Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Battery Remote Switch

I see where Moss sells a remote located battery disconnect switch. My TF has the braided ground cable, Neg ground on the driver LHD side.
Does anyone know how these switches work and how connected etc, and any suggestions if the Moss $90 one will work? They don't really describe it well.



Normally one disconnects the ground from a battery. There are a lot of battery switches that are much cheaper, and install on the battery post. Most are for negative ground. My TD is positive ground, so I put it on the negative side anyway (as it fits that terminal), although it is still possible to short the positive terminal to ground.

I use the switch to take the battery off line, and just make sure to remove the negative cable before servicing the battery if necessary.

Here is an image. I got it on ebay for about $4.

The types that Moss sells: (145-785 $59.95) is meant for in the cockpit use. It requires a solid mounting location and heavy cables to route from the battery, to the switch, and back to the main lug on the starter switch on our T-Cars, where all the power comes together. I believe it is intended for the power supply side (not the ground). The handle is sometimes removeable. The battery switch is required for racing and is an accessory that makes your car look more serious. (145-770 $84.95) is meant to be tucked away. I would imagine that the switch is designed to interupt the ground side, so you would run a heavy cable from the ground to the switch and from the swtich to your body ground. The engine and gearbox should get their ground from the braided line that runs from the A-tower (the body is attached to the A-tower just at the firewall) to the bolt on the gear box or oil sump. (145-795 $11.95) is like the switch I got on ebay for much less. It fits the negative post only.



Dave Braun

Dave, thatís very interesting. In my car there is the same unit installed as shown in your picture. It's installed at the same side as your but my TD is negative ground. That means using the unit the positive cable will be disconnected.
What's the reason for disconnecting the ground from a Battery as you say? Should I change the unit to the neg. side?
Thank you

Keith, And here is the switch hooked up to the ground side. Cheers Phil

Phil Atrill

Thanks for the good comments.
Dave after reading your description, I think I will go with the simple green one. I had one a few years back and it got real loosey goosey. Maybe they have improved.

Guenter, Phil and Keith,

The switch I use is made for the negative battery post which is smaller than the positive battery post. I tried making it wider to fit the positive battery post and installing it there, but my really neat braided positive ground cable would not tighten enough to make a secure attachement, and I decided not to try to alter it, etc. So I installed it on the negative side, which on my car is the power side. Pretty handy to disconnect power before goofing with the starter switch, fuel pump, etc.

The reason you really want to shut off the ground side is that it is the first thing you do when removing a battery, to protect yourself from shorting out the battery against the chassis. Since the ground side is already at the chassis potential, you can't hurt anything if your wrench slips and you hit the chassis and the ground terminal at the same time. But if you happen to do the power side first (the positve terminal on a negative ground car, or the negative terminal on a positive ground car) and accidently hit the chassis or body with the wrench while still in contact with the power terminal on the battery you'll melt something.

One time I was helping Diane change her oil on the B. She has a bad shoulder, and shifted around to get better leverage. All of a sudden there was a loud 'pop' and she was out from under the car in a flash. I think she might have levitated. She had struck the positive lug on the starter with her wrench while connected to the oil plug and she blew the lug connector (a pretty substantial copper terminal) right off the cable. Easy fix, but now she disconnects the battery before she starts an oil change.

Dave Braun

Dave B is correct in stating that when removing the leads from the battery, you always disconnect the ground side first. This is done because if the wrench slips and strikes ground while still touching the battery terminal, there won't be any arcs and sparks produced (and the wrench in your hand doesn't go into the incandescent mode, leaving ugly and painful marks on your hand). This is just about the number one safety rule when working on cars (right nest to removing the ground lead from the battery when working on a car). This said, there is no reason that a battery shutoff switch has to be installed on the ground side of the battery. The cut off switch removes the connection to the battery without having to use a wrench so there is no chance of shorting a wrench to ground. I have the battery shutoff switch in our MGB (one of the black plastic switches with the red key to turn it on and off) on the positive side of the battery (negative ground vehicle), because the switch is located where the positive cable enters the engine compartment where it was easier to install the switch. the only time that this become an issue is when removing the battery (only if the switch is not turned off) and anyone removing the battery should be following the safe method of removing the ground lead from the battery first anyway (I would not trust a switch when removing a battery anyway. Cheers - Dave
David DuBois

I have same switch as Phil's picture.
However I routed mine out to fit positive cable and dipped the "green knob" in RED plastic coat, (so there is no confusion that it is + lead).
My TF have been converted to negative ground.
Just like the idea of bieng able to kill all voltage to the cars electricial systems at one east to get at point.
David Sheward

One advantaged of installing the switch on the "hot" terminal and switching there is that then there is no voltage seeking ground anywhere on the wring harness. Switching at the ground terminal does isolate the battery from ground, but the other terminal of the battery, if still connected, could discharge to ground through any connected circuits.
Be aware of the cheaper versions of these switches showing up at Harbor Freight and discount type stores -they are made off shore, are cheaply made, and fires have been reported.

Dallas C Congleton


The battery should be connected to the car at two points. The main starter lug (hot as you call it) and the side of the battery box (the firewall). Discnnecting either will prevent a voltage seeking ground.

Dave Braun

Attached is a picture of how I widened the opening on the disconnect switch to accommodate the larger diameter positive battery post. Hold the nut stationary while advancing the bolt against the temporary stop. You may have to file the battery post slightly to allow the clamping bolt to be inserted through the holes in the disconnect switch.

Also shown is a picture of the disconnect switch as mounted on my positive earth TD.


Larry Shoer

Thanks Dave for the basic DC circuitry. The point would be to to eliminate the wiring harness, in case of unknowns completing the battery circuit to ground.

Dallas C Congleton

I used to use those inexpensive cut off switches on the TD and a tractor,,,,, untill one failed !! Thankfully, no fire, but it failed to make a good connection,,, I now use a regular racing type cut off switch that I mounted to the "hoop" under the dash... Much easier and more conveinient to use mounted there rather than having to open the bonnet every time ,,,, and much harder for some one else to find!!!

Steve Wincze

dallas, i'm still not clear what unknowns could complete a circuit if either battery post is disconnected at the post. the only possible unknown i see is a combination of faulty circuit coupled with complete failure of the disconnect switch so the circuit is active again. what am i not seeing here? haven't had lunch yet so perhaps i'm not hitting on all cylinders right now. regards, tom
tom peterson

Tom, as usual on these threads, there may be more than one direction or viewpoints taken, and more than one level of interpretation. I am not sure what the original objective was to use a battery disconnect switch- whether one of isolating the battery to deter discharge, or for safety concerns - but my comments were reflective of the comments about the wrench contact on the starter lug burning the wire lug off which could have burned the entire wire and adjacent wires in the harness. The thought was to place the switch to isolate the wiring in case of shorts, which can come from the power side to any common ground.
True , if the ground cable is removed, the battery is no longer in the circuit - but as circuit breakers in house panels are interrupting the power feed, not the neutral or ground side; and the fuses in your car are on the power feeds, not the ground return wires - it seems to be "normal" practice.
Either side will suffice on the battery to interrupt a completed DC circuit, your choice, as is the quality of the switch.
Sorry for complicating an issue.
I don't use a switch, just disconnect the cables and then hang them down out of the off gas area, as I usually leave a battery tender connected.

Have some lunch ;^)
Dallas C Congleton

OH! So you were being sarcastic, Dallas, when you thanked me for "the basic DC circuitry".

Dave Braun

This thread was discussed between 14/08/2010 and 16/08/2010

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.