Morris Minor Owners Club

I've been having trouble with Fenchurch's battery gradually running down after a few days of driving to and from work with the headlights on, or after a long night journey. To cut a long story short, I've traced the problem to the cutout contact adjustment on the voltage regulator. If I set it such that the maximum battery voltage is 14V with the headlights off, then when I turn the lights on the maximum battery voltage drops below 13V and I get a constant small loss on the ammeter even at high revs.

If I turn up the cutout adjuster screw so that I get a maximum of 14V at the battery with the headlights on (enough to put a charge into the battery even with everything switched on!), then when I switch everything off the maximum battery voltage goes up to about 15.5V - which sounds a little too high for comfort. I tried adjusting it using the procedure described in the workshop manual, and that also gave me a 15.5V cutout with everything switched off (and a healthy charge with the lights on).

Why is the cutout voltage varying so much with the amount of load on the system?

If you have an alternator you don't need a regulator, or am I misunderstanding?
John

NZJLY wrote:If you have an alternator you don't need a regulator, or am I misunderstanding?

I've still got a dynamo.

Could be the battery on the way out maybe? Dodgy cell or somesuch? Might be getting it worth checked?

I'm pretty sure it's not the battery because it still holds plenty of charge, the problem is the voltage the regulator cuts out at varies depending on the load. The way it was set before I started adjusting it, the dynamo was able to charge the battery with the lights off but when they were on the regulator cutout voltage dropped to below the battery voltage and there was a constant discharge of a few amps. After adjusting it, it charges fine with the lights on but the voltage is worryingly high when they're off.

I've got a spare (used) regulator here that I might try fitting tonight.

Well FWIW been mucking about having a big battery sort out over the last week or so, and have seen some really strange goings on regrdng charging. One even reversed polarity !

It's safe to set the cutout to 14.8V with no loads (typical alternator output is 14.5 to 14.8V) This setting should give you about 13.5V with headlights on. Adjustment must be completed within 30 seconds from cold start, or the shunt coil will cause a false reading.

FWIW Standard Lead Acid Battery life is normaly between 4 to 5 years depending on battery quality.

This is why the later Morris 1100 etc cars had a better 3 bobbin voltage regulator - to control the charge under all circs. But I seem to remember (and this is from way back) that the headlight connections are supposed to be taken from a specific terminal on the regulator - so that extra charge is automatically pulled from the dynamo only when the headlights are in use. This is from 50 years ago - so I may be talking mince - and it may just be for the 3 bobbin regulator - but it does ring a bell with me! Meantime - try your spare regulator and see if it's any better. A method I used on my 'rally' Minor was to have a switch which fed full battery voltage to the dynamo field - and this was used when on a rally with extra lights etc - but normally was kept off allowing the regulator to control things as normal. You really shouldn't need to do this for just normal headlight use - but it could be short term solution for you. If you do it - you MUST switch off when running without lights - and before stopping the engine !

Ah, I think you're onto something there. The cutout bobbin has both an inner winding that measures the voltage and an outer one that measures the dynamo output current. The dynamo current will increase when there's more load on the system, and the magnetic field from the current sensing winding will add to that from the voltage sensing winding, lowering the cutout voltage. The cutout voltage will be inversely proportional to the dynamo load, which is pretty much exactly what I'm seeing.

I think I'll try reducing the off-load cutout voltage to 14.8V and see if it's still able to charge on-load.

Maybe just need to give it a few minutes to settle down after each setting - pretty sure there is some tempearture compensataion in there somewhere.