Archived unstable power to fuel pump thru factory harness...

[TheDodgeMan69]

Have a 98 24v with a fass fuel pump that was installed about 6 months ago. It is just the fuel pump, not the air fuel seperator kind. noticed the truck was surging alittle and saw the fuel pressure was at 0psi. Immediatly shut truck doen on the side of the road. The pump was wired in using the fass harness hooked to the factory fuel pump power port. My question is: What circut or relay powers the 'factory' plug to the fuel pump? Rightnow the truck is disabled and is my daily driver, leaving me driving a v10 dodge at 9. 9mpg



Ps fuel pressure gauge will raise to 15ish psi when you click the key on, then it will drop to 0psi and stay there, even if the truck is running... Thanks for the help!!!

 

[Sticks]

Is the pump running? Just because the pressure is 0psi does not mean the pump is not running.

You could have a pressure relief valve stuck.

IIRC the FASS systems only use the factory harness to control a relay.

Start the truck and make sure if the pump is running or not. Then see if you have voltage to the pump.

 

[TheDodgeMan69]

I understand the factory harness only runs a relay... I installed it... Once the truck is running thou there is no power to the factory harness, therefore the relay is not turned on and the fuel pump is inop... I havnt seen any blown fuses, and switched some of the relays around under the hood with the same result...



As far as p[ower to the pump; no. as soon as the key is turned on, the pressure goe sto 15ish and drops to 0, like it should... But when you start the truck the pump wont kick on... I tried switching the fass out with the carter 'relocation' fuelpump, with the same result. The only way i had it working, to get it home, was by hot wiring the pump directly...

 

[TheDodgeMan69]

I hate the vp44!!

 

[Bob4x4]

I understand the factory harness only runs a relay... I installed it... Once the truck is running thou there is no power to the factory harness, therefore the relay is not turned on and the fuel pump is inop... I havnt seen any blown fuses, and switched some of the relays around under the hood with the same result...



As far as p[ower to the pump; no. as soon as the key is turned on, the pressure goe sto 15ish and drops to 0, like it should... But when you start the truck the pump wont kick on... I tried switching the fass out with the carter 'relocation' fuelpump, with the same result. The only way i had it working, to get it home, was by hot wiring the pump directly...

The OEM pump feed is directly from the ecm...

 

[TheDodgeMan69]

#@$%!



Ok... Hot wired the fass today. With the truck off, it made 15psi. Started the truck and it dropped to about 13ish. thought everything was 'hunky-dory' and was just going to find a new switched wire to tie into when the radio shut off, and the gauge read 0psi. when i shut the truck off it goes up to 15, but EVERY time i start the truck now it dops to 0psi!!!!!'





AAAAAAAAAHHHH!!!!!#@$%!

 

[Sticks]

Mechanical fuel pressure gauge or electric? Is your fuel pressure gauge the one that is flaking out, not the FASS, or ECM?

If the radio died and your gauge read 0 psi... . Was the pump still getting power at that point?

 

[TheDodgeMan69]

Mechanical gauge tied right into the supply line at the vp44. The gauge sorks fine, as soon as you shut off the truck the fuel pressure goes back up. Im at a loss and installed another pump last night. now 26 psi of fuel pressure with the truck off, 0 when its on!!! aaahhh!!! is there some sort of a pressure regulator in the vp? or should i say; what controles the fuel pressure when the vehical is on... ?

 

[Sticks]

There is a pressure relief valve built into the VP fuel return banjo bolt, but it is mechanical and does not care if the truck is running or not.

 

[Sticks]

After more consideration on the matter...

You posted that the radio died and the pressure dropped. What do you have your primary power to the FASS tied into. If the radio died and you did not blow a fuse, I am thinking a bad ground or power connection. Are you still hot wired or did you try another switched connection. There are other power sources that only have power for a brief period at key on & start - Grid heater circuit for example.

Verify your fuel pressure with a separate gauge. You may have something that is plugging your line to your primary pressure gauge.

Again, verify voltage at the pump (s) when the pressure drops. If you have no voltage, then verify at the source.

I forgot, there is also a relief valve built into the FASS pumps as well.

 

[TheDodgeMan69]

Ok, went to rockey mountain cummins today and explaned the situation... they say its the mechanical pressure relief... found one online for 27. going to but it and try it. Just replaced the grounds and low and behold, the radio came back... so two completely different senarios... voltage at the pumps reads 13. 2 -ok- ... so i will reply back when i get the new pressure relief installed...

 

[TheDodgeMan69]

replaced batterys and cables... truck runs five now... must have been some (+) cable corrosion or somthing... new group 31 batterys and some modifying of the stock batt box. works like a champ now.



thanks everyone for the help.