Negative Or Positive Earth

[kevin the chicken]

Nothing to do with kit cars but here goes.

After my 1956 fordson major radiator sprung a leak and needed replacing while I had the front end off I decided to change the fan belt and whilst doing so noticed that the dynamo bearings were a bit noisy so decided to replace it as they are not an expensive item. After a couple of hours of wrestling with rusted bolts I have fitted it all back together bar changing the connectors on the loom which fit on the dynamo when I noticed it was marked negative earth. This tractor is the only thing amongst my extensive fleet which is positive earth. What is involved in changing it to Negative? Being a diesel it only has lights and a horn. The dynamo was one described as being suitable for all fordson majors, some Fergusons etc.

An easy fix would be great, it's cold out there.

[fry61]

Just a change-over of battery connections as well ?? Think preference for negative earth is to reduce electroletic ? corrosion.

 

Check instrument polarity as well -- maybe a more difficult change-over here if positive earth case.

Edited January 6, 2017 by florin metal works

[IanS]

The dynamo is not intrinsically positive or negative earth.

If it is a lucas with a seperate mechanical voltage regulator (some times called a control box) then all that is needed to change polatity is to flash the field winding.

Positive earth was comon in the late 50s through the early sixties to allow the early transistor radios to work.

The dynamo starts chargeing useing the residual magnetic field, if the residual field is wrong the dynamo will fairly rapidly discharge the battery.

Flashing the dynamo (which should be done every time the dynamo is taken to bits) involves taking a wire from the battery live (negaitve if you have positive earth) to the field terminal on the dynamo and leaving it connected for about one second to establish the correct residual field.

[kevin the chicken]

I like the sound of this! It is a lucas dynamo and the control box is indeed separate, behind the instrument panel. There are two terminals on the dynamo, one larger than the other. Which one would be the field Terminal?

[kevin the chicken]

I could have answered my own question. The terminals are marked. All done and reconnected. Thanks very much for your help.