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MG MGB Technical - Is this a regulator (control box) problem?

Just when I thought I was off and running in my early 3br Mk1, I am stuck with charging problems.

First symptom after a few trips - flat battery. Didn't have ignition light working at first (no globe) - when I did I realised the light stayed on when the engine was running.
I watched the John Twist video and checked the field polarisation ofv the generator and also disconnected the batterywhile the engine was running (1500 revs) with headlights on - the engine died immediately.
So I pulled the generator off and had the wiring, brushes etc checked, and the unit bench tested perfectly.
Reinstalled, making sure fanbelt was not too tight, polarised the field again to be sure, and started the engine.
The light stayed on.

So , have I got a regulator problem? It is a very old CAV regulator that may have been original on this very early Australian Mk1 (where many components were sourced locally). I had it checked and adjusted some years ago but until very recently, it has just sat there in the engine bay - only being called on to operate when I got the engine running (total running time maybe 2 1/2 hrs in total)

Now, with the engine running I viewed the cutout contacts - they were still open . So I assume this is the problem - ie the batteries are not connected to the generator and hence are not being charged.
Or is there another likely explanation before I look for a replacement regulator?

John
John Minchin

John. It has been a long time since I worked with an external, mechanical voltage regulator--a control box in the workshop manual.

But, the first thing I would check out would be the system voltage at the batteries (should be 12.0 to 12.5 volts with the ignition off). The second thing I would check out would be the system voltage with the engine running (should be 13.5 volts or more).

If you are not showing at least 13.5 volts with the engine running, I would go to section N of the factory workshop manual and begin my tests with section N.4 and continue on through the manual until I had identified the problem. The control box is adjustable, although a "special tool shown in figure N.7" may be needed to do so. It may be that your control box is out of adjustment, or needs the points cleaned, rather than broken in a manner that requires replacement.

Les
Les Bengtson

John - Just to insure that the generator is putting out the correct voltage, run the tests as detailed in the article, Testing a Dynamo/Generator in the Other Tech Articles section of my Homepage at: http://homepages.donobi.net/sufuelpumps/ This will tell you if the generator is putting out sufficient voltage, if the voltage is getting to the regulator and if the regulator is doing its job. Cheers - Dave
Daved DuBois

Hiya,

Check the dynamo is charging first.

Simple test.................

Pull of the thick Brown/Yellow and the thin Brown/Green (D and F terminals) from the control box.


Link together with a short length of wire.

Clip one end of a 12volt test light to earth and the end to that link.


Start and run the engine, but dont rev it up or you'll blow the test lamp bulb!


The light should light brightly if the dynamo is ok.

If it doesn't link the dynamo terminals and do the same test, if it now lights you have a prob wirh the wiring between the dynamo and control box.


Check you have a good earth on the black wire as well, can cause all sorts of probs.


Good luck! Hope that helps.


SR Smith 1

"Start and run the engine, but dont rev it up or you'll blow the test lamp bulb!"

More importantly damage the dynamo from over-voltage, or not realise it is under-charging. Use a voltmeter, don't let it go over 20v, and it shouldn't have to go over 1000rpm to reach this.

With the dynamo wiring reconnected and the engine running you should see a similar rise in voltage as the revs are increased slowly from idle, until the control box cut-out relay (the one with the normally open contacts) operates at about 13v. Revving further should increase the voltage to about 15v, at which point a finger placed lightly on the voltage regulator relay (furthest from the cut-out relay) should show it is vibrating, unless the battery is heavily discharged or there is some other heavy load on the dynamo.

As your ignition warning light isn't going out you shouldn't get that far. If the dynamo voltage hasn't gone up then the wiring and the voltage and current relay contacts that effectively link the D to the F terminal as in the test are open-circuit somewhere. You will need to test at various points around the control box to see where the problem lies, and need a control box diagram to see where to test.

If it does but the cut-out relay doesn't operate then the relay winding or the control box earth could be faulty, for the latter see if you have any voltage on the E terminal with respect to a good earthing point on the body or engine.
PaulH Solihull

Hey John,

I had a similar problem with the charging circuit on a TD last summer. It drove me crazy trying to source the problem. In the end it was caused, as noted by a couple of folks here, by a faulty ground wire from the control box to earth
JR Ross



Hi Check out this manual.

www.atcc.org.nz/tcc/LucasFault.pdf

Herb
Herb Adler

Yes it was a voltage regulator (control box problem)
I carried out lots of tests as advised, but in the end decided I would invest in a new regulator rather than persevere with the old one. But it made no difference. Double checked every cable and connection, charged battery fully, sound earth, etc...
In the end I got an expert "old time" auto electrician to come and have a look - after 10 mins checking cables, connections etc, and establishing that the generator was in fact generating, he whipped the cover of the regulator.

"what are these black marker pen lines" he asked. I said, they looked like marks showing correct settings as per Rick Astley's book. Anyway, he fiddled about and 5 mins later it was all adjusted and working fine. He said it looked like someone had purchased the device, changed the adjustments then returned it to the vendor.

Now for some serious driving while our summery autumn (fall) weather holds out!

Thanks everyone for advice as usual!
John
John Minchin

This thread was discussed between 03/03/2013 and 15/03/2013

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