Dynamo troubles (a Lucas 3-brush model C35A)
Author: Bob Humphrey
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Its funny how windscreen wipers pack up in the rain, and dynamos pack up in the dark – there must be a moral there!
We had had a good weekend travelling about 150 miles up to Angoulême for the "Rallye Des Remparts" (15-17 September), then a lovely run of about 85 miles with the MG club of France on the Saturday and watching the racing on Sunday. The weather could have been kinder but the spirit of adventure kept us all interested. I am sure the weekend will be documented elsewhere, but we were the ONLY standard Seven there amongst 800 or more classic cars.
We left Angoulême on the Monday morning and after about 15 miles along the road (headlights on as it was foggy), the ammeter started to fluctuate from 0-15 amps like a thing possessed. Pulling in, I checked the wiring, cutout etc, but couldn't find anything amiss. As the fog had lifted, I reckoned that if the coil pulled 1 amp, we would easily make home (135 miles / 5 hours away) without the dynamo charging and using the starting handle instead of the starter. Just to make sure, I bought a 6v dry cell at the next bricolage shop.
We made it home comfortably, and soon had the voltmeter out and established that there was a problem with the field circuit inside the dynamo. I won't go into how to test dynamos as that has been well covered and if in doubt, look up on the net / Cornwall Austin 7 club. What follows is what I haven't seen documented before. Before removing the dynamo, note the orientation of the rotor arm and distributer in relation to the dynamo/engine – perhaps take a photo on your phone?
On stripping the dynamo down, the first thing I saw was the third brush to terminal wire had been rubbing on the armature, and had been getting hot as the soldered connection at the brush end had melted. The field coils seemed OK and the armature insulation OK, so I replaced the wire and terminal end. I was about to put things back when I noticed that the little insulating bush on the third brush was broken (black bush in photo) C35A017, and its spring shorting to earth. I made a new insulator from nylon and put that in place.
I started to re-build the dynamo and noticed that the plain bearing bush at the distributer end was badly worn with about 0.015" slop in it, so had to replace that. Interesting that the end cap has a hole in it for lubricating the bush, but it is capped off. Was this hole for plain bronze bearings which have now been replaced by Oilite bushes?
Back to the re-assembly and try as I might, the little woodruff key would not stay in position when I slid the distributer drive gear onto the shaft. The cunning plan to get around this problem was to clean the shaft and key, then hold it in place with a little super glue – worked first time! Putting the dynamo back and timing was as per other articles using the notes and sketch I had made before removal.
The cause of my dynamo woes was the disintegration of the small spring insulators on the dynamo back plate. These in turn shorted the 3rd brush to earth causing the wire to the 'F' terminal to heat up and sag onto the commutator etc. Moral – on your next visit inside your dynamo, replace these insulating bushes as they are probably 80 years old!!
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