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MG MGB Technical - charging sytem warning light

When I fired up the 64 today (with stock generator, voltage regulator, positive ground system) the ignition warning light would not go out till 1500 to 1800 rpm. It glowed full on at 1200. Said nothing to Kathy, and off we went on our 20 mile ride around the county. Got back, everything was as it should be. Slight glow at 600 rpm and out by 800. All conections are ok. Battery water levels ok. Generator is fresh with new armature, brushes, bearings and bush. Regulator is a year old with about 5K on it. Storage temp 65F, and its been four days since the car was driven.

Normally, I'd just let this one go but it did the same thing last week. Maybe this is one of those "know when the car is going to break before it does" indicators.

Any thoughts?

Thanks--Paul
Paul Hanley

Paul: sounds like a bad regulator, non-electronic/mechcaical regulators have points similar to ignition points which is why they have for the most part have been replaced with electronic units. Points fail especialy when the car sits around for a while.Now you know why there are so many posts on non-Lucas altenator instalations RIC
R E L Lloyd

Is fan belt tight?
glg

Paul,
You can try cleaning the contacts in the regulator with very fine emery. Follow the emery with sliding a business card or similar item between the points to clear out any grit that may be remaining from the emery. I have an MGA where the light was staying on and this cured the problem. The points can get a slight corrosion that is not even visible, and takes very little to remove. If you have a workshop manual or Haynes manual, it also explains how to adjust the regulator and test the generator output, but I would clean the points before doing anything else. Since the light goes out I would think that the generator is probably working properly.
Good Luck,
Ralph
Ralph

A good electronics store (something beyond Radio Shack) should carry a Points File for cleaning relay points. They use a very fine grit or file cut and are quite thin, to fit between points without bending their support arms. These used to be more common years ago, and may be a bit harder to find today. FWIW
Bob Muenchausen

The warning light not going out until a higher rpm is reached indicates that the generator may not be producing the voltage it should at lower revs as well as the possibility of the cut-out relay failing to operate at the correct voltage. All being OK when warmed up could again be either cause. If it happens again when cold connect a voltmeter to the D terminal of the generator and see just what voltage it is producing at the usual 800rpm (I assume) when the light goes out. If in the order of 12.7v to 13.3v then it is the cut-out relay that has the problem or the voltage may not be reaching it. If the voltage doesn't reach the above until 1500 to 1800 rpm then it is the generator not producing the voltage it should, or possibly the self-exitation voltage (normally about 1v) is not getting through the regulator back to the field winding, compare the voltages on the D and F terminals.
Paul Hunt

Same sort of thing happened to me in a '73B. the generator wasn't working at all below 3000 rpm, and so the battery was draining. First to go was the indicators, followed by the dash lights, headlights, and then the ignition system, forst missing a little and then getting worse and worse until it cut out completely. Got a second hand generator for £12 from Andy Jennings, and everything's great now.
dave pothecary

Turn your ignition on and see what your voltage reads, then if low check all motors to see if any are warm. That will be the culprit. My 77 would only put out 10.5 volts at idle and just over 11 at 3000 RPM. I just happen to be working on the windshield wiper motor for an unrelated problem and felt the heat. I disconnected the motor at once and the voltage jumped up to 13.8 at idle. Voltage drains seem to be quite common on the negative earth systems.
JC Nelson

Large voltage changes like that are usually due to bad connections (not drains) in the system. These cause motors to run slowly, which paradoxically makes then consume more current than they should and hence get hotter than they should. It's quite true that the motor itself could have been partially short and drawing too much current, but that would be more likely to blow the fuse than drag the voltage down by that much in a system without bad connections.
Paul Hunt

Paul Hunt--did the test you suggested after the problem returned yesterday. From cold, the D terminal read 1.7 volts at 800 rpm and light would not go out until 2000 rpm. At =/- 3000 rpm voltage read 12.7 volts. F terminal read .35 volts at 800 rpm with D terminal reconnected. Everything in the generator is new (1000 miles)except the shoes and they tested to spec. Could I have done something wrong on my rebuild? Bad parts?

Yes, RIC, I'm begining to see why people switch to alternators!

Appreciate the help,

Paul
Paul Hanley

1.7v on the D indicates the residual magnetism in the dynamo is producing the priming voltage, but this is not getting through the regulator and back to the F terminal. So check the voltage on the D and WL terminals of the regulator, then the moving contact of the cutout relay (you should see battery voltage on the fixed contact), both sides of the contact on the voltage regulator relay (opposite end to the cutout relay), both sides of the contact on the current regulator relay (in the middle), the F terminal on the regulator, and the F terminal on the dynamo. Somewhere along that circuit you may see a drop from the 1.7v to zero. If it is either side of either of the regulator relay contacts then the contact is dirty, other than that it will be where a spade connects to a terminal or a dry-joint in the internal wiring of the regulator. If it gets all the way back to the F terminal on the dynamo then the field winding is suspect, but only seeing 0.35v on the F terminal indicates the problem is in the regulator or the wiring to it. My best guess would be one of the regulator contacts.
Paul Hunt

For the record, once again you're right on the money, Mr. Hunt. Thank you also RIC, Ralph, Bob and all. It was the voltage regulator. Openened it up and indeed the contacts were pitted. Now, lights out at 700 rpm.

One note that may help others. At first I was hesitant to drill out the black plastic rivets holding the cover on. I learned that the rivets were actually two pieces and that the center peg can be drifted out from behind with a small punch and reused. Had to have the original rivets, ya know!

Cheers,

Paul
Paul Hanley

This thread was discussed between 01/01/2004 and 09/01/2004

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