Circuit openening relay (COR)
What is it?
A circuit opening relay (COR) may as well be called a fuel pump relay, as that is what they control. They simply switch power to the fuel pump and are fitted to most cars with an electric fuel pump (recently cars have begun to use fuel pump ECUs for the job)
Why do you need one?
Because you need to know when to run your fuel pump. But most importantly you need to know when to stop it. You could run your pump from ignition power (runs whenever the key is in the ON position) and things would work, however there are two main problems with this:
- The pump will continue running if you crash. If you break a fuel line then fuel will spray everywhere and not be fun for anyone involved.
- If you leave your car not running, but with ignition on then you’ll flatten your battery.
How to wire one.
A COR is simply a relay with one switching contact and two coils. The switching contact controls 12V to the fuel pump. The two coils are there to close the contact when either the car is starting or the car is running.
The starting coil is simply triggered when the key is in the STA position.
The running coil is triggered differently on AFM and MAP engines. For AFM there is a contact in the air flow meter which closes as soon as the flap opens at all (meaning the engine is sucking air, so must be running). For MAP there is a pin (usually FC) on the ECU which is earthed whenever the engine is running.
If you aren’t sure which pins on the AFM to connect to then they are easy to find with a multimeter. Just find two which are an open circuit when the flap is closed and then go to zero ohm as soon as the flap is opened. They also tend to be the two pins at one end of the connector.