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MG MGB Technical - Fire / Theft Protection Relay

I've been reading a few posts lately talking about both the starting of MGs without a key and possible fire hazards due to old possibly frayed wiring that is always live.

This got me thinking. I suspect it would be possible to insert a high amperage relay close to the battery, interrupting the cable from the battery to the starter. Since it appears on the schematic that all "always live" power through the car comes off the positive starter terminal, interrupting the cable from the battery to the starter would cut power to all other components.

This relay would have a 12v DC coil, with the positive side of the coil attached directly to the battery's positive post, and the negative side of the coil going to ground through a switch hidden up by the driver.

I have read some great anti-theft switch suggestions, including using the overdrive switch when no overdrive is on the car. This is good for anti-theft, but does nothing for fire prevention against aging 12v live wires. The relay would cut all power throughout the car except the 12v going to the new high amp relay.....which would have new, very heavy duty insulation.

An althernative would be a smaller relay off the starter. This could cut power to all circuits as well, but would leave the battery cable live....perhaps a less complex and yet adequate solution.

Any thoughts on this idea? Anyone done it before?

Thanks,
BH
BH Davis

It is more simple to use a manual main switch close to the battery, cutting off the line to the ground.
There are high currant releys offered for industrial and maritime usage, but you should calculate the load crefully

Ralph
Ralph

BH - Ralph is correct, a manual cut off switch in the battery circuit is a much simpler way to go and accomplish the same results. I installed on in the passanger foowell, where it is realtively easy to access, but out of casual sight. You can then wire a five or 10 amp fuse across the switch so that power is still supplied to radio keep alive circuits, clock, etc. but will blow if the starter is energized. For theft proofing, nothing beats changing the hand brake to a fly off style as used in the T series and MGAs. I have never seen anyone not familiar with these earlier cars who could release one of this type of hand brake. Cheers - Dave
David DuBois

Ralph,

It sounds like you are interrupting the positive battery cable with the manual cut off switch. Does that mean you are running a cable the size and weight of a battery/starter cable up to the passenger foot well and then over to the starter? Or is it the negative/ground side of the battery that is being run up to the foot well? If so, seems you would still need a very large guage cable.....like the battery to starter cable.......to do this.

Am I missing something here?

Thanks,
BH
BH Davis

BH,

The switch is mounted close to the battery box. I cut off the negative line from the battery to the ground as this is the safest option and is the normal way it is done on commercials.
It is also easier as you do not need to cut the positive line to the starter on negative ground cars and you can use OEM wires as used on most actual Ford passenger cars for the ground connection to the chassis.

Ralph
Ralph

BH - That was me that has the cut off switch in the passanger footwell. the switch can be mounted anywhere you see fit to mount it and in either the negative or positive side, it really doesn't matter where it is. I chose the passanger side foot well because the battery cable in our 66 MGB runs up to the inner fender on the passanger side anyway and it was just a small detour to run it to a switch in the footwell. It could just as well be mounted on the rear bulkhead just ahead of the battery boxes, I just wanted a position that was a little less visible. Regardless of where the switch (or relay) is mounted, the battery cable has to be routed to it. Cheers - Dave
David DuBois

The RHS front footwell is a pretty good idea and you could just interrupt the browns that run up from the solenoid. In that location the switch wouldn't protect any of the main cable (except the extra bit you would have to provide to get back down to the solenoid) and a problem with the solenoid contacts sticking closed is very unlikely.
Paul Hunt 2

My 1955 ZA had a wiring harness that was a fire waiting to happen. I replaced the harness, which was a relatively straightforward task and added a kill switch with a removable handle in the passenger footwell as suggested above. To power accessories such as the radio memory and the clock I added an auxiliary fusebox which made the wiring of modern conveniences a bit simpler.

Cheers,
Rich
Rich McKIe

I fitted one of the big red keyed cut off switches to my car as well. I mounted it to the right rear heel board behind the drivers seat (right hand drive). You can easily reach behind the seat and switch it off and remove the key even if you want (a bit of anti theft protection there). The main reason I fitted it though is for when I am working on the car so I can disconnect the battery. I switch the positive lead.

I did recently see an anti theft circuit that used a magnetic reed switch hidden under the dash. You pressed a button to arm the circuit then to disarm it you waved a magnet in front of the switch casually when you got in the car to disable the circuit.
Simon Jansen

I have a cut-off switch in the same place. It becomes an automatic process to reach behind the seat with my left hand (RHD car) at the same time as my right goes for the ignition switch. Trouble is sometimes I turn the cut-off switch off before I have turned off the ignition. No problems so far, but it isn't to be recommended!

Now a reed switch in the seat and a magnet in your back pocket would be neat.
Paul Hunt 2

This thread was discussed between 21/08/2007 and 31/08/2007

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