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Fitting a battery cut off switch


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I want to fit a battery cut off switch on my TR3a - not primarly as an anti-theft device just for practical reasons.

 

I am converting the car to positive earth and have fitted an alternator so I am doing away with the regulator box. I was thinking of fitting the isolating switch on the bulkhead approximately where the regulator sat. It's one of those with a removable key so I was planning to put the key on the cabin side up under the dash next to the steering column and the terminals would then be on the firewall adjacent to the battery and above the started solenoid.

 

Before I start cutting/enlarging holes in the bodywork, does anyone have any better ideas pls?

 

Thks Ian

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Ian I fitted my isolator switch on the negative side of the battery, I routed the Plastic covered Neg cable through the bulkhead and mounted it inside the cabin behind the parcel shelf sorry I dont have a pic.

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Two precautions. Use good thick grommet for live cable through firewall (or proper connector block). Dont ever switch engine off with the isolator switch -pop goes the alternator diodes...

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Behind the brake cylinder: there's a plate behind the 'brake pocket'. Unscrew the plate, install the cut off switch. Route the cable from the plate to the battery and the other cable can be routed to the starter motor.

 

You can reach the switch behind / under the dash, behind the steering wheel. And to make things easier: I've used very flexible welding cable.

 

Menno

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Ian I fitted my isolator switch on the negative side of the battery, I routed the Plastic covered Neg cable through the bulkhead and mounted it inside the cabin behind the parcel shelf sorry I dont have a pic.

 

 

Pete, Why would you put the isolator on the earth? To me the logical place for it is on the live feed.

 

And did yo umount the switch on a bracket so that both the switch key and the connectors were inside the car or did you have the key sticking out on engine side of the firewall?

 

Questions, questions.

 

Rgds Ian

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Ian,

 

I wanted to have a battery disconnect as well. Found this simple unit at a parts store and

mounted it on the battery. A quick twist of the green knob and the power is disconnected.

Very easy to mount and use and no hot cables through the firewall.

post-7985-0-53967900-1298351540_thumb.jpg

 

John

'54 TR2

post-7985-0-53967900-1298351540_thumb.jpg

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Behind the brake cylinder: there's a plate behind the 'brake pocket'. Unscrew the plate, install the cut off switch. Route the cable from the plate to the battery and the other cable can be routed to the starter motor.

 

You can reach the switch behind / under the dash, behind the steering wheel. And to make things easier: I've used very flexible welding cable.

 

Menno

 

 

I like it - good solution Menno. BTW What size are your cables (mm2 cu)?

 

Thanks Ian

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If you are fitting an alternator, you will be swapping from positive to NEGATIVE earth.

 

It doesn't matter whether the isolator is on the positive or negative side of the battery, but it may be easier to put it on the negative side and have a direct feed from the battery's positive terminal to the starter motor.

 

I have a rotary switch mounted on a bracket fixed to the return lip (underside) of the dashboard on the passenger side (see photo - I know it's a TR4, but one could do this on a TR2/3/3A). The switch is within reach of my left hand when seated in the driving seat. One can find such switches (brand new) for as little as £10 in an autojumble.

I always isolate the electrics when parking the car.

 

As Peter Cobbold rightly advises, NEVER use the isolator to turn off the engine (unless the car's on fire, in which case a damaged alternator won't be a major consideration!).

 

Ian Cornish

 

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I like it - good solution Menno. BTW What size are your cables (mm2 cu)?

 

Thanks Ian

 

 

I don't know... The cables are thicker than the original cable. During the restoration of my TR I frequently visited the body shop. There I noticed the welding cables: thick and very flexible. I went to a hardware store where they sell welding equipment. They also sell welding cable per meter. I'm sure you can find the relevant data about current. I figured that industrial quality welding cable would be good enough for transporting the power from the battery to the starter motor...

 

Menno

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. Dont ever switch engine off with the isolator switch -pop goes the alternator diodes...

 

Sorry chaps but there must be something wrong here because most race cars only have the battery cut out switch to control the electrics.Also when the cars are scrutineerd they make you start the engine and then turn it off by the switch to make sure it works.If this was doing damage there would be an awful lot of damaged race car alternators out there. Enlighten me.

If using the fia type switch make sure that water cannot get at them.

Roy

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The "theory" is that an alternator (unlike a dynamo) requires a reference voltage, and that reference is supplied by the battery. If the battery is isolated, the alternator will try to wind itself up to an ever higher output voltage. However, if the isolator is turned "off", it is likely the engine will die at the same time as the alternator is winding its output upwards, so you may get away with it (i.e. engine stops before any harm done to alternator)!

 

Also, I believe that isolators provided for racing may have some sort of ballast resistor, so as to quench any wind-up. The bog-standard, cheap, device which I use is just a rotary switch, and I would not use it to turn off my engine.

 

Ian Cornish

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Hi Ian,

 

all the isolators I've ever used are just that, simple chop the power supply, nothing else - no sophistications or complications. If the car has an onboard plumbed-in extinguisher system, that utilises a separate trigger circuit. I've knocked off engines hundreds not dozens of times, and never had an issue.

 

Cheers,

 

Alec

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Pete, Why would you put the isolator on the earth? To me the logical place for it is on the live feed.

 

And did yo umount the switch on a bracket so that both the switch key and the connectors were inside the car or did you have the key sticking out on engine side of the firewall?

 

Questions, questions.

 

Rgds Ian

 

 

Ian my car was Negative earth, every German Hydraulic crane has an Isolator switch on the Neg side, if you put the switch on the "Live" side and there is a short, dial 999 and ask for the fire brigade, but by the time they get there its all over!

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Ian

 

Well in the years I was in motorsport it had to be positive cut out due to throttle cables etc,neg cable marked yellow ,maybe due to alternater run on? and why is the key red?

Edited by ntc
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Ian,

 

Must admit I'm probably with Pete Fenlon on this. So far as effectiveness is concerned what difference does it make which side of the battery is switched? If the battery is isolated from the chassis on the earthy side the whole lot is just as dead as if the live is switched off, but safer in the event of a short.

 

I only really thought about it when I bought a car with a switched earth and my first thought was that it had been done on the wrong side.

 

However, what a switched earth does prevent is the use of a separate single feed to any separate items such as a clock or cigarette lighter, which can be done with a switched live and constant earth. It also doesn't allow for the FIA approved, alternator safe, type of switch that Stuart mentioned.

 

Guess you pays your money and takes your choice, but I don't think there's an absolute right or wrong!

 

Malcolm T

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I'm with Ian Cprnish on this, and the evidence is three cooked alternators (diodes). The MSA rules demand the live side be switched, and the battery earth lead marked with yellow tape. I fitted ballast resistor but that did not protect diodes.

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