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MG TD TF 1500 - Fitting a battery cut off switch

Reading about electrical fires in the archives I've decided to fit a cut off switch under the dash. I have a "quick release" fitting on the "-ve" terminal of the battery (+ve Gnd car)but these days I don't seem to be able to move too fast so thought it would be safer and quicker to reach for a switch under the dash. X marks the spot that I plan on fitting the switch in the image below. I should be able to reach it easily from the drivers seat.

This will however mean longer battery leads probably 15- 18" from battery to isolator and 15" back to the starter switch. Not being electrically inclined should I be concerned about increased voltage drop and heating the cables up. Does anyone know whether 16 mm sq cable will still be good enough for the job?

Thanks

AJ



A R Jones

AJ, it's a long time since I did my physics but I wouldn't have thought the voltage drop over 18" of extra cable would make that much difference (didn't some cars have the battery in the boot - pain in the whatsits, I know, but they still worked, and that was a very long run, in comparison).
Provided the cable is of sufficient gauge and the connections are solid and sound, I wouldn't have thought there would be a problem. If someone wants to do the maths, I've found http://cse-distributors.co.uk/cable/voltage-drop.htm which looks like it might help. A quick scan suggests it's in the order of mV/amp/metre. If I'm reading that right, I don't think you'd notice any voltage drop at all.

I don't know what switch you'd use, however. I've got one of those which piggy-backs on the battery terminal, to isolate the battery when I'm not using the car. Presumably someone out there knows of something which will suit what you want to do.

Beautiful weather here of late, BTW. Wall-to-wall sunshine, light southerly breeze and dry roads. Perfect for a last run or two of the season, I think.

- Tom.
Tom Bennett - 53TD 24232

Make sure you can reach that spot with all the wiring and stuff back in....
And you would want the switch on the +tive wire wouldn't you....
gblawson(gordon- TD27667)

I would add a caution that some of the battery disconnect switches sold here in the US by the regular auto supply stores, are made in China and are the source of some fires themselves due to poor alignment and tension of the electrical contacts.

I would get a good quality "brand name" switch, not one of the cheaper ones.

FWIW
Dallas



Dallas Congleton

I agree with Dallas. I've had two of those plastic cutoff switches fail on me. If you look at the racer supply sites you can find a more durable metal-bodied switch.
David Littlefield

Instead of reaching into the mass of wires that is under my dash I put the cutoff in the glove box.

Mort 1950 TD Möbius

Mort's installation is one of the best I've seen for a T series car. The added length of cable involved in this modification will introduce negligible voltage drop in the primary system. Just use the same gauge of cable as what is in the car already. The main thing to be careful about is that the lugs on the ends of each cable involved are properly crimped. The switch can be in either the hot side of the battery of the ground side, it makes little difference (I put the switch in the hot side in our MGB because I wanted the switch in the front portion of the passenger compartment and the battery, and ground connection is over the rear axle). Cheers - Dave
David DuBois

Thanks guys.

I have a TF. No glove box! It'll be there for all to see!

AJ
A R Jones

On my TF I installed a Hella battery disconnect switch, felt comfortable with the quality. I changed my car to negative earth, routed the positive to the cutoff switch. I have heard it is better to interupt the ground, but by using the positive on my car, the cable run was much shorter. Soldered a helmet end on the cable going to the battery which I routed to the switch, then connected the other cable from the switch to the starter switch.
I made a small bracket for the switch, painted it black, and located it on the "H" brace under the dash. The switch is out of sight, but I can easily reach it from the drivers seat with my right hand. It has become automatic for me to turn the battery off after I stop the car, and turn it on before restarting. If one has a clock that actually works, you can of course run a hot wire to the clock so it will continue to run with the switch in the off position. Works for me.
Larry Brown

This begs the question .. which post does one isolate? The Positive or the Negative? Apparently, there's a definite difference. I have noticed that on most batteries, corrosion always seems to start with the negative post.

Gord Clark
Rockburn, Qué.
Gordon A Clark

If the car has a clock, or any other device that needs power to it continuously, one can install a Poly Switch Resetable fuse device that will open at a set current, around 5 amps and then reset itself after the excess load is removed. The one needed is RGEF200 (4.7 amps) to RGEF 500 (8.5 amps). See www.circuitprotection.com for a source. Cheers - Dave
David DuBois

I wanted mine to be quickly accessable by the driver,,, sooo I mounted it to the "Hoop" under the cowling,,,
We are now in the habit of turning it off every time we stop the car,,, With out having to open up the bonnet,,
On one of our tours, we had one car's starter switch short out catch fire,, it would have been much easier to cut the electricity if a cut off switch was near the driver,,,,
Even in the glove box, if something happened while under way, it might be difficult for the driver to open the box and still keep control of the car whilst frantically trying to bring it to a stop,,,

SPW
STEVE WINCZE

I use a battery disconnect switch whereas you have to unscrew a green knot that breaks the circuit. I always do this when shutting the car down but lifting the hood each time certainly is inconvenient. I have been considering a remote battery disconnect made by Painless Performance that uses a solenoid and remote toggle switch for inside the car to open or close the circuit. Has anyone else looked at this?
http://www.painlessperformance.com/webcatalog/largeview.php?SearchField=30204

thanks,
Bill TD24570
Bill Brown

I think if I did use that, I'd put in a timer function to make sure it got switched off a while after the ignition was shut off. Did you read the whole thing?

"Note: Solenoid draw is 2.5 AMP while engaged. If left engaged (battery connected) with vehicle not running it will drain the battery."
but I wouldn't use it.
A W Parker

AJ I have ny cut off switch mounted on the steering column under the dash. Easy reach from the drivers position. I always cut off
the battery whwn i stop the Engine.
Thoralf. Norway TD 4490
Thoralf Sorensen (TD4490)

Yea, I see what you'r saying, I guess there is no free lunch. I wanted a disconnect in the cockpit but don't want to drill additional holes in the firewall or foot plate for the battery cables.

thanks,

Bill TD24570
Bill Brown

This thread was discussed between 20/11/2011 and 21/11/2011

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