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MG MGB Technical - 68 ignition light flickers

I have a 68 MGC (same elect system as the B) -- 16AC alternator with external regulator on fenderwell.

Problem: ignition light flickers. Light is NOT on at idle. Starts to happen at higher RPM, say 1500 & up, light comes on dim at first, building to brighter glow, then goes off. Happens about once every second or two. It's pretty consistent.

If I turn on ANY electrical device, the ignition light stops doing this. Even just the brake lights "fix" the ignition light flicker. I normally drive the car with headlights on for safety, and the ignition light never flickers when headlights are on -- the ignition light stays off. The flicker only happens when I am driving with no electrical loads imposed on the system (well, no lights on, at least).

Also, it goes away at idle -- ignition light is off, no flicker. If I drag the clutch to reduce engine speed to, say, 500 - 600 RPM, then the light will come on steady as expected.

Any ideas? Battery terminals are super-clean. Maybe clean up terminals on alternator, regulator, fuse box? Or is this a common problem with the 16AC external regulator?

Mark Palmer
S.M. Palmer

Mark. I am no expert in this area. I have a 68 BGT which came with the 16AC alternator and external voltage regulator. Problems the first two years of ownership. Had the alternator rebuilt by an alternator shop. Replaced the voltage regulator (which is soldid state, by the way) with a new one. I finally decided that the best solution was to replace the rare 16AC alternator with a more common Lucas 18ACR which I did. That corrected the problems I was having which were similar to yours. Perhaps Paul Hunt can explain what is happenings. Les
Les Bengtson

Mark, I seem to remember the same on mine when the brushes were going. It was a very cheap fix.
Steve Postins

It could be the brushes - easy to check, simple and cheap to fix. I had similar symptoms, but didn't diagonse to the same level of detail as you did - I took the thing apart as soon as I noticed an occasional flickering and I was sure the belt wasn't slipping.

My brushes are definitely worn, one in particular is short enough to only be grazing the slip rings. Won't know for sure if this solves it till the weekend, but I'm pretty confident - right now anyway :).

Found some great info here: http://www.mgcars.org.uk/electrical/alternator.html

and was able to verify the guts of the thing was okay from this: http://www.mgcars.org.uk/electrical/rectifiea-service.html

Good luck!
Mike Polan

The big thing is what is your voltage doing? Another thread elsewhere has similar flickering, going off with load, but whilst the voltage seems normal when the light is flickering it seems to be off-charge altogether when any load is applied. The Workshop Manual doesn't cover how the indication varies with changes in load but says a glow increasing with revs can be caused by an open-circuit live-side output diode which can damage rotor windings, regulator output stage, overheat the brush boxes, and blow the warning light. With pulsing on and off fairly consistently at about 1 sec intervals I'm inclined to think it is an electronics problem rather than just worn brushes or dirty slip-rings, even though at least one does seem to be well worn.
Paul Hunt

Rechecked the rectifier pack on mine, found a shorted diode. Swapped it for new along with the brushes. Just back in from re-installing it - all back to normal now. I'm pretty happy, I hate going for rebuilt/exchanges. Hopefully you can get yours sorted too Mark.
Mike Polan

This thread was discussed between 21/02/2005 and 24/02/2005

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