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MG MGB Technical - Follow-up: Dead battery, or...

Last week I posted a thread about a dead battery in our 1977 MGB. The car lterally stopped running once the battery drained completely. I had the alternator rebuilt (only $35!) and reinstalled it last night.
In a short test run in the car things seemed fine. The car started and ran well. However, the headlights didn't brighten or dim when the engine revved.
Question: Is there a test to see if the battery is being charged by the alternator?
Cheers,
Randy Olson
randy olson

I'm a novice compared to others on this board, but this is how I would do it. With the engine running, disconnect the battery. If it stays running, the alternator is doing it's job. Then, with a voltmeter, check the incoming volts at the battery terminal. If you're reading 12-14 volts, you're alternator is doing the job. Double check the same at the alternator output terminals.
Brian Johnson

I hope that I am in time!!
Do not disconnect the battery with the alternator running. I will immediately blow the alternator diodes. This is mentioned in all manuals. Please do not give advice that you haven't actually carried out successfully.

Mick
M F Anderson

To check the alternator just leave everything connected and check with a voltmeter for about 14 volts at the battery, at just above idle.
The correct voltage and RPM for testing should be in your Workshop Manual.

Mick
M F Anderson

Many appologies. Maybe I should have referred you to the experts.

http://www.mgcars.org.uk/electrical/alternator.html

scroll to the bottom.
Brian Johnson

No harm, no foul. I haven't done anything yet.
I'll do some checking tonight or tomorrow and let you know what I found.
Thanks for the input so far.
Randy
randy olson

A car with an alternator will not run without a battery; unlike cars with generators. The alternator requires power from the battery to excite it's field. (I could use that as well). If you take a voltage reading of the battery before you start the car it should be somewhere over 12 volts, and again while it's running, it should be around 14 volts as mentioned. The increase shows you it's 'putting out'. (I won't comment on that)

Doug
Doug Campbell

"Inch by inch - row by row - we're gonna make that MGB GO!"

Good news on the rebuild. As to the headlights, from what I know, if they don't dim or brighten -- that is stay the same intensity -- that indicates a good charging system.

In other words, there's something wrong when headlights, (because of their electrical draw) dim or brighten with engine revs.

Sounds like you are on your way. But keep a new battery on your wish list. Over the winter, bring the battery indoors and keep it charged -- a cou[ple of hours a month with standard charger.

Thanks for the update.
glg

I checked the voltage at the battery. With the car OFF the voltage across the battery was 12.8. Then I started the car (ran beautifully!)and the voltage across the battery was 13.3. Not much change.
Then I put the battery charger on, and the meter on the charger read CHARGING, but it was close to the CHARGED range.
My son just got his learner's permit this afternoon, so the MGB is about to get a real test!
Let me know what you think about the voltages.
Regards,
Randy
randy olson

Randy. As I have mentioned before, there are two components of alternator charging--output voltage, which should be between 13.5 and 14.5 volts and amp output. You alternator should be capable of putting out about 41 amps maximum. But, this is not an all the time thing. The system puts out the necessary amps and volts to run the systems that are being used (when it is working correctly). Thus, it is possible to have adequate voltage output, but inadequate amps, thus requiring that the car run partly on the battery. When the battery is no longer capable of making up for what the alternator is not providing, the engine dies. In other words, exactly your original problem.

Since you have tried to have your alternator tested at various parts houses, without success, you need to take the car into a professional mechanic who has a charging system tester. This test does not require the removal of the alternator from the vehicle and takes only a few minutes. The place where I have my American cars serviced will do this test, for free, on my MGs. This would be your logical next step.

One thing that has not been discussed is the ignition warning light. Does yours work? The light is similar to a balance beam or beam type scale. When the alternator and battery are in balance, the light should be off. When the key is first turned on, the light should illuminate because the battery is powering the systems and the alternator is not. My experience is that, with a marginal alternator, the light will glow very dimly when the battery is augmenting the alternator output. What is your light doing? Les
Les Bengtson

Well, the MGB survived the driver's test. I didn't know it could "burn rubber" until my boys learned to drive! My neck still aches from the whiplash. I only had to grab the wheel and return the car to the road twice. Tommy actually did an outstanding job, and is well on the way to Octagonland.
So far, the car seems to be running well. I'll watch the battery charge for the next two days and see what happens. I think the alternator is now charging as the battery charger is edging towards the CHARGED range.
Fingers crossed; more later.
Randy
randy olson

I may be able to have the system checked tomorrow at the local parts house if it's not raining too much.
As for the light on the dash, I haven't seen it. Time to check the bulbs and fuses, I guess.
Thanks for the advice.
Adios amigos,
Randy
randy olson

randy
I was told at a local tech session that the alternator will not work properly if the ignition light is burned out. This was from a MG mechanic. Others may have a different opinion.
Jim Lema

Hi.

An alternator needs to be 'started up' by a small current from the battery.

The wiring is arranged so that this small current goes from the battery to the alternator via the 'ignition' light. (and the ignition switch / relay, of course).

When the alternator starts turning and producing power, it no longer needs the 'start up' current, and the ignition light goes out (or goes dim if the alternator is only producing marginal power).
This arrangement provides a useful indication that the alternator is operating, but failure of the ignition light bulb does (usually) result in no alternator output the next time the engine is started.

I believe that some vehicles have a resistor (an inexpensive electronic component) connected in parallel with the ignition bulb, so the alternator still gets a 'start up' current if the ignition light bulb fails. Has anyone out there calculated the appropriate value for such a resistor ?.. I would guess that a value a little higher than the 'hot' resistance of the ignition light bulb would provide adequate 'start up' current without dimming the bulb too much... maybe around 300 ohms ?.

HTH.. Don
Don

Maybe this will help:

http://www.vtr.org/maintain/alternator-overview.html
Dan Masters

This thread was discussed between 26/09/2003 and 27/09/2003

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