1G Fuel Pump / MPI

[English_Rugby]

Should my fuel pump run when I turn my key to the accessory position? I am having a problem building fuel pressure in my DSM - somebody told me that it should be running in the acc. position - I can only hear it when I crank it and it's a wally so I definitely should be able to hear it when it is running. Thanks.

 

[bigpatofpa]

fuel pump only gets power in the on key position. no power to the fp in acc ign switch position

 

[luv2rallye]

Not exactly. The FP only gets power when the ECU see's CAS pulses which is when engine is cranking or running. Just key on without engine turning doesn't do it.

 

[Pale Horse]

fuel pump only gets power in the on key position. no power to the fp in acc ign switch position

I don't think this is true, a lot of people modify their cars to be setup this way but from the factory the fuel pump is only powered when the car is cranking or on.

Edit:

Hell I never refreshed the page before entering my reply and didn't see luv2rallye's better explanation. My bad.

 

[steve]

On a 1G there are two inputs to activating the fuel pump side of the MPI relay.

One from the starter, while the starter is running the fuel pump will get power.
The other from the ECU, when the ECU sees a pulse from the CAS it will turn on the fuel pump and as long as it keeps seeing pulses it will keep in on. Once the pulses stop for a few seconds the ECU turns the pump off.

Some DSM's seem to generate a random pulse on the CAS line when the ignition is turned on and those cars will have the pump start and then time out.

 

[English_Rugby]

Sorry I mean't on not in the accessory position. When I crank I get fuel but then when I stop the starter the fuel pressure dies - this leads me to believe it's a problem with my CAS? Thanks for the information.

 

[steve]

Could be the CAS but it's also very likely an ECU problem where it can't turn on the relay.
A datalogger will let you trigger the pump for testing and that would establish one way or another.