positive and negative ground

[spankyway]

I have another good question, every schematic I have looked at for my 75 MGB shows a negative ground, my car has a positive ground. Can someone enlighten me?

 

[jlaird]

Someone changed it.

 

[rick_ingram]

Why would anyone in their right mind convert a car over to positive ground that was originally a negative ground?

Is the car running....or has someone mistakenly hooked the battery up with the polarity reversed?

 

[tony barnhill]

If the polarity were reversed, would the tach work?

 

[rick_ingram]

I think that there would be a multitude of electrical components that may have fried if that is indeed the case...which I believe is what has happened here.

 

[danmas]

Is it really positive ground, or has someone just stuck a "positive ground" sticker on the car?

 

[Kimberly]

Are you sure it is a positive ground, you can't go by the colors of the battery cables. Do you have an alternator or a generator? If the cables were/are hooked up wrong to the battery, that would explain why your alternator light on the dash is on and possibly why you had trouble hooking up your timing light.

 

[PAUL161]

Whew, I sure would like to hear the outcome on this one. /ubbthreads/images/graemlins/crazyeyes.gif PJ

 

[spankyway]

I have checked the battery cables and the positive side is on the frame of the car. The car does have an alternator and I checked the charging to see why the light stays on. The alternator is putting out about 14.8 volts at idle, and it does idle a bit high around 1100-1300 rpm. While driving the car the tach is a bit jumpy and the temp guage does not work. All other guages seem to work fine. The guy I got the car from was adament about the positive ground. I set up my timing light the way I always do and I din't get any light. I switched the lead to the other side of the coil and everytime I pull the trigger the engine dies.
I am not sure if I should change the ground back to negative or not. If yuo have more advice let me know

 

[dklawson]

Very curious. Hooking up the alternator "backwards" should let the white smoke out (if not the black smoke). Not many positive ground alternators were made and I find it hard to believe they would be in use as late as 1975.

What kind of timing light do you have and how are you hooking it up? Is this a real old timing light? I don't ever remember seeing a timing light that hooked directly to the low tension side of the coil.

 

[spankyway]

My alternator is a Lucas and it has the "wide-wide-narrow" plug on it. I cannot remember the exact color combo on the wiring but I do remember on heavy brown and one thin brown.
My timing light is an Actron and has a red and black lead and one that goes to the number one cylinder that is green. My light is not inductive so I have the little spring that goes between the plug and wire.
Just out of curiosity, what would happen if I switched the ground now?
One more point that may be of interest, last night while I was driving the negative side came loose of the battery and the engine died. I don't know if this is important or not.

 

[Rick_Stevens]

I know from hard experience that if you hook up one of the Actron lights "backwards" it will kill the car immediately. The instructions for these lights say that black is ground and that the red should be at "battery positive" - which can be any hot power source. They usually warn not to hook up to the coil LT side, but not always.

I know what I'd be doing right now, but it's not my car. /ubbthreads/images/graemlins/nonono.gif

As long as you've absolutely 100% confirmed that it's currently - no pun intended - battery positive-to-ground, it sounds like things should be swapped around. Before doing that, though, I'd want to run down every important circuit - instruments, lights, and charging system - to make sure what went where. Use a multi-tester and double-check everything against the diagrams from Advance Auto Wire, including color coding. Very time-consuming (again, based on experience) but necessary, methinks.

Take your time, be methodical and patient, and have extra fuses on hand.

Rick

 

[Dave Russell]

It might just be possible that the battery is installed backward but is working. Measure the voltage at the battery terminals, nothing connected, & verify that the polarity is still as marked.

A fully discharged battery can be recharged backward so that it's terminals are reversed but will still work, at least for awhile.

As Doug said, it's very unlikely that the alternator is actually positive ground. Verify the alternator polarity with the voltmeter.
D

 

[spankyway]

I know what I'd be doing right now, but it's not my car.




What would you be doing now?

 

[DrEntropy]

This is a new one on me. I've swapped many of the old cars to negative earth, but I have NEVER seen a later car wired "bass-ackwards" as a "retrofit." Rick's right: chase all hot leads to termination, use the wiring diagram for the year of the car, and take the time to do it right. I'm ~STUNNED~ you haven't let the smoke outta the harness!

WOW! /ubbthreads/images/graemlins/nonod.gif /ubbthreads/images/graemlins/crazyeyes.gif A new DPO low., IMO

 

[spankyway]

I am a little ignorant to engine electricals (I do paint and body work), how do you check the alternator polarity? The battery is a brand new one, the guy I got the car from had just put it in. I am looking over some information the previous owner gave me from www.mgcars.org.uk and the "3 terminal" alternator it shows is the one in my car.

 

[DrEntropy]

I kinda think your PO had his apples and oranges mixed up. It would take a major rewiring effort to wire the car for positive ground... Does this car have a radio? Does it WORK?

 

[spankyway]

I does not have a radio. The only "weird" things I see are the red indicator light stays on and the alternator charge is 14.8. The temp gauge does not work and at high revs the engine sputters bad. The turn signals will not work, and I have checked the relay by plugging it into the hazard wires and they work. The green wire and the Light green and brown wire both have continuity from the turn signal switch to the relay but they don't work at all, the hazards do though.

 

[Nunyas]

I'm not sure if this is the safest way, but this is how I would do it. With the car running, I would take a nice sharp positive (+) voltmeter lead and poke it through one of the fat brown wires coming out of the alternator, and then I would stick the other lead on the engine block. If the voltage comes out positive then you have negative earth in your car, and the battery is probably charge backwards.

It'd probably be easier and less prone to shock to just measure the voltage across the battery terminals to figure that out though. Positive lead to positive battery terminal and negative lead to negative battery terminal should give you positive voltage on the meter; Positive lead to negative battery terminal and negative lead to positive battery terminal should give you negative voltage on the meter, if it doesn't then the battery's polarity has been changed

 

[DrEntropy]

...umm... that'd be with one'a them new-fangled DIGITAL multimeters, BTW.

...I'm highly tempted to say: "Hook it up with the negative of the battery to ground" and wait fer th' smoke... but I'm a Cretin. My opinion is you'll have a tach and all the rest then... and if not, you've identified the parts which have been fried from the wonky backwards connection. Take it with a grain of salt... and have the battery terminal *Loosely* connected and a fire extinguisher handy just in case.