Positive or negative earth?

#1
I have taken temporary care of a P6 2000 registred in may 1966, with a suffix C chassis number.
The car's been off the road since 1983 and last time the engine ran was in 2005.
Amazingly the engine hadn't stuck, and I'm now trying to make it run, but there are some snags.

What I'm trying to find out first is, if the car's got positive or negative earth.

As a suffix C it should be positive, but the generator's been replaced by an alternator from SEV Motorola.
How can I find out if + or -? Below some findings, hopefully they can be understood.

- The battery earth cable clamp was missing.
- I replaced this with a new cable intended for negative earth.
- The battery cable to the solenoid is black and the clamp is a tad too small to fit the posititive battery pole.
- In spite of this, I connected the battery using negative earth. No sparks or smoke, and amazingly the starter turns the engine. The distributor rotates the right way too.

I'd much like to know, before I go further; does the starter turns the same way regardless of polarity.
The alternator is confusing too - were there ever alternators for positive earth?

I can also mention that some electrics worked: horn, windscreen wipers, headlamp flasher, fan and interior lights. And in case you wonder, the car will probably have to be scrapped due to rusted sills, and who knows what other horrors, though amazingly the floors seem ok, as do the doors and suspension pick-up points.
Rather sad though.....
 

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roverp480

Active Member
#3
Some alternators can be insulated return so don't earth through the body but by a separate lead, also yes there have been positive earth alternators ( very few). Connecting a alternator the wrong way usually damages it, so if yours works and charges, I would suggest it is connected the correct way. Can you see if there are any markings on the alternator connections?
 
#4
Thanks for your help! I did manage to find evidence of SEV/Motorola selling alternators for + earth in the sixties. After connecting the battery the correct way, + earth, a lot more electrics sprang to life, including most of the lighting, and the starter still turns the right way. The use of a test instrument revealed that the ignition coil HT circuit is defective, so a new coil is the next step. Perhaps not unusual, but still a bit surprising on an old wreck is, that most of the ignition system seems to be original parts, for instance the HT leads are stamped Lucas Suppressed blur-blur (!)
I'll probably have shot the alternator, but won't know for certain until the engine is up and running.
 
#5
polarity wont effect what does and doesnt work, we have converted numerous cars from positive to negative earth, the only changes needed baring swapping the battery round is re-polarizing the dynamo ( or changing to an alternator) and switching connections on the ammeter
 

harveyp6

Well-Known Member
#6
polarity wont effect what does and doesnt work, we have converted numerous cars from positive to negative earth, the only changes needed baring swapping the battery round is re-polarizing the dynamo ( or changing to an alternator) and switching connections on the ammeter
And switching the LT wires on the coil.
 
#8
The alternator must be a later addition, I'll bet the car was converted to negative ground when that happened.
Sorry, but this doesn't seem likely in view of what I found out about the battery cable clamps. Also, the (original) coil was connected with "CR" towards the distributor.
Even if in poor condition now, due to having stood outside under a tarpaulin for 12 years, it seems like the car was taken very well care of before being taken off the road, and it probably hasn't done any great milage.
 
#9
All the battery cables will be black, so that's no indication. And the clamp may not mean anything, if it was a split clamp style I would never change them, just spread them out a bit.

And keep in mind that converting to negative ground in a car without electronics simply means swapping the battery around & repolarizing the generator & regulator (or replacing the generator with an alternator).

Yours
Vern
 
#10
A bit late and pure guesswork on my part but I'd suspect the car was probably converted some time in the 70s and probably not by the first owner, based on the assumption that the owner of a new Rover wouldn't be fiddling with it and most 'upgrades' seem to be carried out by susequent owners. Were many +ve earth alternators around then?

Many cars were altered to -ve earth when owners wanted to fit new radios, new-fangled 8-tracks or cassette players etc, remembering that a radio wasn't a standard fitment. By the 70s most, if not all radios were -ve earth, although IIRC some, but not many, were switchable, therefore, if you wanted a new radio you had to have the polarity changed as well.

Does the car have a radio or cassette player, if so that might assist.

I had my +ve earth P6 converted to alternator in the mid-70s and don't recall even having an option to retain +ve earth.
 
#11
I just bought some heavy duty booster cables for my 1977 V8 Rover

but the instructions printed on the cables bag are amazingly cautious

first of all it says
both vehicles must have the same polarity i.e both must be positive earth or both must be negative earth
it then goes onto say that all vehicles made after 1980 are negative earth

am I right in thinking that all P6 rovers were pushed out from the factory positive earth?
 
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