i have just put in a new sti engine but it wont start, checking to see if there was any power to the injectors i found that both wires to the injector plug were 12v constant, is this wright, if so how do they know when to open if both of the wires have a + feed, cheers phil.

 

anyone know?

 

Try measuring the voltage of each wire with a volt meter and you will find that one is marginally lower than the other, which is negative.

 

Fairly common to have constant 12V to one side of the injector then the other side is a switched earth in the ECU. This means open circuit when the ECU is not triggering that injector, then earth it thro a power transistor to trigger the injector.

If this is the case on the STi then checking the voltage with the engine not running will mean both sides of the injector wiring will be at 12V, one side from the 12V supply the other side because, without an earth at the ECU end, the whole lot of wiring will be at 12V.

Disconnect the plugs from the injector and you will find the supply side will still be at 12V but the other side will not. If it's sequential injection (ie all four injectors fired independently) then just disconecting one injector will do. If it's batched they're all in parallel and you'll need to disconnect all four.