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MG MGB Technical - 1969 GT Alternator Smoke escaped

A friends car, it is up on axle stands in a barn for the winter and he replaced the battery last week looking to get it going for the season, the battery (single 12v) had been on a conditioner where there is mains available. There was some smoke and now the ignition light is permanently on. Would this happen if he got the polarity wrong? also the chassis number starts GHD4 so it may be still fitted with an early alternator with separate regulator, i wont see the car until Monday. I looked on the B+G website and they only carry the 18ACR now, I imagine there is a plate that tells you which is which. how hard is it to swap them out if it is the old style unit? I can post some pics on Monday if it isn't just 3 bolts, over to Summit at Maidenhead and job done
Stan Best

Stan
Yes that sure can be caused by polarity issues
The guy that lives next door to me had his battery hooked up to one of those little solar battery minders for two months while he was away
When he got back home he noticed the leeds were around the wrong way but thought it would be ok
Fitted the battery into his car, turned the key on and poof- wrecked the alternator, the ECU, the radio and the body computer-----expensive mistake in his case being an almost new car
With an MG it has probably fizzed the alternator and possibly the tacho and maybe radio if it was turned on
Rotten luck that
willy
William Revit

Stan. The 68 used the 16AC alternator with a separate regulator on the right inner fender. Whether any of those carried over into 69 production is an unknown, especially in places, like the UK, where cars were registered as a later model year when sold.

Years ago, I switch from the 16AC to the 18ACR by simply using a later model wiring diagram and some general guidance from Paul Hunt. System has worked well for quite a number of years.

Les
Les Bengtson

Indeed, it sounds like the positive and negative cables, at the battery, somehow got reversed. From past experience, this will damage the alternator and cause a short circuit in the wiring harness leading from the alternator to the starter motor. The harness must be opened up and the damage will be obvious, but easy to repair with some new short lengths of wire, solder and electrical tape. As Les mentioned, it may be the earlier five wire alternator that is lo longer available. The later Lucas 18 ACR is a direct replacement, but it requires an adapter to switch it over to three wires. It's available from Moss, and many other places I would imagine, for under $5. RAY
rjm RAY

Thanks a lot guys, that is really useful, I know zip about MGB alternators, my car is still dynamo.
The existence of an adapter is interesting. I may not need it, the car was rebuilt into a heritage shell 4 years ago with a stage II 1950cc engine so I may get lucky and find the dead one is an 18ACR. It may take a bit longer to find the damage in the loom and fix it.
Stan Best

Stan, the electrical damage, in the loom running from the alternator to the starter, should be relatively easy to repair. You'll most likely find three wires that have melted together. Separating them and replacing the damaged sections, with new wires, should take no more than an hour. Good luck with your project. RAY
rjm RAY

67/68 cars i.e. Mk2 before roadster No. 158371 and GT No. 158231 used a 16AC alternator with external regulator on the inner wing, which looks a bit like the control box for dynamos.

Unlikely to get one of those now, and both the alternator and the regulator may well have been damaged by incorrect connection.

You might like to look at this which shows the original 16AC wiring http://www.mgb-stuff.org.uk/charge2.htm, and this for conversion to the later 16/17/18ACR http://www.mgb-stuff.org.uk/charge2a.htm.

Incidentally the early 16ACR alternators had five wires on two plugs. The 16AC had four wires, maybe two plugs, I haven't seen one.
Paul Hunt

Thanks all, I got an 18ACR alternator at Summit Maidenhead when I was that way and went over to check out the car today. The battery was the right way round and the fault was easily found, removing the small spade put the ign light out when the engine was running and the loom looked fine. Then when I pulled on the 2 larger spades the wire just fell out of one of them and the terminals on the body were loose. The smoke must have been from the high resistance. New alternator on and a replacement terminal crimped on properly and all well, possibly the poor crimp caused heating which damaged the terminals. Interestingly although there are 3 wires the 2 high current ones are common and +12v so the return must be via the chassis.

Stan Best

Nothing like a poor connection to ruin your day. Fortunately, no major wiring damage was done. RAY
rjm RAY

Although several diagrams show alternators with -ve terminals the earth return was always via its mounting points to the engine and from there to the body, just like the battery.
Paul Hunt

This thread was discussed between 06/02/2016 and 13/02/2016

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