Low power to fuel pump.

[MrShadetree]

I recently became the owner of a 1996 Ford Explorer XLT,4.0,auto. I did some work for said vehicle,knowing that it wouldnt start due to a fuel problem,but I figured that I would be able to repair it. Well,electrical isnt my area of expertise,and now Ime sort of stuck. heres whats happening... All fuses and relays test out ok. I turn vehicle over,and I get priming pulse to fuel pump of 12 volts,and then voltage falls to 6.80 volts.[measured at inertia switch, and fuel pump with the key in on position.Voltage dissappears when key turned off. Now,I remove fuel pump relay, and place jumper wire from socket 30 of relay, to socket 87 of fuel relay. I then can measure 12 volts at inertia switch, and fuel pump connector... The problem is that 12 volt reading is present with key on or off. Anyone have any ideas on this? This is the first Ford product I have ever owned as I am primarily a Datsun 280z collector,but this rig is really nice for being a 1996. Any help or advice would be greatly appreciatted.

 

[Runnin'OnEmpty]

Sounds like the fuel pump relay is at fault.
You can swap it out with the A/C relay for
testing.

Without looking at my wiring diagram, I'd say
the 12v with key on or off is normal. One of the
sockets you're jumping is probably constant 12v,
since the relay does the switching.

 

[2000StreetRod]

Fuel Pump Relay monitor

The Fuel Pump Relay power output (pin 87) goes to the Inertia Fuel Shut-Off Switch via one DK GRN/YEL wire as shown below and to the PCM Fuel Pump Monitor pin 40 (not shown in the photo) via the other DK GRN/YEL wire.

During normal operation when the ignition is switched from Off to Run the voltage at the Inertia Fuel Shut-Off Switch is battery voltage for a few seconds until the PCM de-energizes the Fuel Pump Relay. Then the voltage drops to a few volts which is the input voltage of the PCM pin when not driven by the Fuel Pump Relay output. When the ignition is switch to Start, the starter motor rotates the crankshaft and the crankshaft position sensor notifies the PCM the crankshaft is rotating. The PCM energizes the Fuel Pump Relay anytime the crankshaft is rotating provided the crankshaft position sensor output is functioning. If there is no signal from the crankshaft position sensor to the PCM the Fuel Pump Relay is disabled for safety reasons.

Fuel Pump Relay Pin 30 is hot at all times. Installing a jumper from Pin 30 to Pin 87 defeats the PCM control of the Fuel Pump Relay and the fuel pump should run continuously no matter what ignition switch position is selected.