extinguishes completely at around 2000rpm.
I've checked and cleaned the connector at the back of the alternator. The rest of the wiring harness "looks" good but I guess you can't tell what's inside the insulation.
The only modification is a pertronix ignition system, otherwise the wiring is stock.
Any help would be appreciated.
D O'Brien

Does the charge light come back on when the rpms drop below 2000 rpm after have gone out? If not, you are probably ok. Take the alternator to a local auto electric shop and have it tested. Cheers - Dave
D.W. DuBois

Yes, it comes back on when the RPMs drop. It's like my throttle is a dimmer adjustment. At idle it is about half brightness. The more throttle, the dimmer it gets until it extinguishes.
D O'Brien

Sounds like the regulator (internal) is flaky.
D.W. DuBois

Have you replaced the alternator / water pump belt recently? I had exactly the same problem on my 1980 MGB some years ago. I put a new belt on and problem solved so that may be the answer.

One of the diodes, in your alternator's diode trio, has likely failed. New ones are available, fairly cheap, and aren't that difficult to replace. RAY
rjm RAY

I think the battery might be the cause of the problem. Or I have an additional problem.
I disconnected it and checked with a multimeter. 12.2V.
After leaving it plugged into my charger for most the day it was reading 13.4V.
Disconnected it before I went to bed and left it disconnected overnight. This morning it is reading 12.5V.
Is it possible the battery is on the way out and so the alternator is struggling to charge it? Would this cause the warning light to behave the way it is?
I'm going to let it charge to at least 13V again and see if the warning light still misbehaves with the battery near full. Shops are closed today so I can't buy diodes or any parts until tomorrow.
D O'Brien

IMHO, I would do what the others say. 12.5V is ok for the battery. You need to check the charging output from the alternator. If its not putting out 13V+ then the battery isn't getting charged.

Something internal in your alternator is flaky. Just have it rebuilt. Brushes etc, and tested. Inexpensive and great insurance. You can leave your battery for a couple days without cables on it. If the voltage continually drops then, yes, your battery might have issues.

I've had batteries that were 11.5V and worked fine. The light refers to your alternator.

As a rule I always attend to grounds and make sure they are clean and working. I build electronics and learned the importance of a good working ground. My car was much happier when I went through and cleaned all the grounds.

Battery was just a red herring. It's okay.
Meter was showing around 12.5V with the ignition on. I had warmed up the engine earlier so it started without any choke or throttle, idling at 1000rpm. Meter didn't change still showing 12.5V.
I slowly revved the engine up to about 2000rpm and the meter climbed to 14.2V. After letting the throttle settle back to idle the meter was still showing 14.2V and didn't change.
I could understand now if the light stayed off after the engine had been revved a bit because the voltage test suggests it is charging properly after I hit 2000rpm.
But it still behaved as before. Dimmer as the throttle increased then brighter as it decreased. Even after going for a short drive as soon as I let it idle the light starts to glow dimly.
Battery voltage after a short drive had also increased slightly to 13.0V but after letting it sit for 10 minutes disconnected it had dropped to 12.5V and stayed there.
D O'Brien

A fully charged battery will show between 12.5 and 13 volts. The alternator's internal voltage regulator keeps the voltage produced, by the alternator, below 14.6 volts. Your alternator is capable of producing 46 amps at full output. The fact that your alternator comes to life at 2,000 rpms indicates a diode problem. Depending on your mechanical aptitude and experience, only you can decide whether it's best to rebuild the alternator yourself, have it professionally done or to install a new or rebuilt unit. RAY
rjm RAY

Is the alternator drive belt nice and tight and not worn out?
SR Smith 1

I'm pretty good at soldering, built countless Short Circuits and Silicon Chips electronics kits. I might have a go at the diodes. I've never pulled an alternator apart but I don't fancy buying a new or rebuilt one. Hopefully it isn't too difficult disassembling the unit and getting to the diode.
D O'Brien

HI, You can get the complete diode pack to repair the alternator. Have a look at this link, which gives a nice picture of the pack and also some info on MGB alternators.

