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MG MGA - sat nav and positive earth

How do you fit a sat nav to a positive earth car? I don't really want to change to negative earth at the moment. I am assuming that I need to wire in a cigarette lighter for the power. My brother had one fitted by a garage without changing his car to negative earth but he does not know what they did (he no longer has the car).

Many thanks,

Phil
P Parmenter

The sat nav devices I have seen have had no grounding issues, so power is your only concern... together with the issue of using it in other vehicles.
One solution would be to install an additional cigar lighter wired with the center being positive and the shell being negative. This could be challenging with a dash-mounted unit as they tend tyo want to earth themselves in the mounting, but there are any number of add-on kits in the auto parts stores here, including extenders, which would work (best if concelaed, however as they would not look "correct" in an MGA.
Another possibility would be to buy a short extender , cut the wires and reverse them, resulting in a "convertor".
Or you could get a spare power cord for your unit to be used only in the MGA and modify it accordingly.
In any event, your biggest concern is that you never connect it in either your A or your modern the wrong way...
David "BTDT" Lieb
David Lieb

You might want to try a boating supply store. You can usually find a plastic "cigarette lighter" socket that has two isolated wires coming out. Like David said, most GPS, cellphone and other handheld devices don't have any exposed metal that is connected to negative, so they are not a problem. However, you do have to be careful when the charging connector is not plugged into the device because the exposed metal on the barrel connector used on many devices will be connected to negative.
Jeff Schultz

You could just mount the lighter socket to the wooden bulkhead in front of the batteries, behind the seat, next to the transmission tunnel.
Neil McGurk

Also, it is very important that you put an inline fuse in the negative wire going to the socket.
Jeff Schultz

Any time you try to isolate the device from the chassis of the car for the purpose of polarity change, you are tempting fate for the first opportunity for a battery short to ground. This can work for devices that are inherently double insulated with no exposed metal parts, but the connection point, like a lighter socket for instance, can still have an isolation problem.

The proper way is to use a voltage inverter to power the new device. This sounds a little tricky, but the voltage inverter doesn't actually change the polarity. What it does is to double the battery voltage to apply +24 volts to the "hot" side of the new device while "grounding" the new device on the original +12 volt chassis. The car still uses the "+" battery post for the "floating" ground reference. See details here: http://mgaguru.com/mgtech/electric/et207.htm
Barney Gaylord

What a complicated way to do a simple insulation job Barney. If the cigarette lighter is installed well then there will not be any chance of a short, or no more than is already inherent already in an MGA. I wouldn't class the inverter as the Correct way. The correct way would be to SIMPLY convert the car to negative earth.
Why anyone would not want to convert to neg earth is beyond my understanding, but I am all ears.. :-)
Bob (robert) Midget Turbo

As long as you have a fuse of 2 amps or so in the negative wire to the socket, the downside of using an isolated socket is very limited.

Jeff Schultz

With a device like a GPS, you have just as much of a risk of shorting it out in a negative-earth car as a positive earth car. I agree that the invertor would be the better way to install a stereo (given a reluctance to convert to negative earth), but you run more risk of damage just having the invertor circuitry and extra battery there as you are avoiding.
David Lieb

Bob, I agree, it makes more sense to convert the car to negative earth, which I have done to my own MGA many years ago. But that wasn't the question. Maybe he wants to keep it positive earth for concourse, or maybe he's a bona fide member of the Positive Earth Society.

If you attach the inverter to a negative earth radio it effectively becomes a positive earth radio, and it installs the same as any positive earth radio. It can ground on the dash, and no shorting hazard anywhere.

The GPS unit installs like a radio (or like any other negative earth device). You can install the inverter behind the dash and connect it to a cigar lighter socket, go ahead and ground the lighter socket on the dash. End result is a negative earth lighter socket in a positive earth car, and both have a common ground so no possibility of any ground short.

You can also install a standard lighter socket in the positive earth car, grounded on the dash. Then plug the inverter into the lighter socket, and plug the GPS into the inverter, and still no possibility of a ground short anywhere. That would satisfy a concours car as original even if you left the inverter plugged in and the inverter and GPS units sitting on the dash. It's just a plug in accessory like a cell phone.
Barney Gaylord

My GPS unit has a small USB style connection at the device for the power input. Usually, The shield (case) of the connector is grounded. I don't know if this one is or not. If you were to isolate the cigar lighter on a pos ground vehicle, and reverse the connection at the lighter socket, then if you disconnect the cable and let the free end touch metal on the car, it could short to ground causing a lot of excitement. Presumably the 2 amp fuse in the ground wire should prevent too much of a "surprise".

