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MG MGB Technical - Alternator 'reds'

I had a continuously lit alternator light on my 70 B. I replaced the alternator but am still seeing "red". When the ignition is turned on the light glows brightly as expected. When started the light dims and will go out @ 750 RPM. Lower than that it glows lightly. Higher, it continues to glow lightly through higher rpms but doesn't really get brighter. Any ideas on how to get the red out????

Thanks,
JAP
John Pflugi

This is mentioned in the archive a few times. The original Lucas alternators don't cut in until 900-1000 rpm or so. If you manage to tune your car really well, you can idle comfortably below this, however, and you end up with the red light glowing a little. The solution, as far as I'm aware, is to either: 1) live with it, if the car doesn't feel like its about to die; 2) raise your idle ; 3) upgrade to the Bosch alternator. I did, and I'm happy with the results - no more throbbing headlights at idle!
Adam Birnbaum

John, Check the archives. There are many threads about this problem. I have the same issue my 73 MGB. The archives gives detail inspection information about amperage and testing methods. Most of the time it is either a ground or bad diodes in the alternator. Good luck.
Robert Browning

I had my alternator repaired and it went again after two weeks. Red light flickering and then stayed on. I went back to the guy who fixed it and he admitted he hadn't got the right diodes and had cobbled soemthing together. To be fair to him it was a Saturday and I needed the car for the Sunday. He said to return the Unit and he would fit new diodes and then it should be "as good as new".

John
J.A. Gearing

Whilst the alternator only *starts* charging when the revs are raised to 900 or so rpm, once it has started it will continue to do so down to about 600 rpm or so under all 'normal' electrical loads, so once started and blipped over 900 it should charge at its idle speed of 750 or whatever with no warning light. It should switch off and on at those speeds, and not flicker or glow dimly at any speed. If it does either of those you still have a problem. Yours sounds like another faulty alternator, not unusual for rebuilt units if that is what it is. You need to measure the voltage on each side of the light when it is glowing dimly. The light acts like a pair of balance scales and glows if there are different voltages on each side, going out with the same voltage on each side. In theory the alternator puits out 14.5v on the brown/yellow to one side of the warning light and the same voltage on the brown to the battery and the rest of the cars electrics, including via the ignition switch and the white to the other side of the warning light, hence no glow. But if there is a bad connection some where on the white side its voltage will be lower than the brown and the brown/yellow at the alternator. If there is a problem in the alt the brown/yellow could be either higher or lower than the brown and the white.
Paul Hunt

I have a question for you, then, Paul, about charging below 900-1000 rpms. If a seemingly good alternator charges a battery at any speed above 900 rpms, and maintains correct charging voltage throughout the rest of the rev range, is it faulty if it does not then stay at correct charging levels once it falls below 900 rpms?? Or is that an indication that there is an overload or some other problem in the system?? I ask because for the life of my first Lucas alternator (a long lived rebuilt which seemed to be OK in all aspects except low idle charging - also a 16AC with separate VR) it would not handle loads effectively below 900-1000 rpms. This was the impetus for my changing over to the Bosch alternator in the first place. I am curious to know, as it may be worth noting for anyone going to my Conversion site so that they may fix what they have, if possible, rather than doing a conversion that may not really be necessary. Thanks!
Bob Muenchausen

It depends on the load, but the max charging current of the Lucas (and V8 Delco) does drop as the revs drop and therefore the system voltage drops below 14.5v. But even at idling speeds i.e. 750rpm it should still be capable of holding the system voltage above battery voltage (12.8v) to some degree. I've monitored both cars system voltage at idle with lights, heater fan and HRW on the V8 and been satisfied that the battery is not being discharged. I have heard one person comment that he sat and watched his lights dim right down and flatten the battery in a long hold, but quite frankly I doubt that comment was strictly true. Would anyone be so daft as to watch it happen without doing anything about it, like turning off headlights or pulling the choke to fast-idle? And it would have to be a helluva long hold.
Paul Hunt

In my case, Paul, the "long hold" was night time, dead of winter, snow storm, and every high current thing on - wipers, heater blower, rear window defogger, head and running/brake lights, and ignition. In that instance, I too could watch as the lights dimmed, the fan and wipers slowed way down and the voltage gauge registered about 8.5 Volts! Eventually (about 30 - 40 seconds like this at a stoplight, -25F temps) even the engine began to bog down and only nudging the accelerator up over 1000 rpms really helped to get things back to Normal and keep the engine from quitting. As you know, I then changed to a Bosch unit (tho it could have been any other of higher output at low rpms, really) and the problem went away for good, even before installing relays for the headlights and rear defogger. FYI
Bob Muenchausen

I am told that there is another retro fit alternator from a Mitsubishi. Can anyone help me with this?
I am in the midst of doing the Fiesta Bosch retro.
But the Mitsu alternator is even more reliable and LIGHTER!
Safety Fast
Dwight
Dwight McCullough

Dwight,

if you find out more info on the Mitsubishi alternator, please share. All the other conversions (including my Ford Fiesta Bosch one) are dealing with OLD alternators. Use of a newer or current production alternator would probably bring us several advantages besides just lighter weight. I note that one of the British suppliers, Cambridge Motorsports, I think, sell what looks to be Mitsubishi or Hitachi alternator of recent vintage for our cars, which, if true, says it can be done, and probably for good reason.
Bob Muenchausen

This thread was discussed between 18/08/2003 and 21/08/2003

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