Changing early cars with positive chassis to negative chassis

[John A. Ross]

Changing early cars with positive chassis to negative chassis

A process spoken about but very rarely done these days, however it is a procedure that you SHOULD do if fitting another dynamo from an unkown source as you never know if it came from a positive or negative chassis car.

There is more to it on a conversion than checking a unknown dynamo however the bits "in the middle" are the same.



The page in the service manual states (see manual page HERE)

Round terminal batteries, take note the terminals on postive and negative are DIFFERENT sizes ! and sometimes marked + and - and please if you change these fit new terminals correctly marked for the fella that tries to fix it after you !



TO CHANGE POSITIVE TO NEGATIVE CHASSIS

• First disconnect the battery
• Fit your new battery terminals if needed but do not re-connect the battery at this time.
• Remove the "D" terminal from the rear of the dynamo (it is the big one)
• Remove the "F" terminal from the rear of the dynamo (small one)
• Make sure the "D" and "F" are safely tied away and cannot short to chassis ! as sometimes the insulation sleeves are lost and the terminals are bare.

GENERATOR RE-POLARIZATIQN

A replacement generator (dynamo) of the type described in this Section is suitable for use on either positive or negative earth systems, provided it is polarized to suit the vehicle's electrical system after fitting.

This is the point you need to change the battery terminals around, in the case of rectangular terminal batteries it is not so bad, for round terminals however note the terminals are NOT the same side and marked, as mentioned above the cost of new terminals is buttons and they would be correctly marked.

Once you ensure the battery terminals are in order you can then connect the battery back up, negative to chassis and positive to solenoid

To do this, fit the generator to the vehicle but do not at this stage connect the cables to the "D" and "F" terminals.

Determine which battery terminal is earthed on the vehicle and then temporarily connect a jumper lead to the battery positive terminal (fer negative earth systems) or negative terminal (for positive earth systems).

We know now the car was positive chassis so now we have changed the battery configuration so the negative terminal is connected to the vehicle chassis

Make sure the battery is secure and the terminals are tightened

BE VERY CAREFUL DO NOT ALLOW THE END OF THE LEAD
IN THE NEXT STAGE TO TOUCH ANY METAL AS IT IS CONNECTED
DIRECT TO THE BATTERY AND YOU WILL SHORT OUT
THE BATTERY AND CAUSE FIRE !

Connect a small temporary lead from the positive battery terminal long enough to reach the "F" terminal on the dynamo

Flick the other end of the jumper lead several times against terminal "F" this serves to re-polarize the generator.

The temporary connection can now be removed and the original cables connected to terminals "D" and "F".

Thats it, you should hear sparks from the F terminal as you "flick, I usually do few senonds on few seconds off then a quick few flicks.

[John A. Ross]

NOW the extra bits

Check the polarity of the heater blower and the horn, these need rewired or FUSED (see other article)

[PMCG]

I've been trying to check the heater blower polarity but having some difficulty, can you point me to where it is? :)

[John A. Ross]

Your bonkers, no blower, i will bring a fan down tomorrow and fit it, told you I can fit enough of a fan to work without you even seeing it

[Samuel B Saunderson]

Good article! Should be of great interest to owners of very early cars.