12v basic relay wiring query

[gazm66]

Hi all. Totally new to auto relays so hope this is ok to ask.

Can anyone advise would the below work ok for triggering a standard 5-pin changeover relay to allow the blue circuit to be joined when the orange circuit is joined (by the alarm on a vehicle)? pin87a will be unused. This is how I intend to wire it.

The alarm has multiple "circuit" wires eg circuit1, 2, 3 etc to break circuits such as ignition, pumps, etc, and join them when authorised (pin code accepted in vehicle)

(the orange immobiliser circuit puts out 12v)

This means power is on pin85 and ground is on pin86 instead (normally vice versa), but think reverse polarity is ok on this relay anyway?

Thanks

 

[brianmoooore]

Some relays have a diode between the relay coil connections, 85 and 86 so these have to be wired with the correct polarity. If it doesn't have a diode, polarity is unimportant.
With the circuit above, with 0 volts on 85, 87 will be 0 volts and 87a 12 volts. With 12 volts on 85, 87 will be 12 volts and 87a 0 volts.
Is this for an older car? The best way to immobilise a car with an electric fuel pump and an ECU that predates immobiliser chips, is to arrange for the alarm to short circuit the fuel pump supply to chassis AFTER the fuel pump fuse.