Can I wire my pump this way? / Diagnosing fuel pump issues.

[jon95dsm]

I'd like to see if I can wire my fuel pump a certain way but first here is some information about why I'm asking.

Ever since I installed a Walbro 255 and Aeromotive fpr I've had an issue with the fuel pump randomly cutting off. I usually can't re-start immediately but I can after waiting 10 minutes or so. I rewired the pump with the STM kit and nothing changed. About a month ago I posted asking about this problem and after a discussion I decided to replace the OEM relay. That didn't fix anything either.

What I have to figure out now is whether I have an electrical issue or a faulty pump. I have read some threads about diagnosing the electrical system but all of those threads were about no-start issues. Since my problem is so intermittent I'm not sure if I would see the electrical problem even if that ends up being the culprit.

What I'd like to do to determine if my problem is electrical or the pump is re-wire the power wire that acts as a signal wire to the STM relay. I would do this by putting a fuse on a 12v switched source, running that to a manual on/off switch, and then connect it to the signal wire on the STM relay. This would allow me to have full control over whether I'm sending power to the STM relay or not. I would be bypassing the oem sensors, ecu, and relay. If I still get random pump loss, I know I have either a bad pump or a loose/dirty connector at the pump. If I don't, I know my problem is either a sensor, wiring, the ecu, or the oem relay.

Would there be any adverse effects to running the pump wiring this way? I didn't see any threads that mentioned ECU-controlled fuel flow for cruising or anything like that but I wanted to be sure I won't mess anything up by doing this.

 

[steve]

Factory wiring uses a switched ground from the ECU to the MPI (or Fuel Pump) relay to activate it which in turn sends battery voltage to the fuel pump.

Depending on where you want to interject your control will determine if you want to add a switch to ground or a switch to 12v. Assuming that the STM relay is after and switched by the factory relay your plan of adding an additional switched 12v source to the STM relay input should work. I'd go as far as including two diodes so neither source (factory relay or your switch) could power the circuit in reverse.

Don't forget that the ground for the STM relay could also be questionable causing it to be intermittant.

Unlike other Mitsubishi's the DSM only either turns the pump on or off. No PWM or high/low voltage.

 

[jon95dsm]

Unlike other Mitsubishi's the DSM only either turns the pump on or off. No PWM or high/low voltage.

I thought this was the case but wanted to be sure. I'll look into the rewire ground again but I had this issue before the rewire as well. My STM relay is run from the factory power wire in the back, just before the pump. To clarify, my plan would be to not run the factory power wire at all. My switched power would be the only source connected to the signal on the STM relay. In this case I would not need to worry about adding diodes correct?

 

[Murdertalon]

My fuel pump actually got much stronger (psi) and comes up to pressure faster with a rewire. I added a heavy ground wire to a back seat bolt, then tied that to: the pump, relay and the wire coming from the ECU. It seemed to help my car actually start faster it used to have to crank a little before it would fire, now it fires right up.

 

[dustyboner]

if your using a standard 4 post relay like this:

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normally you would have the relay hard wired to ground (- ) so it would be triggered when supplied with by B(+).

the easiest thing to do is
wire it like this:

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i usually just use a mitsubishi relay.

on the mitsu relays, the copper terminals will be for the main load.

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[Murdertalon]

That ground wire is fine for triggering the relay. I would add another ground from a bolt to the pump. Everything else seems like it will work out as long as that little jumper wire is just running the relay and has nothing to do with the pump feed which looks like is the case. I would place the switch somewhere accessible from the drivers seat. That setup will force the fuel pump on bypassing the ecu.