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Positive or negative earth?

Discussion in 'Electrics' started by J.Lackland, Nov 12, 2017.

  1. J.Lackland

    J.Lackland New Member

    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    As a suffix C it should be positive, but the generator's been replaced by an alternator from SEV Motorola.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.....

    Attached Files:

  2. Penguin

    Penguin New Member

    starter motor will turn same way regardless of polarity
  3. roverp480

    roverp480 Member

    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. J.Lackland

    J.Lackland New Member

    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. Penguin

    Penguin New Member

    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
  6. harveyp6

    harveyp6 Well-Known Member

    And switching the LT wires on the coil.
  7. Vern Klukas

    Vern Klukas Member

    The alternator must be a later addition, I'll bet the car was converted to negative ground when that happened.

  8. J.Lackland

    J.Lackland New Member

    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. Vern Klukas

    Vern Klukas Member

    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).


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