I wouldn't try it. Best way is to convert to ground. Even if you are 100% sure it is OK for the GPS unit, what about the next accessory you try to plug into the available outlet.

Chuck (with neg ground car).

C Schaefer

Barney has the only real solution - an inverter or modify the vehicle to negative ground. Mounting a standard lighter socket in the metal bulkhead leaves a metal (conductive) surface exposed on the wooden mounting and will rise up to bite you at any moment of lax attention. A fuse in the positive (or negative) side of the circuit will prevent total melt down, but becomes a pain in the back side after about the third time of having to replace it. I used one of the auxiliary power plugs sold at marine supplies, wired appropriately in our TD for such use and since it is all plastic except for the contacts, it works well and since my wife and I are the only ones to drive the car, it is fine because we are well aware of how it is wired and what to be watchful for. The problem will come if we ever sell the car and the new owner is not aware of what has been done. The best approach, even if I am not following it, is to follow Barney's advice - you will never be sorry that you did. Cheers - Dave
David DuBois

I neglected to mention one point. One problem with installing the inverter behind the lighter socket is that it becomes non-functional as a cigar lighter, unless you have at least a 10 amp inverter (or may be 20 amps). You also cannot charge the battery backward through the inverter like you might do with one of those cigar lighter plug jumper gadgets. In other words, a cigar lighter powered by an inverter becomes an accessory power jack only, no longer a cigar lighter.
Barney Gaylord

It's OK, I am not a concours buff or a member of the P.E.S. It's just that I am going on a trip in the car shortly and didn't want to attempt swapping to negative earth before I go due to lack of time and not wanting to risk messing anything up. Thanks for all the info but I am now a bit confused. I have a 2 pin plug on the rear wooden bulkhead that I use to charge the batteries (after some good advice from this forum). Could I use that to power the sat nav? If not it looks like the only safe way is to buy one of these inverters.

Phil
P Parmenter

You could get a socket like this, and wire it with a 2-pin plug to it the fit the one on your bulkhead. You would also want to put an inline fuse in the negative wire.

http://www.maplin.co.uk/Module.aspx?ModuleNo=1055&C=Maplin&U=SearchTop&T=lighter%20socket&doy=18m3

http://www.maplin.co.uk/module.aspx?ModuleNo=20624&doy=18m3
Jeff Schultz

Also, like Dave said, you just have to be careful with the exposed metal on the charging cord when it is not plugged into the GPS. As long as the negative wire is fused, contacting the metal barrel on the charging connector to the chassis will just blow the fuse.

Jeff Schultz

Barney Sez:

"...or maybe he's a bona fide member of the Positive Earth Society."

I don't know about you, and you can call me a cockeyed optimist if you wish, but I just couldn't stand to live on a Negative Earth.




Yeah, I know. That's deep.
David Breneman

Phil

A few months ago I installed a cigarete lighter in a 58 +VE EARTH MGA so that I could use a Sat Nav. I obtained it from a Caravan/camping accessory shop and had no problems fitting it. It is well insulated by its plastic black holder and I installed a fuse to be on the safe side. Works a treat and cost about 6.00.

Dave
D J ROUGET

Dave (Rouget),
Thanks for your reply. So you fitted your cigarette lighter with positive earth - did you then just plug your sat nav straight into this or did you need to alter the wiring in some way?

Phil Parmenter
P Parmenter

For the concours buffs out there, is there a market for a battery that is marked in reverse? That is the neg terminal is marked + and the neg terminal marked -. Along with this I could provide a grounding cable in Red. This would allow the neg ground crowd to enter concours judging without loosing any points. What judge actually takes a voltmeter to check?

I know that this would present a number of other problems that are just a bit more serious. I'm ONLY JOKING!

Chuck
Negative attitude and proud of it.
C Schaefer

Chuck,
Do the judges actually look under the battery access hatch in concours?

Neil
Neil McGurk

Niel,

I can't say, I've never entered one. There was some talk some years ago as to the inspection criteria for an MGA, but I don't know if that is what is being used. There seemed to be a lot more exceptions than absolutes if I remember correctly.

If they don't look under the access hatch, how would they see a single 12V battery vs 2-6 volters?


Chuck
C Schaefer

Phil

Sorry for the delay in replying. Actually it was a 12v socket that I fitted which I guess is like a cigar lighter but without the lighter. Because it is totally insulated by its plastic surround you just connect up the +ve to a +ve supply and -ve to -ve.

Dave R
D J ROUGET

This thread was discussed between 16/03/2008 and 31/03/2008

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