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Conversion to Negative Earth/Ground

Braman Wing, 18 Oct 1996:

Converting to negative ground is very easy, and allows you to run all sorts of modern electronic equipment (alternator, electronic distributor, etc.). I believe the only items on a Mk II that are affected are:

  1. coil - will work reversed, but not as well
  2. ammeter - switch leads
  3. water temp, oil, and fuel gauges - switch leads(not sure about fuel, I think you need to switch)
  4. clock - usually does not work anyway
  5. fuel pump, depending on type of pump - I have an aftermarket fuel pump
  6. that is positive ground, it's possible to switch the diodes in the old ones
  7. around, but I don't know the details

I believe this is everything, if nothing goes drastically wrong it should take less than an hour to do. It is actually possible to run a negative ground radio in a positive ground car, I did it for 2 years. You need to isolate the chassis of the radio from the body and run the ground wire to a positive terminal and the power wires to ground. This is a bit dangerous, since if the body of the radio is grounded, it will short. However, if you're careful it will work. If you want more details of my installation, let me know.

John, 18 Oct 96:

To change polarity to negative to earth, when a generator is fitted:

  1. Refit battery with only the earth fitted to the negative battery terminal then with jumper lead ,for a instant connect the battery positive to the generator field terminal, this changes the polarity of the residual magnetism in the generator field.
  2. Swap the primary leads on the ignition coil, this causes the electrons (spark) to emit from the centre electrode. The need to do this is because it takes about 2-5000 less volts and the engine may misfire under some conditions if not changed.
  3. Connect the power lead to the positive battery terminal - and the job's done.

21 Oct 1996 and later: composite of messages, Jag-Lovers, from Egil

Kvaleberg and Vince and maybe others:

  • It is actually possible to run a negative ground radio in a positive ground car, I did it for 2 years.
  • Response: Sounds pretty dangerous - what about the ground wire to the antenna?

Actually, most of the older vibrator-type radios were polarity-insensitive and could be used without modification in either positive or negative ground cars. The exceptions were those with synchronous vibrators and some of the signal-seeking radios that used motors instead of spring/solenoid drivers.

The real problems arise when trying to use a radio with any solid-state components in it. In many cases, however, a voltage converter can be installed to allow positive ground operation of a negative-ground radio. The main requirement is that the maximum current draw requirements of the radio be relatively low (less than 3 amps).

Egil Kvaleberg, Norway, 21 Oct 1996:

Re item 3 - water temp, oil, and fuel gauges - switch leads (not sure about

fuel, I think you need to switch):

  • Comment in response: No need to do anything with any of
  • these, they are not polarity sensitive! And remember you have to "flash" the generator, i.e. apply battery voltage of the "new" polarity so that it runs like a motor - one second is enough.
  • It is actually possible to run a negative ground radio in a positive ground car, I did it for 2 years.
  • In response: Sounds pretty dangerous - what about the ground wire to the antenna?

Probably not much of a problem, wanting a new radio is usually one of the main reasons people switch to negative ground anyway.

Cleo Bay, 6 Nov 1996:

A note on alternators: The Delco Delcotron has a built in regulator which makes the alternator negative ground. You would need an external regulator setup or a specialty alternator is needed instead if you want to stay positive ground. I suggest switching to a negative ground system to allow easy use of radar detectors, CD players, radios, etc. Also, you will be able to get replacement alternators at the local auto parts store. :) The fuel pump and amp meter are the only electrical parts that need looking at for a positive-negative change. Whatever you do, don't sell or get rid of the Jaguar parts. If you ever sell the car, you will be glad you kept them!

Cleo Bay, 17 Nov 1996:

John, Yep, you are right about the coil. I also forgot about the battery! You might look at a early to mid eighties GMC truck for a Delcotron. Make sure you get one with the v shaped pulley. Re John's comments that the fuel pump and amp meter are the only electrical parts that need looking at for a positive-negative change:

Response: Not as true for XK 120s and XK140s.

