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MG TD TF 1500 - Chargeing system and ammeter

My ammeter was reading just into the + area at idle and high rpm, so after reading thru the archives I decided to polarize the Gen. after doing so the ammeter is now in the + at idle and way into the neg once its reved up. like its working opposite of what it should.

I am not sure if the car was + or - ground and if someone changed it, I bought the car with a 12v batt installed with - ground!

SL Salvatore

SL the best and quickest way to get a sense of it's previous polarity is turn on the headlights. If the ammeter goes toward minus then the battery is correct. If it goes toward plus then the battery is not correctly connected.

The above assumes that if the polarity was changed it was done correctly and the wiring to the ammeter was reversed.

Let us know what you find.
cheers,
Bob
Bob Jeffers

SL
Is the (-) terminal of the battery, connected to the starter switch on the firewall? Also, is the (+) terminal connected to the metal battery box?

Edward
E.B. Wesson

No, The - is conected to the battery box and the + is conected to the starter switch..
SL Salvatore

I turned the lights on with the eng off and the ammeter goes to +. but as stated above the + from that batt goes to the starter switch! what does all this mean?

SL Salvatore

You have a negative ground setup , and when you "polarized" your generator, you changed it back to positive ground....
I'm pretty sure that you can repeat the polarizing process, and return the system to negative ground.
Of course, that's assuming you want to have a negative ground car.
Edward
E.B. Wesson

Easy Edward. The generator gets polarized in terms of the polarity of the output voltage (current). This is done by briefly connecting the ungrounded terminal of the battery to the Field terminal of the generator. If the negative terminal of the battery is grounded this can only result in a positive-polarized generator, i.e., positive output voltage. Seems as if the ammeter is wired backwards. Bud
Bud Krueger (TD10855)

So should I flip the wires on the Ammeter?
SL Salvatore

If you're going to stay with a negative ground system then you should flip the wires. Bud
Bud Krueger (TD10855)

One thing --- before you go fiddling around behind the instrument panel, do yourself a favor and disconnect the ground cable from the battery. After all -- you wouldn't want to let the smoke out of the wires. Bud
Bud Krueger (TD10855)

It you don't have a radio or any other solid state devices wired up, you might just switch back to positive ground.
Jim Northrup

I'll try switching the wires first! So was this orig a + ground set up and someone switched it?
SL Salvatore

All pre-alternator MGs were originally positive ground.
Bud Krueger (TD10855)

Bud
I understand what you are saying, but SL had a Negative-ground car to start with, so what did he do to "flip" the ammmeter, and can't he just undo what he did to begin with, if he wants to keep it negative ground?
Edward
E.B. Wesson

I will be keeping the - Ground!
SL Salvatore

Edward, read SL's first post again,"..I bought the car with a 12v batt installed with - ground". I suspect that the car needed a battery and someone automatically connected one in as negative ground. I don't think the PO was intentionally converting the car from positive ground to negative ground. At least that's MHO. Bud
Bud Krueger (TD10855)

Ohh boy, I have created a monster here. Apparently the car was at some time changed from the original positive ground configuration to negative ground. Now the headlights ON produce a + reading on the ammeter. That says that the conversion to negative ground was not done correctly (or completely). The statement was made that the + terminal of the battery goes to the starter. That says that the car is now negative ground.

The ammeter should go to the minus side when the headlights are turned ON. If it does not then the conversion to negative ground was not completed. The two wires connected to the ammeter need to be swapped. Then the ammeter will read minus when the headlights are turned on. Next the spark coil should be checked. The terminal marked CB should now go to the white wire ( plus 12 volts when the ign.is turned ON). The other terminal should go to the distributor (usually a white wire with black tracer). This should put the car to a completely negative ground wiring. All the rest of the electrical stuff doesn't care which polarity it gets as long as it gets the 12 volts.

This then leaves the charging system. Now polarize the generator by connecting a wire temporarily from the + terminal of the battery to the small terminal of the generator(dynamo). This will change the residual magnetism in the case of the generator so that the generator will produce plus voltage. This will show as a plus reading of the ammeter when the engine is reved above about 2500 RPM.
Bob Jeffers

Ok, I will give it a try and see what it does!

Thanks guys!
SL Salvatore

This thread was discussed between 12/07/2011 and 13/07/2011

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