As you can see, they are not very expensive in the UK. There is only a few wires to solder and if you are careful, it's fairly straightforward. I changed mine a couple of years ago and had no problems. There are lots of threads on how to change them as well.

Andy Robinson

I have the alternator on the bench and I've just checked all the diodes with my multimeter and a test light.

All 3 plates gave a reading with all 3 stator wires (I reversed polarity on the innermost plate as per the guide I am using).
Then I did the reverse check and both inner and outer plates still showed a reading (albeit different value) but middle plate showed zero.
Just to double check I used a test light doing the same tests. Inner and outer plates appear to be the problem. Test lights up in either direction. Middle plate only lights in one direction (as expected from diodes).

So I guess some of my diodes are faulty. I think I'll replace the whole rectifier rather than solder new diodes. Thanks for the link Andy.
D O'Brien

I spoke to soon.
Once I unsoldered the stator wires and took the rectifier out of the alternator I checked it again. All 9 diodes are good. I put it back on the alternator and checked again. Still good.
The only difference was an orange wire connecting the front plate with the yellow wire from the regulator (the two join at a screw in the middle of the case between the regulator and rectifier). Once I reconnected that, my earlier "faulty" results were repeated.
So is my regulator the problem (not the rectifier) or am I chasing another red herring with the whole alternator?
D O'Brien

"After starting the car, at idle it still glows faintly. As I increase the throttle it gets dimmer until it extinguishes completely at around 2000rpm."

FWIW the Workshop Manual says that if the lights glows at a standstill, dims appreciably at 850rpm, and gets progressively dimmer as speed is increased, with poor output (voltage) at low speed increasing to slightly below normal at 3300rpm, an earth-side output diode is open-circuit. Accompanied by excessive noise but normal temperature.

On the diode packs I have seen there are interconnections between various diodes even with the diode pack fully removed from the alternator, so the readings from one may well be affecting the readings from others.

At the least I'd go for a new diode pack.
Paul Hunt

Actually it's a bit more than $20 for a replacement rectifier. $44 to be exact.
I'd normally take any advice from Paul Hunt as gospel but in this case I'm pretty sure testing each diode individually with a light rules out anything the other diodes might be doing. The connections look complicated but once I traced everything it's pretty simple really.
I did think of one obvious ground connection I forgot to check earlier. The one from the warning light itself. I have pulled some gauges out of the dash recently so it's possible the wiring loom got moved.
If nothing else, I guess $44 is better than the cost of a new alternator. This one is only a couple of years old.
D O'Brien

Sorry I didn't see this prior to you pulling them but as you now know diodes are impossible to check in circuit.

If you can swing it, at least take it to an alternator shop to be tested before you throw parts at it.

There is no specific earth/ground connection for the ignition warning light, this light definitely must not be earthed anywhere or it causes problems.

One side of the potential difference to light it comes from the ignition switch, the other from the voltage regulator inside the alternator. I've worded it like that as under some conditions 12v comes from the ignition switch and an earth from the alternator i.e. when the ignition is first switched on before starting the engine. At others voltage comes from the alternator and an earth from the ignition circuits as when you switch off a spinning engine. Both these light the lamp. Under normal driving conditions there is 14v both sides so it is extinguished, and when parked up there is 0v each side of course.
Paul Hunt

Next time I'll remember not to doubt you Paul. And as usual most the other comments were correct too. It is (was) the rectifier.
I found a friendly auto electrician who was nice enough to do some thorough tests and not just try and sell me a new alternator. In fact he was happy to do the tests knowing I was planning on fixing it myself and I told him up front if it was going to cost much to repair I wouldn't bother.
He even took the time to show me exactly why it was faulty. Two diodes, one on the outer set and one on the inner set.
Thanks for explaining how the light is connected too. I should have worked that out from the circuit diagram. I'll blame my ageing eyes for missing that.
D O'Brien

This thread was discussed between 13/06/2015 and 17/06/2015

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