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MG MGB Technical - Alternator Light

Has anyone any ideas as to why my alternator light starts to glow when I reach about 5000 rpm. it is so dim it can only be seen at night. The car also stumbles a little at high 5-6000 rpms. My car is a 1970 mgb roadster.
JH Clark

Sorry I don't know, is your alternator standard with standard pullies?

The electrical connections to all in good condition and all clean, secure and protected?

Ignition switch and earths all as above?

Any misfires, tacho twitching?

Light on coasting or engine pulling?

Are the two things connected or separate and just noticed together(?).

Those that know will be along





... that's if you've not scared them away with putting driving at 5-6,000, in a MGB, you'll be thrown out of the club !! 😮

🤣
Nigel Atkins

Alternator light glowing as the revs are raised, especially if it gets brighter with more electrical loads like headlights etc. turned on, is an indication of diode pack failure in the alternator. Output can be excessive which can damage other internal parts.

Not likely to be directly associated with the stumble.

When you say 'dim', is that when first turning on the ignition before starting the engine? If a roadster then that could be sunlight turning the dash-mounted lens opaque, the glass 'jewels' in the earlier tach and speedo suffered less from that.
paulh4

Could also be worn/stuck alternator brushes losing contact with the slip rings at revs--this will/can also cause a missfire at high speed revs

willy
William Revit

Doesn't that just reduce the output so the ignition is powered from the battery?
paulh4

yep-12v instead of 14ish and the coil will break down under load at high revs at that, specially if the plug gaps are a tiddle on the wide side
William Revit

I get the theory, but if the V8 can run very easily into the red with half the points closed time of the 4-cylinder I'm surprised that makes a difference.
paulh4

Not a theory Paul, a fact - haven't you ever driven a car with a dead generator/alternator, turn the lights on and you go backwards real fast
William Revit

Never had a dead one (so far ...), but I did leave one unplugged once. That ran for a good few miles before the voltage drop was enough to cause a problem ... at which point I instantly realised what I had (or hadn't in this case)done. Unless the battery is already weak to begin with, lights being on will only drag voltage down relatively slowly, at which point it probably wouldn't restart as mine didn't. I got the impression from JHs post that this had been happening for a while so was charging to some extent, but maybe not. Neither is it clear that his comment about the 'so dim' warning light was at 5000 or whether it was generally i.e. at switch-on and he had to be in the dark to see it.
paulh4

When I built up my midget using a recon Alt I had the dim alt light (not pushing it then as new engine) - sent it back and confirmed dodgy diode.

R.
richard b

You would expect an old pushrod engine to stumble at 6000 rpm.
Roger Walker

This thread was discussed between 11/06/2021 and 15/06/2021

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