• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Negative Ground Conversion- inconsistent guidance

Share this page

TulsaFred

TulsaFred

Jedi Warrior
Country flag
Offline
I've seen two explanations on the internet for converting my B/E from pos to neg ground.
The first explanation says to flash the A or A1 terminal on my Lucas voltage reg to the D terminal. It says not to touch, under any circumstance, the F terminal or it can damage the regulator.
The second explanation says to disconnect the two terminals on the generator itself, then to flash the smaller "field" terminal with a hot lead.
These two sets of instructions seem inconsistent.
Which is correct?

BTW, can anyone elaborate or show a pic of the "Field" terminal on a B/E tach drive generator?

thanks
Fred
 
The whole idea of polarizing the generator is to create a residual magnetic field in the field coils of the correct polarity, so the generator can start. So, you run a lead from the battery (or the A or A1 terminals of the regulator--same thing, for these purposes) to the field coil and just make the connection for a couple of seconds. This creates the magnetic field and magnetizes the metal around the field coils.

The problem is that the generator needs a magnetic field from the field coils in order to generate power, but the field coils need a working generator to be energized. It's kind of a chicken-egg problem. However, if the generator has been working normally, it magnetizes the metal in the field coils a little, so there is some residual magnetic field, allowing it to start up. If that residual field hasn't been created, or worse, is in the wrong direction (i.e., if the car was previously positive ground), you need to create that field. Thus, flashing the generator.

If I were doing this, I'd disconnect the generator from the regulator, just so you power the field coils and only the field coils. However, I doubt that you could do any damage to the regulator if it stayed connected. I doubt that anything short of a major lightning strike could hurt one of those suckers.

Sorry, I don't remember which terminal is the field. I seem to remember that the D lead is a screw terminal and the F is a spade, but probably someone else should verify this info.
 
Thanks Steve,
I've never seen an explanation for the need to polarize a generator. Now it make sense.
Fred
 
The field terminal is the smaller of the 2. Never tryed it but have heard that if you just drive the car the field will eventually repolarize itself.

Think I'm right on this, if not I'm sure someone will pipe up!

Kurt.
 
It will repolarize itself in about 20 seconds at about 2000 rpm.
All I do is to reverse the battery, fire it up and bring it up to 2K for about 20 seconds. The red light will glow real bright and then suddenly go out and that is it.
Real easy, don't overcomplicate it.
BillM
 
I've always suspected you could do this. I've never been completely certain what would happen if you tried to run the generator with it magnetized to the wrong polarity. So, since the process of "flashing" it is pretty simple, I've always just done that.
 
Cool,

So all the machinations and multistep instructions for conversion to negative ground can be reduced to:

1. Turn battery around so that negative goes to ground and positive to the starter solenoid (redo battery cable ends)

2. Switch coil wires to opposite terminals (so that the negative terminal goes to the distributor, and positive from the ignition switch to the other terminal).

3. Start car and and drive or run it to 2000 rpm for 20 seconds

Can't get much easier than that.

Fred
 
I thought there were some wires inside the Tach that had to be switched around as well? Maybe the B/E tach is different than my Mk3 so it is not needed?????

Jeff
 
All above are correct- the tach must be modified but on a mechanical drive tach that is all that you do. For an electronic tach you must modify it. For tach mods I go to the Moss catalog and follow the instructions (2 wires to unsolder/resolder).
BillM
 
I have done this too. Just switch the battery around. My old TR3, TR4, Land Rover, Bugeye, never a problem. (all had mechanical tachometers except the Rover which had none)
Of course now I have switched to an alternator. I couldn't deal with the 22 amp generator any more with halogen headlights.
 
Great info but I'm looking for clarity on a couple questions.

Is there a way to test the generator to see if the polarity is right after you re-polarize it? I don't want to hook it back into the warning harness until I know it's right. I'm scared it will burn up my new electronic ignition if it's wrong.

Are you supposed to reverse the two wires going to the generator? I'm getting mixed feedback on this question from the Internet.

Any help would be appreciated.
 
You'd have to bench test the generator before installing it on the vehicle. Don't switch the wires at the generator. You won't damage the electronic ignition.

Fred summarized it best above. You don't even need to polarize it with a jumper wire. Just run the engine at 2000 rpm.
 
Well Dave, if a guy with a positive ground Pertronix were going to negative ground to put in a radio, for example, then yeah, he would need to get a negative ground Pertronix too. Seems to me. Charlie
 
Back
Top