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MG MGB Technical - alternator woes (continuance of topic)

Ok, many of you know about my alternator probs, its in the White smoke post. Basically my light comes on sometimes, an not others, but is charging fine (I assume). Heres how it has come, I still havent tested with my multimeter yet because, well, I simply forgot today to do it while I was working on the car. Anyways I found that If I choke it and it revs over 2500 on initial startup, the light turns off, it will also turn off if I drive it past 2500rpm when I get into first gear. Sometimes if it sits for a bit at idle the light comes back on. the problem is, even if the ligths on (say I start it without revving or driving) it still puts out power. I drove for like 40 minutes the other night with all lights on including brights, and sat in a lot for like 20 minutes, with the car idling, and no dimming occured. I was doing a ton of wire fixing today getting all the taillights and such to function correctly, and never ran the car during this time. When I was done, and turned the lights on with the car off, the headlights would barely light up more than a flashlight, but as soon as I started it they got ultra bright as they should, but the alternator light was still on.

Is there any connections from the light to the feed that I should check to see if it is JUST the light thats the prob? Thanks again, let me know if you need any clarification or info. I checked out the wiring diagram on the electrical help site, but didnt see any way the light would me misfunctioning with the alternator running correctly.

Also, it used to be a lot harder to get the light to go off, higher RPMs, and more warmup time was needed, and it would come back on under easy driving. Now it seems semi-quick, but not as quick as it should be... could this just be the alternator warming back up after not being used in 8 years? ( I dont know if that makes sense, but thats all I can think of. )
Bryan Corey

I had similar symptoms and eventually I had the Alt. itself tested. Replacement of the field diodes did the trick
























































































































































ian thomson

Bryan,
I will try to explain the system as I understand it. The red light is a warning for low voltage. Between the alternator and the bulb there are some electrical devices that sence the voltage. It is set to go on at something like less than 12.5 volts. It turns off at something like 13.5 volts. These are just guesses not exact specs. One the light senses a drop in voltage, at idle most cars do not produce the same amount of carging power, the light switches off. Once the voltage goes back up, depending on the alternator this may be 1000 RPM's or 2000 RPM's depending on how well it works the light turns back off. So you see it is possible for the system to charge, anything higher than 12.5 volts will charge a battery somewhat, while the light is still on, not at the 13.5 cut off yet.
There is a diode on earlier cars that can be replaced, but it would be probibally easier/cheaper to replace the older alternators unless you are very familiar with motors/alternator/electronis/ect.
As for the alternator, there should be 3 or 5 wires comming out of the back of it. One is noticibally larger than the others. This connects to the battery via the starter and is the one charging the car. The other wires do various things, one being the output to the red light. All you can really do here is check that all the clips in the alternator itself are clean and that the clip is clean and that there is a good connection between the two. From what you have said the alternator may just be old and not charging well anymore. If you get a voltmeter and check the voltage at idle and then at 2000RPM, 3000RPM you should not expereince a lagre voltage increase. This is what old generators(early sixties MGB's) had and these produce more current/voltage as the rev's increase. A good alternator should produce current/vlotage at low RPM's and should not increase too much as the engine rev's higher.
At this point you have two choices(you need to check the output with a volt meter at several RPM's): If the engine is producing current at low RPM's and the light is still on(I had this happen to me) you can unscrew the bulb and drive it untill it fails. Mine lasted another two years, although you will get no warning and may end up stuck on the highway. Or you can replace the Alternator and the light will only come on with the engine off. After I finally did get a new alternator I screwed the bulb back in and I have had no troubles since. If the alternator voltage is around 12 volts at idle and progressively goes to around 13-14 volts at higher revs your alternator is on borrowed time and I would replace. It will work when the car is run at high RPM's like on the highway, or on country roads, but in traffic the battery will have to be used. It can therfore become discharged and die, and even if it does not this strain will dramatically shorten the life of you battery.
If you do decide to get a new one and are low on funds like I was when mine went kaput call local alternator/starter rebuild shops. These places do most of there work to auto-shops, but most will still do a walk in and I have found them to be significantly cheaper if you find the right one. Some even have rebuilt alternators there and willjust swap out with you if your old one is intact with no housing or coil damage.
Hope this helps you out. I am not sure this is a completely correct explination so if anyone can help me out please do.

John
J Arthurs

Bryan,

There's a reasonably good description of how an alternator works, including the function of the warning light, at:

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

How do I take it out? It seems like all the bolts are hard to get to.
Bryan Corey

Bryan,

This would be a good time to invest in a service manual. Haynes is adequate and can be ordered through most auto parts stores. The Bentley reprint is available at Moss Motors or Amazon Books and is supposed to be better. Where most of the people on this board are quite helpful, when it comes to details of nut and bolt removal, they may choose to move on to something more challenging.

To answer your question further: Disconnect the battery ground terminal. Disconnect the wires from the back of the alternator. Remove the one adjustment bolt at the bottom front of the alternator. Draw yourself a diagram of any washers and spacers at the two nut and bolt combinations at the top front and top rear of the alternator. Then remove these nuts and bolts. Don't drop the alternator when you pull the bolts out.

Matt Kulka

duh, I didnt even think of checking my Haynes, stupid me, thanks though!
Bryan Corey

This thread was discussed between 24/06/2002 and 27/06/2002

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