Steal Stopping - Part Two

In the first part of this series we devised how we going to secure our Nissan 180SX for a total cost of under AUD$250. Now it’s time to get stuck into the install.

We’ll begin with the stand-alone supplementary immobiliser - something which can be added to any car, regardless of whether a security system is already fitted.

Necessary Components

A supplementary immobiliser can be a very simple system comprising just a handful of components. Our installation required only a relay, momentary pushbutton switch, some terminals and hook-up wire.

The total cost for components was around AUD$20.

Installation

Point of Immobilisation

There are three common points of immobilisation – the ignition, starter motor and fuel pump. Wherever possible, it’s best to choose a point of immobilisation that a thief would find inaccessible. Note that it’s pointless to immobilise the starter motor in a vehicle fitted with a manual gearbox – the car can be simply push-started...

In the case of our 180SX we chose the starter wire of the ignition circuit as the point of immobilisation. A diagram is useful for finding the appropriate wire, but you can’t go past a multi-meter. The starter wire of our car showed 0V, except when being cranked when it showed around 11V.

Once you’ve identified the correct wire, back-track it through the vehicle’s wiring loom - the immobilisation circuit should be buried as far into the car’s wiring loom as possible. Cut the appropriate wire and solder a length of hook-up wire to each exposed end. These hook-up wires will connect to the immobilisation relay. Note that the hook-up wire should be the same gauge as the wire that you’ve cut.

The relay we used is a conventional normally-open automotive relay that’s rated at 30 amps. A thirty amp current rating is ample for this application. Hide the relay under the dash where it’s inaccessible for a thief and connect the hook-up wires from the ignition circuit.

The wires from the ignition circuit must connect to the switched output section of the relay. We used insulated crimp terminals to connect each wire to the relay.

There are two more wires to connect to the immobilisation relay. An earth (anywhere convenient on the metalwork of the car is fine) and the input for arming/disarming.

Arming/Disarming

The arming/disarming circuit serves to energise the relay when you want to start the vehicle. The circuit can be anything from a simple hidden switch to a more elaborate arrangement, as used here.

The approach we took with our 180SX incorporates two separate switches that must be closed simultaneously for the vehicle to start. Furthermore, the use of momentary, normally-open switches means the immobiliser is effectively self-arming – it doesn’t require separate switching on/switching off.

The switches to arm/disarm the immobiliser can be dedicated switches hidden throughout the cabin or – more daringly – you might decide to make use of a factory switch.

The two arming/disarming switches should be wired in series. This means both switches must be closed simultaneously to energise the immobilisation relay. Connect the arming/disarming switch circuit to the immobiliser relay. Note that, like most good security systems, we used all-black wiring for the entire immobilisation circuit.

Now for the moment of truth.

Try starting the car with the immobiliser switch(es) closed. If the circuit is correctly oriented, the car should start as normally. You will hear the immobilisation relay click into action. Now try starting the car without touching any of the switches. If you hear nothing, congratulations - you have completed your stand-alone supplemental immobiliser.

In the third and fourth (final) parts of this series we'll take you through the installation of an off-the-shelf remote control security system.