Rev Counter on a Positive Earth Car? - Change the car or the tacho?

C

chrisyork

Active Member
This post on behalf of a member who doesn't do internet.

He has an early positive earth 2000 and wants to fit the tacho and clock pod from a TC. Seems to me there are two approaches - change the car to negative earth or change the tacho wiring to suit the car.

My recollection is that changing to negative earth can be a bit fraught thanks to the dynamo and control box. I seem to remember it's necessary to "flash" the dynamo to get rid of residual field magnetism from the positive earth? Can anyone put some flesh on this and/or provide a procedure to change to negative earth?

Modifying the tacho witring to suit positive earth sounds fraught as well! Clearly no problem with the instrument bulbs or clock. The two other circuits are the power supply to the tacho and the signal cable to the coil. The power supply is taken from the 10V voltage stabiliser at the rear of the speedo. Also the wire to the coil I know likes to be on the right side of the coil. Will it all work simply by swapping the coil wire to the positive terminal (it normally lives on negative) so that it matches the swap on the power supply?
 
Hi Chris,

i put my bet on converting to negative earth, unless he is determined on originality.
The conversion is very easy and harmless.
Below is the procedure copied from a friend's website for ADO16s.
www.austinamericausa.com


"How to convert from positive ground to negative ground:
-Converting from positive to negative ground is very easy. Here are the steps:
1) Disconnect the wires at the generator (dynamo)
2) Swap battery connections around
3) Swap Coil leads
4) Swap Heater leads
5) Re-polarize the generator (dynamo)

Re-polarize the Generator (dynamo):
To re-polarize the generator, connect a jumper wire from the positive side of the battery and touch it several times to the small terminal on the generator. You will see a small harmless spark. The generator's magnetic field is now reversed."

Demetris
 
I'd change the car to Negative earth rather than try to convert the tacho. Apart from anything which is polarity sensitive (radios normally, can't think of much else offhand) the coil LT terminals need turning around, as well as the battery (thought I'd better say that :;): ) as well as new terminals for the battery,then IIRC flash the new live to the field terminal to repolarise the dynamo. (Personally I'd change to an alternator at the same time)
 
Thanks for that chaps. I've passed the comments on and he is now considering how much he values originality. His car is a '64 so the reve counter pod would be non original anyway.

Another solution would be to have the rev counter converted to positive earth - JDO their webpage could almost certainly do that for him.

Chris
 
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