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MG MGA - Volt meter behaviour

I have a Smiths voltmeter connected that normally shows around 13.5 at anything over 800 revs. Lights get brighter when I rev up, ignition light starts to flicker at around 650 idling. All this has been fine, batteries charge well.

However, I've just noticed for the first time that when I am driving with the lights on (don't often drive after dark in the A) the voltmeter drops to about 12 volts (ignition light stays out) is this normal?

Positive earth, generator.

Thanks all

Chris.

Chris Currah

It should maintain at least 13 volts with the lights on and rpm above idle. If it is only running at 12 volts you aren't charging the battery. Get a second voltmeter that is known to be accurate and compare it's reading to yours, it could just be the gauge.
John H

John,

Gauge is fine John, it's just when you put the lights on that it drops to 12. Without the lights on it shows 13.5 and above. Problem with battery?

Chris.

Chris Currah

More likely the voltage regulator is not properly adjusted or the generator output is not up to spec. Also check that the regulator is grounded. Have you added any other electrical equipment? The charging system on these cars did not have a lot of excess capacity. If you have a factory workshop manual it has step by step instructions for checking both the generator and regulator.
John H

First thing to check is the tension and the condition of the fan belt. The extra load with the lights on might be causing the belt to slip. Might be a bad generator, but I doubt the regulator would cause this problem.
Jeff Schultz

"First thing to check is the tension and the condition of the fan be" - Do not tighten the fan belt! All that will accomplish is to destroy the rear bushing in the genreator. Generators are not like alternators that develop a lot of back emf torque when a lot of current is drawn. Generators don't get more difficult to turn when current is drawn and do not require a tight belt. With the lucas generators that use a bushing at the rear of the armature and a very light tension on the belt is required so there is minimal side force developed by the armature on hte bushing. See my artical on this subject at: http://www.omgtr.ca/technical/tdtf_fan_belt/loosen_up.htm
The volt meter should read closer to 14.5 at 1000 RPM and probably drop to 13.5 with the lights on. John H is correct in the assesment that the problem may well be a misadjusted regulator. The shop manual give fairly detailed instructions on adjusting the regulator or take it to a local auto electric shop for adjustment. Good luck - Dave
David DuBois

Chris, - Your situation seems perfectly normal for a generator. Just don't let it idle too long with full electrical load.

Much depends on how and where you are measuring the voltage. By the book, at room temperature and running speed (1500-3000 RPM) with no electrical load, voltage output should be about 16 volts at the output terminal of the dynamo. Make that a half volt higher if the control box is cold, or a half volt lower if the control box is hot.

When you connect and switch on all vehicle electrical loads (not including starter or hooter) the voltage will be lower. This may be around 15 to 15.5 volts at the dynamo output terminal, possibly as much as one volt lower between the starter switch terminal (main battery cable) and engine ground, and another half volt lower across the battery terminals (possibly as low as 13.0-13.5 volts with full load).

Fully charged battery open circuit voltage is 12.6 volts. Higher voltage at the battery terminals will be charging the battery. Lower voltage at the battery terminals will be discharging the battery. Battery terminal voltage from 13.0 with full electrical load up to 14.0 with minimal electrical load is reasonable for a generator running at road speed. With a higher output alternator you can easily get another half volt. Anything higher than 14.5 volts at the terminals will overcharge and boil the water out of the battery.

If you have very low voltage, disconnect and check the dynamo function first, by the book. Jumpering D and F terminals together on the dynamo should produce up to 20 volts just above idle speed. Do not let it run above 20 volts when testing. If the dynamo works, then go on to the control box.

If your voltage readings are not within the expected operating range you can adjust the regulator relay in the control box. These are notorious for drifting out of adjustment over a couple years time with regular use, usually toward lower output. Very small motion of the adjusting screw makes a big change of controlled voltage. As little as 15 to 20 degrees clockwise rotation of the screw may increase the output by one volt. If you cannot adjust the regulator to the required output range, see how far you can throw it and buy a new one.

One caution for the original style MGA control box is, when the regulator relay fails it can remain with contacts in the closed condition. This will cause dramatc run away voltage and current output at road speed, which can cause internal meltdown of the generator in a matter of minutes. Any time you replace the generator, always check function of the control box and the regulated voltage output before driving the car.

If you have the correct controlled voltage output, but the battery runs down anyway, it is most likely a faulty battery, maybe not capable of holding a charge. You can check the battery condition with no special tools. Charge the battery to full capacity, preferably with an automatic shut-off battery charger. Disconnect the charger and switch on headlights only. The low beam headlights, parking lights and dash lamps will draw 12 to 14 amps. The battery should have capacity of at least 40 amper-hour. So the headlights should burn brightly for a few hours before they go dim from a low battery. If the lights go dim in an hour or less the battery is shot and you need a new one.

Good battery but slow engine cranking is a different class session.
Barney Gaylord

Thanks for that all,

So it looks like I may not have to much to worry about as this is not much different from normal, perhaps a bit low under load.

I will have a look at the regulator, maybe a slight tweak is in order. I'll dig out the manuals as I haven't been there before.

Thanks Chris.
Chris Currah

Sorry forgot to mention, the Voltmeter is attached to the switched side of the ignition switch.

Chris.
Chris Currah

This thread was discussed between 05/10/2006 and 06/10/2006

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