General MG elect. prob.

[Louis 52MGTD]

I just bought a 1952 mgtd, it has a charging prob. slightly below normal, when I looked under the hood I noticed the batt. Plus cable is going to the starter switch and the batt. neg cable goes from the batt. to a ground on the gen. case. the wiring chart says the car is a positive ground ? it has been driven like this for a while. should I switch the cables like the book says or will that cause a prob. with blown fuses ? I am weak on elect. thanks for any help. Louis

 

[NutmegCT]

Lou - many of the TDs were changed from the original positive ground to negative ground over the years. So negative ground isn't likely the source of your charging problem.

If your TD is using a dynamo (a generator, not an alternator), you'll get low voltage from it when the engine is idling.

Just to be sure, what tells you the charging is slightly below normal?

Tom M.

 

[Louis 52MGTD]

Tom,
Thank you for that info. about over the years some were changed to a neg. ground, I 've never heard that, why would people do that ?
The amp meter shows slightly to the left of the center [neg] even when driving at 45-55 MPH but the batt seems to hold up, voltage ck at 12.40 to 12.45
the red light has not come on in a year, so I'm told by the seller.

 

[NutmegCT]

Thanks Lou. When the ammeter is slightly negative while driving, are any lights, especially headlights, on?

 

[Louis 52MGTD]

No lights on, day time driving only, and not very long. BTW it is a gen. not a alt. I have only driven it about 4 -6 miles, had to work on a leaking front brake.

 

[NutmegCT]

Quick test: start engine, then let it idle. See what ammeter shows (0, minus something, or plus something).
Then rev the engine and hold it around 2000 rpm. See what ammeter shows while it's up at that high speed.

 

[Louis 52MGTD]

Tom, I can't run the engine at this time, waiting on brake parts [wed ?] the last time I drove it was fri and the amp. meter needle stayed slightly to the left of center [neg.] while driving it never went to the plus side, with the engine off the amp. meter show's slightly to the left of center.
In the one pic. you can see the red cable coming from the pos. post of the batt to the starter switch, the neg. cable is grounded to the gen. case.
You said sometimes people would change this to a neg. ground ? why would this be done ? Thanks for your help. Louis

 

[NutmegCT]

Hi Lou - not sure what you mean by "what would this be done". If you mean "why", you open an interesting philosophical discussion! It was often done to make it easier to add modern electricals which require negative ground (as the change has little if any effect on the engine itself - a spark is a spark).

Does the ammeter move at all when you switch on the headlamps?

I see other cables on your battery coming from that positive battery post beside the big red cable What "new stuff" has been added to the car? It *might* be that the ammeter isn't even functioning. A good photo showing the dash gauges might give us a clue.

Tom M.

 

[Louis 52MGTD]

Tom, sorry about the typo. someone did add a radio which is in the glove box, and their is a trickle charger cable wired in to plug in the T/C.
I will send in a few pic's of the dash with the switch off and on showing the amp. meter. about an hour from now. Louis

 

[DrEntropy]

The switch to negative ground may have a couple reasons. My guess is the person(s) who made the change neglected to change the ammeter. Radios of modern vintage are negative ground. Tach is mechanical? The later tachometers were electric and required rewiring if the positive-to-negative switch was made.

EDIT: Yep! The radio installation is the reason for the changeover.

 

[NutmegCT]

Thanks Lou.

Tom, sorry about the typo. someone did add a radio which is in the glove box, and their is a trickle charger cable wired in to plug in the T/C.
I will send in a few pic's of the dash with the switch off and on showing the amp. meter. about an hour from now. Louis

Those additions are the likely reason for the polarity change, and the cause of your ammeter woes. The "on and off" photos of the ammeter face will give us a clue.
Tom M.

 

[DrEntropy]

Read this article, it will explain:

How to: Convert a vehicle from Positive Earth to Negative Earth

Positive to Negative Earth Conversion First up, why do we have Positive Earth and Negative Earth in cars…? During first half of the 20th century some manufacturers used positive ground, while…

blog.simonbbc.com

 

[NutmegCT]

Lou - Note the section on changing the ammeter wires in Doc's linked article.

 

[Louis 52MGTD]

Here's a pic. of the dash [amp. meter sw. off] you ask for, I'll have to wait until I get it back on the ground to show it running.
After reading the article put up by DrEntropy it sure sounds like the amp. meter wire might be the prob. that was a very interesting article and would like to thank the Dr. for posting it, very helpful.
Now that I think about it when driving it I would look at the amp. meter and it would be slightly to the left toward neg. with a slight wiggle as if it was working, but only in reverse.
Have learned so much here in only a few days, what a great forum.
I will let you know what I find when I get it back on the ground and can start it.



.

 

[NutmegCT]

Lou - don't worry about starting it just now. See if the ammeter moves at all, when you turn on and off the headlamps or blow the horn.

 

[Louis 52MGTD]

Tom,
When I turn the head lights on the amp. meter will jump from 0 to -15 or about 1/2 way to -30
And when I cut the head lights off, the needle goes back to almost 0.

 

[NutmegCT]

From that, it sounds like the ammeter is working, but isn't "centered" on zero. I wouldn't worry about it.

But I would worry about the charging system, if the ammeter doesn't move to positive when the engine revs up.

When you get the engine going again, measure the voltage coming from the dynamo/generator. If it's showing a healthy 12 to 14 volts at engine speed around 2000 rpm, the dynamo is probably good. If not, could be that the dynamo brushes need replacing - which is easy to do.

Keep us posted after you can get the engine running.
Tom M.

 

[Louis 52MGTD]

Well, I got the front brakes replaced but now the break lights stay on when the switch is on, the break ped. is very tight, no free play at all, I never messed with the master cly. can you over fill the master cly. after bleeding the brakes ? could that cause this ?

And I still have my charging prob. guess I'll end removing the gen. and taking it to a repair shop, the few shops around here that work on old cars with gen's. have a two month waiting time to get in.
It's still not showing any charge on the amp. meter or using a voltmeter.

 

[LarryK]

A lot of the cars were changed because the younger mechanics(techs) would not recognize positive ground, so damage was done and the result was.the change to keep from happening again. Why we work on our own cars.

 

[Louis 52MGTD]

Thanks Larry,
Should I change it back to a pos. grd. or just get the gen. worked on and leave it alone ? the only added accy. is a radio in the glove box which I can do with out and remove. from what I understand it was the modern accy's that people went to a neg. grd. ?