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MG MGB Technical - Alternator Going??

69 B - Seems like my Alternator light is stays on longer and longer as I increase the revs. Used to go off around 1,000 rpms and now goes off around 2,000 plus, nearly 2,500 sometimes. When car is cold it goes off sooner (it was 52 degrees on the way to work this morning with the top down). #&*^% thing is pretty new. Replaced it 2-3 years ago and I don't drive that much...maybe 3,000 miles a year. Oh, once it goes off then doesn't come back on till around 1,000-1,200 rpms on the way back down.

Any similar experience out there or opinions? Thanks.

JTB
J.T. Bamford

Hi..

We seem to have had a few threads about failed recently rebuilt alternators recently.

Having got the ignition light to go out, does it stay out when you drop the revs to tickover ?.

Don
Don

JTB. Yes, it sounds like the alternator is going. I have had rebuilds (two) go out in less than a year. I also have had rebuilds last for several years in regular usage. The ACR series of Lucas alternators have an internal, semi-conductor, voltage regulator. There are some very poorly made semi-conductors available which do not hold up well. Since none of us know what quality of parts are used in the rebuilds, it is difficult to determine who is using good quality parts and who is using the parts from the low bidder. Its a five minute job to have the alternator checked out on the car and relatively inexpensive. Probably worth doing. Les
Les Bengtson

Gents,

My rebuilt alternator is gone after just 18 months. I found that NAPA sells their premium rebuilt alternator for my "B" with a lifetime guarantee for $109.00 plus a $7.00 core charge. A deal that's hard to beat. See www.napaonline.com to check it out.

Cheers, Doug
Doug Keene

Les,
Checked out on the car? Meaning just put my multi-meter on it or are you talking about a test the parts place can do? Thought you had to remove 'em and take 'em in. ??

Doug,
Thanks for that. It was kind of what I was thinking. Find one with a guarantee. I HATE doing the work again much less spending the money again. Easy job but I don't have much time anyway and would rather spend it improving the car instead of on remedial c_ _ _.

Don,
Tickover? Is that "idle" in "Amurrican"? If so then "No". When it drops back to my normal idle at about 900-950 RPM it comes back on. Is that symptomatic of something specific? Thanks.

JTB
J.T. Bamford

Generally speaking, I and some others have had fairly consistent good luck (notice all the qualifiers to this?) with rebuilt alternators from Autolite. They rebuild both their own brand AND others such as Lucas, Bosch, Delco, etc. and seem to do a good job most of the time. However, they are the premium rebuilt with the lifetime warranty where I live and you pay about what Doug mentions for them.

Many rebuilts are done overseas or across the border, container loads going to the far east, or trucked to Mexico where it is the discretion and oversight of the rebuilder that guarantees how quality is determined.

One thing that makes judging quality harder these days is that often better parts come from places we might not expect, and so, labelling goods from anywhere bad/cheesy because of bad past performance 20-30 yrs ago, is not always relevant anymore. Just as we have always done in this country, now countries like Malaysia and China can now turn out both excellent and sleezy copies of the same parts - you (or the contractor for rebuilds) pays your money and takes your pick.
Bob Muenchausen

Hi J.

Yes, tickover is 'real English' ;-) for idle.
Apologies for not spotting the answer to my question in you original question.

The alternator is effectively three phase, a bit like having three alternators in one unit.
I am no alternator expert, but my guess is that one of the phases has gone open circuit.

Have you suddenly aquired an engine speed related whine on the radio ?.

Don
Don

John. While it is possible to check alternator voltage output by using a multimeter, the "real" charging system test equipment will check out both the charging voltage and charging rate (measured in amps). This test is performed with the alternator on the car, only takes a few minutes to perform and is far more extensive in the information it produces than what the average hobbyist can do by using a multimeter. A multimeter can tell you if the alternator is bad, but cannot guarantee that it is good. (You can have the correct voltage output, but insufficient amp output which will not show up on a voltmeter.)

It is also possible to remove the alternator from the car and take it into a auto parts store to have it checked out for free. Two problems with that. First, if the alternator is good, you have removed and replaced it, wasting time. Second, while I can get the local Checker store to check out my Lucas alternators, some other posters have mentioned that they could not find someone who knew how to check the Lucas alternators and could not have the "free" test done. I prefer to drive down to my local mechanic and pay him a few dollars to run a charging system check, then give me the print out from his test machine. At that point, I will be able to determine how the alternator is responding both under load and not under load and at various rpms. Worth the small amount it costs, at least to me. Les
Les Bengtson

With the light going off later and coming back on sooner it is possibly a voltage regulator problem, a missing phase would normally cause low output and a increasingly glowing light as the electrical load increases. A simple voltmeter check with everything switched on gives some idea, but the workshop manual output check consists of bypassing the regulator and measuring the resultant current, which needs a 30amp plus ammeter (which I don't have) and involves significantly higher voltages than normal (which I'm not prepared to do). An alternative (ho ho?) would be to add a known external load to the system and confirm the output voltage was still in limits, but how many of us have a 0.5 ohm 400 watt resistor kicking around? I have heard several cases of testing houses declaring an alt as good but replacing it anyway clears the problem, so I suspect, as with many things, there are testers who have a proper test rig and testers who just stick a voltmeter on it.
Paul Hunt

Hi all.

I use a 'loose' 30-0-30 dashboard type ammeter when I want to measure more current than my multimeter can handle. It isn't very accurate, but it is usually good enough.
Be careful not to short anything, high current shorts can be quite dangerous.

If you need a low value high current resistor you could rig something up using diesel glow plugs (from the breakers).

I personally would just get another alternator from the breakers and use it for substitution tests, you can also use it to tide you over until you get a new one.

HTH.. Don
Don

This thread was discussed between 08/10/2003 and 11/10/2003

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