  1. Water and oil gauge is mechanical.
  2. Original pump, if it is a non-diode non-condenser type, requires no changes. If it has a condensor or diode, reverse them.
  3. Clock requires no changes.
  4. Fuel gauge is a question, but do not think there is any changes required.
  5. Coil wires reversed.
  6. Battery terminals reversed.

Not had any problems with my charging systems on my XKs so do not plan to do changes with my cars. The only reason that I would change my charging system is to get more amps to run additional equipment. I'm happy with mine as it is. It works and is reliable (for the past 10 years!) :)

Larry Schear, 17 Nov 1996:

The case of the alternator (most US units) is connected to the negative terminals of the diode bridge that converts the alternator's alternating current (AC) output to the direct current (DC) required by the car's electronics (lights, horn, and solenoid don't care, but starter, some motors, and electronics DO care). The positive end of the diode bridge typically goes to the red or positive insulated bolt on the alternator case. Doesn't matter if the regulator is internal or external. Best solution is to find a positive-ground alternator, second best is to change car electronics to negative ground, worst is to try to electrically isolate the negative-ground alternator in a positive-ground car (insulated bushings (nylon) at all bolt and bracket attachment points. Possibility is to take the alternator apart and reverse diode polarity connections! This might be dicey, as the heat sink plate holding the diodes (usually in sets of three) is usually used for both mounting and grounding. Alternative thought is to maintain positive-ground and generator (with new brushes and a cleaned commutator), and keep a seldom-used car on allow-current trickle charge.

Cleo Bay, 17 Nov 1996:

Don, You are right about the coil. The clock does not have a diode and will work with either polarity.(if it works at all!) I rebuilt my clock but left it unhooked.

Cleo Bay XK120, XK140

Mike Plechaty, 17 Nov 1996:

The stock Lucas electric fuel pump doesn't care if the polarity is reversed (i.e., it won't run backwards). Yes, the ammeter will read backwards, but it's easier to let it be than disassemble the dash to get to the leads (at least this was my observation when I changed polarity in my 120 FHC). Fuel gauge will function normally, regardless of polarity. Wipers work OK, etc.

Don Tracey, Australia, 17 Nov 1996:

Yes the coil needs wiring the other way, also the clock if it still works which I believe has a diode in it (unlikely). All cars now that I know of use neg. earth and have for 30 odd years.

Jim Isbell, 20 Nov 1996:

The suggestion made that positive ground would prevent corrosion is obviously false. Other wise since the connector (floating somewhere in a wiring harness) could just be turned around to prevent corrosion. This is obviously not true. The connector has no Idea where the wires that come to it come from physically, only what polarity they are. Polarity will have no effect on corrosion. Polarity will have an effect on the shape (it will be reversed) of the EROSION of the breaker points, but again it wont make any difference the erosion will happen anyway.

As to the combustion, I doubt it, but its possible. But since the spark coil is a transformer it would be easier to reverse the connections on the secondary. This would make the block the "hot" side and the electrode the ground side. So I doubt that it makes any difference or spark coils would be made that way. Spark coils are currently made so the secondary and primary have a common ground. But it would be just as easy to remove the common ground and reverse the secondary to reverse the polarity of the spark and it would work just fine.

While on spark coils, has anyone ever tried adding a second spark coil to an XJ6 the way they do to the XJ-S to increase the energy available. I have wondered if it would make any difference in the combustion.

Johnboy, 18 Nov 96:

Ken, Thank you for your reply, I think I will just install the neg. gnd. Delco if it fits), reverse the battery (make it neg. gnd.) and change to a U.S. coil (I'll probably need a ballast resistor). That should do it as far as I can see.

Dick White, SC, 18 Nov 1996:

You don't need to buy a new coil, just reverse the leads on the low voltage side of the one you are using. The plus and minus should be marked on the top. If it is done properly now, the (+) post should be connected to the points which go to the chassis. After converting your system to negative ground, switch the leads so that the (-) post goes to the points.

31 Jan 1997, from Tom.Carson

Antique Automobile Radio Inc., 700 Tampa Rd, FL 34683; 1-800-we fix am; apparently sells a power inverter that lets you run negative ground equipment on a positive ground system. I used a similar device many years ago on a Mini I had and it simply worked.

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