Battery cut-off switch installation

[Tinkerman]

Need some thoughts on the best place to install a battery cut-off switch in a TR3A.
Moseso's latest story about zapping himself makes me go, himmmnn I have one, ought to put it in, but where?

Any thoughts greatly appreciated, Tinkerman

 

[Moseso]

I bought a very cheap and effective disconnect (which I should use more often) off eBay. It clamps to the ground (in my case negative) terminal on the battery, and the ground strap attaches to the other end of it. It has a green knob that is the switch contact. Half a turn is all it takes. $3.95, I think... I've seen 'em at the local auto parts places too.

Here's one on eBay now. Don't buy this one... His shipping is ridiculous. Link provided only for the photo of the unit.

 

[Tinkerman]

I should have done this first time around. Here is a pic of my Harbor Freight cut off switch. Not at all like the one you linked me to Moses. I kinda think the one I have is not a good design for our TR's. Plus if you lose or misplace the key your up the creek. Even though they give you two keys.
Thoughts?

Tinkerman

 

[mrv8q]

I'm a fan of the type Moses showed in the pic. I've got one on both cars, and haven't had any trouble w/ them....

 

[TR4nut]

And Harbor Freight has the other style too for about $4 in case you want to switch to it - I haven't used one yet, but I like it that there are no mounting issues involved.

 

[dklawson]

My Spitfire has a disconnect very similar to the one Moseso posted the link for. My father-in-law's Formula-Vee has the type similar to the Harbor Freight part with the red key. They each have their purpose. The battery clamp one is easier to install.

If you decide to use your Harbor Freight switch, it's not hard to keep up with the key. Put a lanyard cable through the hole in the key and anchor the other lanyard end someplace close by the switch. However, that defeats its use as a theft deterrent. You could just put the backup key somewhere hidden in the car and take the original key with you when you want to disable your car.

 

[Gliderman8]

I have the same cutoff switch in my TR6. Last winter while doing some wiring, the whole cockpit filled with smoke. Luckily, the hood was open so I just reached in and unscrewed the knob to disconnect the battery. This $4.00 switch saved my car!

 

[MGTF1250Dave]

Aloha,

I also have one like Moses has. My car has negative ground, and I have the cut out switch on the positive terminal of the battery. I'm not an electrical engineer but my reasoning is that if something electrical grounds itself, I can quickly disconnect the power source. If the switch is on the ground terminal of the battery, the disconnect will be after the inadvertent ground and not interrupt the power going to ground. Am I wrong?

 

[Moseso]

Dick --
If it came from Harbor Freight, it won't break the bank to abandon that key switch and get the other kind. The installation hassles of that key switch far outweigh any advantage it might have. The key type are required in race cars, in the cockpit. Fortunately for us non-racers, we don't have to do that. If it's just a service/storage cut-off you need, get the battery clamp one. It installs in one-minute flat.

 

[Moseso]

Aloha,

I also have one like Moses has. My car has negative ground, and I have the cut out switch on the positive terminal of the battery. I'm not an electrical engineer but my reasoning is that if something electrical grounds itself, I can quickly disconnect the power source. If the switch is on the ground terminal of the battery, the disconnect will be after the inadvertent ground and not interrupt the power going to ground. Am I wrong?

Aloha, Dave --
Yes you are wrong. The switch breaks the connection between the chassis (ground) and the negative battery terminal. Even though, you may have +12 volts at any given point, you can "ground" it to the chassis harmlessly, as the chassis is no longer attached to the negative terminal of the battery. Open circuit.

Worth adding --- the switch works your way too. An open circuit is an open circuit, at either battery terminal.

 

[DNK]

More work but easier to use

 

[MGTF1250Dave]

Aloha Moses,

Thanks for clarifying that for me. I had another car that had a radio installed and the in-line fuse was on the ground wire for it. When the radio shorted out, the fuse survived but the radio was smoked.

 

[TR3driver]

Thanks for clarifying that for me. I had another car that had a radio installed and the in-line fuse was on the ground wire for it. When the radio shorted out, the fuse survived but the radio was smoked.

