This a circuit to wire a cheap normally open (NO) Relay (called an spst relay) to set up a momentary switch to cut your fuel pump, starter or ignition so that if someone tries to start your car, they have to find the hidden switch and hold it while starting. The other circuits I found on the net used a NO/NC relay (called an spdt relay) or two NO relays. This one is very simple and uses one standard cheap NO 4 pin relay you can buy online, from your autoparts store or Jaycar.
How it works
The relay works by jumping the current coming in from the ignition switch to energise the little magnetic coil inside while ever you press the momentary switch. This keeps the contact from 30 to 87 closed while-ever you have power coming in. If you turn off the ignition, the relay opens. If you use this circuit and find the relay stays closed even though you turn the power off you just need a relay that has a diode between 85 and 86 to stop the back-feed or fit a 10c diode yourself (there is a pic below to show you diode direction as they are a one-way electrical valve).
Relay basics
In case you didn't know this, relays are designed to take a heavy load (30 to 40 amps) while the switch that operates them only needs to carry a very light load as the relays trigger with about 100mA or less and 9-12v. This means you can add relays right next to the air-horns or headlight with a heavy power feed coming from the battery close by and trigger it with a lightweight switch from the interior. This way you get very little voltage drop, your headlights will be full brightness and your air-horn won't let the smoke out of the wiring. ?
An automotive relay has two parts. The power goes in on 30 and flows across a spring loaded contactor that is held away from the 87 terminal (which is the out) by the spring. When you apply power to the 85 terminal it travels through the little coil and out to earth (86) which creates an electromagnet to pull the contactor down. This is the normal wiring but you can usually reverse the 30 and 87, and the 85 and 86 terminals without upsetting things. This is what it looks like inside.
If you have the standard mustang headlight wiring, your headlights are probably only half as bright as they should be and if you lose your lights after a minute or so on high beam (as my car used to) you need to wire relays in for your low and high beam as the little circuit breaker behind the instrument panel is overheating. This page shows you how to do it if your wiring is in the standard spot. I hid most of my wiring so I wired mine with a $13 evilbay harness and just upgraded the relays to better quality ones. If you use the method from the link above, you can buy a bunch of 5 pin relay plugs on evilbay for $1 each as you can just ignore the middle wire if using 4 pin relays. I usually get the ones with relays for a bit more and have found the relays to be very reliable. You can always chuck a couple of spares in the boot in a sealed plastic bag, then you can swap them over quickly if one ever fails.
Other uses for relays
If you fit an electric power steering pump, they draw up to 50 amps which reduces the current available to the starter and can also drop the voltage so an EFI doesn't get enough voltage to let the car start (they are sensitive little things). Fitting a 5 pin NO/NC (spdt) relay that cuts the power to the pump while cranking will fix this. You may need two in parallel as 50 amps is beyond the capacity of one.
If you think outside the square a bit NO/NC SPDT relays can fix some electrical conundrums for you. I wanted my LED Lights to come on with the parkers but go off when the headlights came on, this relay fixed that problem. My alarm only accepted a negative feed (earth) to trigger it and the ultrasonic motion sensor only put out a 12v feed so I used a relay to convert the 12v to an earth trigger.
Caution
You need to be careful as there are a number of different versions of relays around that swap the terminals. So just because they look the same, the terminal functions can vary. However, the labelling is usually consistent, 87 is switched on with power, 87a is normally closed, however the Bosch 5 pin below found in many common cars labels the second output as 87b which is just a second 87 terminal to save running two relays. The relays usually have the circuit printed on them to show what they do and the terminals labelled underneath.
This is the 5 pin NO/NC SPDT Relay with a diode. Not all of them have the diode and most times you'll get away without one.
This is the 5 pin NO SPDT relay with two outputs.
There are a number of sites around for further info but this one has some nice pics and a simple explanation Basic Car Audio Electronics.
