Car battery polarity

the 1948 Ford prefect was 6v as well.

Indeed. Up to the 1950s, Positive Earth was the normal system in the UK.

If I remember correctly, up to the early 1960's, at least in Austins.

nib

Not since around 1958

I'm pretty certain my 1962 Cortina was +ve earth

I've just finished fitting a dashcam to one of my vintage cars which was made in 1955 and has positive earth. It's just incredible the amount of extra grief it gave me. I'm just so used to doing stuff with negative earth I found having a switched polarity a complete mind-f*ck and so many gotchas to catch you out. You can't use the ciggie lighter plug adaptor for a start and it just gets worse from there.

Supposedly (I see it described but not done it myself) old cars with +ve earth, dynamo charging and no polarity-sensitive devices can be swapped over. Just by swapping the battery around, quickly flashing the residual magnetism in the dynamo by connecting the flipped battery to the dynamo F terminal and starting up. I though of this on our old Austin 1100 and:

The dynamo will produce the other polarity because its field windings will be reversed.

The starter motor having field windings will go the same way.

Instruments were heated bimetallic meters so will work on either.

The electric fuel pump was an unpolarised solenoid so would work either way round.

The ignition will work on either polarity.

All the lights were OK either way round - the flashing indicators used a hot-wire relay and no electronics.

The charging control was three unpolarised relays so would work either way.

As far as I can remember, all of the other motors were field wound: wipers, heater fan.

So back in 1964, no electronic components, no permanent-magnet motors.

As soon as alternators came in with diodes determining polarity, everything went -ve earth.

nib

I had a radio

Wasn't there some thought that +ve earth encouraged body work corrosion. Or was that just a Ford/BL excuse:-)

On 17/07/2024 20:15, charles wrote: ... >> So back in 1964, no electronic components, no permanent-magnet motors. >

It's a long time ago, but I'm pretty sure I can remember car radios of the time having polarity switches, with careful control of earthing on aerials.

I fitted an old radio into my 1966 Spitfire, positive earth. That radio was old enough to have valves for the RF stages and transistors for the audio. It was modern enough though to be all one box, not a dashboard unit plus a box under the bonnet!

nib

I can remember ford 'sit up and begs' and morris minors with +ve, but it all changed around 1960

Dont remember any cortinas that way though.

VW Beetles had 6V electrics up until the mid 60s

I've changed polarity on a couple of cars.

One thing which you had to do was to flash the field winding on the dynamo before starting the engine, otherwise the large current inrush could damage the relays in the voltage regulator.

Other than that, things operate exactly as you said.

Most UK cars used positive earth well into the 60s.

In the late 60s I was involved in swapping (I think) three cars from +ve to -ve earth. As indicated above, it basically involved simply momentarily zapping the generator field winding to reverse the residual magnetism, and reversing the battery.

IIRC, with the traditional points ignition the only impairment is that the ignition spark is slightly lower. This is because when the correct way round, the back EMF of the ignition coil primary adds to the EHT of the secondary. When the battery is reversed, it gets subtracted. I think you lost about 600 volts. [But might be wrong.]

In the early 70s, it was possible to buy positive earth alternators to retrofit to older cars. I fitted one to my 63 Sceptre.

I did also once reverse a dynamo polarity as described to suit a new radio. Early car transistor radios (indeed all transistor radios) used PNP transistors and therefore needed a positive earth, unless they were switchable. The only valve car radio I ever saw used an inverter rather than 12V valves (which did exist but weren't very good).

Quite likely. I was being conservative, without checking the actual dates.

In message <v78mvr$1snrq$ snipped-for-privacy@dont-email.me, at 16:11:55 on Wed, 17 Jul

I had a Series 2 Landrover with 24v electrics.

Twice as much to go wrong!