A different problem. WHen the radio shorted, it evidently shorted to ground rather than drawing current through the fused ground wire. This is why fuses are normally in the hot lead.

But when disconnecting the battery for service, it's generally better to disconnect the ground clamp. The advantage there is that shorting anything to the body (including either battery terminal) will not cause a short. If you disconnect the hot terminal instead, then a wrench (or whatever) between the hot terminal and the body WILL cause a short.

Of course, either way, dropping a wrench between the battery terminals causes a short. But there isn't much to do about that, other than removing the battery entirely when working under the hood.

More hassle to install, but the advantage of the type Dick linked to is that it can be turned off without having to open the hood. Having once had a fire that burned through the hood release cable (on a non-LBC), I'd say there is some advantage to that.

That incident also taught me that dry chemical fire extinguishers don't work worth a hoot if you try to shinny under the front bumper and shoot upwards into the fire. Fortunately, the garden hose didn't suffer a similar limitation, and I got the fire out before it did TOO much damage.

But in my case, it was the battery itself that was burning (caused by trying to charge a battery with a shorted cell to 15v) so the cutoff switch would not have helped.

Immortal words : "Did you know your car was smoking?"

 

[angelfj1]

I should have done this first time around. Here is a pic of my Harbor Freight cut off switch. Not at all like the one you linked me to Moses. I kinda think the one I have is not a good design for our TR's. Plus if you lose or misplace the key your up the creek. Even though they give you two keys.
Thoughts?

Tinkerman

Dick: Before you invest in the HF unit, PLEASE take a look at this unit still available from Lucas. I have one intended for the TR250 but haven't yet installed it. I know how much time, money and sweat you have invested in your restoration and it makes me cringe a little to think that a bad switch could cause a real disaster. Remember, with a fresh battery if you were to close this switch into a short circuit, the switch would need to take up to 1000 amps or more without failing. At least Lucas has a published specification for the make/break and continuous current rating and a short time rating (thermal). I doubt that you can get this data from HF. Don't get me wrong. I have lots of HF tools, but I get a little nervous buying their electrical stuff.

These are still available from suppliers new or on eBay. I have seen prices ranging from $50 - $75.

 

[alfa33047]

As an old SCCA racer I have had to use the cut off switches for many years. There are two problems with these switches that you need to be aware of if you install one in your Triumph. If you cut your cut 0nand off battery switch often, they wear out very quickly, so be prepared. Secondly, if you buy a battery cut off switch, buy the one that also hooks to your altenator so that it kills the power to your altenator when you cut the power off. Best of luck, Bill C. Mesa, Az.

 

[TRMark]

I installed this switch years ago in my 4 when I rewired the car to accommodate an alternator fuses and relays, it disconnects the battery when removed. I make a practice of removing the key along with the ignition key whenever I park the car. I had to drill a hole in the firewall for the battery cable.

 

[Twosheds]

The key type are required in race cars, in the cockpit.

Sorry if I'm being too picky, but the racing organizations I am familiar with require that the cut-off switch be accessable from outside the car.

This is so that corner workers can run over to where you are wrecked and shut off the juice. The cockpit of a closed car might not be considered as accessable from the outside. Inside the cockpit of an open car? I dunno. Maybe they would accept that. What if the car was upside-down?

Mine was on the scuttle above where the cubby box would be on a street TR3. I couldn't reach it when belted in. But I could just turn off the stock ignition switch if need be.

 

[Moseso]

Yeah... You caught me guessing and ill-informed, again. I knew the switch had to be somewhere. I guessed at the location....

 

[PeterK]

Old radio antenna hole makes a good location (on a race car)

 

[TR3driver]

Of course, if it is just shorts you are worried about, it might make more sense to add some overcurrent protection to the electrical system. Practically all modern cars have at least one fusible link for major circuit protection, along with fuses for nearly every device.

Somewhere on BCF, Geo Hahn posted a nice photo of his fusible link installation.