Prior to my recent trip to France I noticed that the low battery light was coming on. On my Acewell dash this is set to come on at 12.5 volts and off again at 13.0 volts. The alternator warning light extinguished normally after starting so there was no issue with the excitement of the field, there was another cause.
I ordered a replacement set of brushes for my Lucas ACR alternator just in case they were worn. When I took the back off the alternator and dismantled the brush box I discovered that the wire attaching one of the brushes to the terminal had broken. I think that the current was being carried by the spring alone, hence the erratic charging of the battery.
Replacement of the brushes is a fairly straightforward task. I also did a quick health check of the nine diodes with the multimeter. Then a quick test drive proved that the full 14.2 volts had been restored. Then it was off to France for the weekend. All was okay until after arriving in France I noticed the low battery light was illuminated again. Oh no. Anyway, long story short I managed to nurse the battery through the weekend and I made it back to the UK without the need for a breakdown truck - phew!
So, alternator out again, back off and then I discovered this:
The red wire had snapped off from its ring terminal. I suspect that I had trapped the wire when previously replacing the back cover of the alternator. I soldered a new ring terminal on and carefully refitted the back cover. Testing proved that the full 14.2 volts had been restored. Hopefully, there are no more alternator problems.