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MG MGB Technical - Diagnostic suggestions - charging

As I was driving home from work recently, I noticed that both the indicated engine temperature and the fuel level had suddenly dropped a bit. These are the only two gauges that are electric, so I thought it might be a charging problem. However, the red charging light did not come on, and there was no stumbling or other issues. Turning near home, I suspected that the signals were a bit more lethargic than usual.

The next day, I find that I can't start, the battery is flat.

I have not yet performed any diagnostics, and was wondering if anyone has good ideas. I can't think what might cause the charging system to work just well enough to keep the charge light off, but would drain the battery. Isn't the charging light on a diode circuit, so any drain of the battery should have been indicated?

I haven't done any further investigation, but it seemed odd enough that someone might know what had happened. Any thoughts welcome!

Thanks,
-Ed.
Ed Morse

addendum - 73B, HIFs, no bizarre mods.
Ed Morse

Sounds like your voltage dropped. Since the light didn't come on could be the alternator is still working. I've had batteries die suddenly probably due to internal shorts. I'd see if it can hold a charge or even take one before going much further.

Once I was pulling into a parking lot and the engine on an EFI car just slowly died. I got it parked okay. Everything was dead. I went to the store in the shopping area and bought a new battery and installed it right there. That was it.

Robert McCoy

I once had symptoms very much as you describe, Ed, on my 73 B. Running lots of errands one day, and each time I started the car the starter seemed alittle more lethargic. Finally just barely got the car started, so I went home. Red light never came on. When I tried to start the car at home again, it didn't turn the engine over enough to start it before the battery was totally dead. Due to the gradual death of the battery, I concluded lack of charging as the culprit, rather than a bad battery (although that probably was flawed logic) so I took off the alternator and took it in to an auto electric place. They diagnosed the windings to be bad, and rewound it and it has worked great ever since. He gave me an explanation as to why the red light never came on, but I couldn't follow it, and don't remember the official explanation. My guess is that the alternator output dropped at the same level as the battery voltage, thus leaving the light unlit.

Erick
Erick Vesterback

All I have to offer is the usual stuff. Clean the terminals, charge the battery, check charged voltage (should be about 12 volts), start the car, check voltage again (should be higher than 12 volts, maybe 13 or a little more).

Someone will probably refine these hints.
Tom

The gauges certainly shouldn't have dropped between chartging and not charging, that is what the voltage stabiliser prevents (or is supposed to).

Have you checked the alternator plug is fully in? The warning light will only indicate no charge if it is in. Does the warning light glow when you turn on the ignition? If not, remove the plug from the alternator and connect a ground to the brown/yellow wire (NOT the brown wire) and that should make the warning light glow with the ignition on. If it doesn't either the bulb is blown (unheard of) or there is a break in either its brown/yellow or white (to the ignition switch) wires. If that makes the warning light glow then the alternator is faulty. Most alternator faults *will* cause the warning light to glow when charge fails, but there are bound to be some where it won't.

Th engine running with no charge will drain the battery, and certain faults in the alternator can cause the battery to drain with the engine off, but usually with much smoking from brown wires.

But the only real test of whether it is charging or not is to measure the voltage on the brown wires at the fusebox. Ignition off and a good battery you should have about 12.8v. Cranking you should have about 10v with a good battery. Running at 3000rpm and no load should give about 14.5v. Running with all *factory* loads turned on will reduce this, but even at idle you should still have better than 12.8v.
Paul Hunt

This thread was discussed between 03/10/2008 and 04/10/2008

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