It's a known problem on Fords of this age-there is a Service letter about it-basically the connections on the back of the alternator are replaced.IIRC changed from push-on to a screw connector-just the thick wire.

This problem is certanly alternator related.

If it's not a fault with the connections then I'm afraid it's a problem with the alternator itself and it will need to be repaired or exchanged.

You can measure the charging voltage across the battery with your multimeter, OP, which should be about 14.5 Volts. Turn all the electrically powered items on that you can think of (headlamps, heated rear screen, etc) and see whether there's very much change. There shouldn't be.

You should be moderately reving the engine whilst carrying out this test. Don't get anything caught up in any moving belts whilst doing so.

My bet is that the alternator will be found to be faulty, but, without the opportunity to actually test the car, it's impossible to be sure. I beleive that many garages and auto electricians will test batteries and alternators for free, or very cheaply, anyway. Perhaps this might be a good direction to go.

Might also want to consider changing the brushes? I also had thnis on an old 1977 Avenger with a Lucas 18 ACR alternator. New brushes fixed the problem.

Easy to change on 16/18 ACR alternators.

Q.