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MG MGB Technical - Not Charging

My 79MGB will not charge correctly unless I rev the engine to about 1300 RPMs. I have it ideling at about 900RPMs and it will not maintain the charge. The voltage slowly drops. The higher the RPMs the more it charges. I found out that 1300RPMs will keep the voltage up 10 13vdc.

Am I right in thinking this is a altenator problem? And what should it be charging at?

Thanks for the help.
bsf Fowler

Once revved to about 1000 rpm an alternator should charge (warning light out) at about 14v - assuming a minimal electrical load - down to about 600 rpm. If the ignition warning light is not functioning correctly the engine might have to be revved to about 3000 rpm before the alt *starts* charging, but a good alt will then charge down to about 600 rpm as before. Most faults described in the workshop manual are accompanied by non-standard operation of the warning light, what is yours doing?
Paul Hunt

The wornoing light never comes on. But I did take the readings with a multi meeter. I will check it again tonight
bsf Fowler

It could possibly be a bad voltage regulator. That thingy that turns the alternator on and off according to how much of a charge the battery has. I used to have an alternator that would charge intermittently, or whenever it would feel like it.I replaced it with a rebuilt lucas unit thatburnt out a diode after about two years or so. At this point I got tired of spending $100 on lucas P.O.S. alternators and bought an AC Delco from Advance Auto Parts. It is from a 78 camaro, produces about 65 amps, charges all the way down to 400 or so RPMs. If my alt. light comes on then the engine has usualy died. The whole conversion cost me about $60, that got me an alternator, mounting bracket,and alternator pigtail. I have been more than pleased with this setup, no more dim headlamps at stopsigns!
Here is a link to a bosch conversion
http://members.aol.com/idbritcars/boschAlt.html
Here is the link to the delco conversion.
http://teglerizer.com/alternator/
I have been more than satisfied with my conversion, especialy since a rebuilt delco unit with core will cost somewhere around 26 bucks- can't argue with that, unless you are a purist.


Robert

The real bugaboo with some OEM alternators, especially early ones (from the late 60s or early 70s) is the speed at which they actually begin to create a charging voltage.

My GUESS is that some of those early units were not capable of generating the right charging voltage at lower rpms (say, 400-600 rpms) and so, they charged well above 1000 rpms, but at idle, the electrical system became dependent on the battery. OK in theory, I guess, but not too practical in the real world, given the uneven status of electrical systems and batteries from one car to another. And I think this is a possible reason why some conversions do work better than some early OEM units.

Even later Lucas units are capable of charging at lower rpms and have higher output levels, sometimes comparable to the conversion units. But I performed my conversion to a Bosch unit on my 68 GT and have been pleased with it too.

I have since come to think that had I replaced that very early 68 16AC alternator with ANY alternator made after about 1975, I would have obtained virtually the same results as with the Bosch or a Delco conversion. A Lucas 18ACR unit would have done just as well, without the hassles of making mounting brackets or chasing down a scarce and relatively expensive "imported" alternator.



Bob Muenchausen

This thread was discussed between 22/04/2003 and 23/04/2003

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