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MG MGA - How does one tell if his car is

a positive ground or a negative ground?
Tysen

Tysen. You look at which terminal of the battery/batteries is attached to the cable, normally black, which is attached to the chassis. That is the ground attachement. If the negative terminal is grounded to the chassis, it is a "negative ground" car. If the positive terminal is attached to the chassis, it is a "positive ground car". The only problem is when one converts an MGA to negative ground, does not tell the new owner (such as an estate sale) and the new owner hooks up the battery backwards.

If you have a good quality stereo system, the car is probably negative ground. If it does not have a modern "sound system", it may be either. If you are totally clueless as to what it might be, hook up the battery as a positive ground system and take the car for a drive. Let it sit over night and try to start it in the morning. If the battery is flat, the system was hooked up improperly, the mechanical voltage regulator is stuck and drained the battery. Such happened to me, back in the mid 60's when I had a new battery installed in my first car--a positive ground 53 Mercury Coupe. We traces shorts, bad connections, etc. for a week, then asked our next door neighbor, a Ford mechanic what he thought. He thought the fellow who installed the battery was an idiot, reversed the battery, hit the mechanical voltage regulator with the handle end of a screwdriver and said, "It should work right now". It did until I graduated from high school and got my first LBC.

Les
Les Bengtson

Just use a test meter between a live terminal on the fuse box and ground

If you connect the leads to the meter red + black - (as standard on the meter), and then put red lead on the fuse and black to ground, a negative reading says you have a positive ground car, a positive reading says it's negative ground.

dominic clancy

Tyson has a good question. If there is not battery already installed, how do you tell which it is? Someone once told me that you could look at the coil connections. If pos ground, the + side of coil is connected to the distributor (I think). I seem to recall that back in the day, the battery posts were different diameters. That is, one was fatter than the other and one could tell by looking at the size of the hole on the battery clamp. I believe that with modern batteries, this practice was abandoned. I'm sure there is another way to tell, but it escapes me at the moment.

Dennis, Az
Dennis

Dennis. The original coil was marked "SW" for the switch connection (negative originally) and "CB" for contact breakers (positive originally). This practice was carried through the MGB Mark I with the Mark II series having the alternator, negative ground system and a coil having a + or - marking.

You are also correct that the positive and negative battery clamps are different sizes. However, same problem as with the coil--people have changed things over the years. All three of my RB MGBs had a positive (larger) battery cable going to the chassis and attached to the negative terminal of the battery. After some searching, I was able to find the correct (smaller clamp, black insulation of the wire) ground cables, but it took a little looking to do so. Perhaps more so than others were willing to do.

In the worst case (no battery, nothing to indicate that the car has been switched over to negative ground and same size battery terminal clamps) one can simply start from ground zero. (Again, unless there is a radio involved which should be removed and examined to see if it is a negative ground instrument or a positive ground instrument.) Hook up the battery the way you want the system to be, polarize the generator and hook up the coil in the manner appropriate for the system ground.

Many years ago, Waverly Dunham, the best sports car mechanic in central Florida agreed to come out of retirement and assist me with an MGA project. I was back from Germany on leave and helping finish up the engine rebuild on the MGA I had given my brother when I was transfered. Waverly suggested converting to negative ground because "sound systems" were coming into fashion. I can still remember what he said. "The fuel pump doesn't care what the ground is as long as it has one. The generator does not care what the ground is as long as it is polarized. The coil needs to have the CB terminals hooked to the switch and the SW terminal hooked to the distributor. Beyond that, it does not matter a bit."

Waverly's advice might be worth of some thought in this case.

Les
Les Bengtson

I would just take off the battery cover and look at the battery. It is marked + for pos and - for neg. Look to see which wire goes to ground. If it is the pos terminal to ground then it is pos ground, and if the neg goes to ground, then it is neg. The other wire goes to the starter switch.
Jeff Becker

This is a good time to bring up a favorite piece of advice regarding changing the polarity of their car. There are many valid (and some not so valid) for changing a T series MG, MGA or early MGB to negative ground, but as can be seen in this discussion, a new owner not familiar with electrics is not going to be able to tell which way the car is grounded, or worse they get the car without any battery in it. Do yourself and the next owner of your car a favor if you do change the polarity on your car or if the MGA that you just purchased turns out to have been converted to negative ground and get one of the lables from Moss Motors that says Important - This Vehicle is Wired Negative Earth and stick it on the battery box cover. Better yet, get two, on for the battery box and one for under the hood. It could well save you from some fried electrics and will be a great help to the next owner. Cheers - Dave
David DuBois

This thread was discussed between 28/06/2006 and 29/06/2006

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