Fuel pump relay

Tiny
FREDSIMON
  • MEMBER
  • 1994 FORD F-150
  • 6 CYL
  • 2WD
  • MANUAL
  • 300,000 MILES
Dual tanks - rear pump works - front pump sometimes - in power distribution box, which relay works front tank?

thanks for the quick reply about the #2 relay - does this mean that there is only one relay even with 2 tanks? It is the front tank that I'm interested in.
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Wednesday, December 10th, 2008 AT 8:44 AM

15 Replies

Tiny
DAVE H
  • MEMBER
Fuel Pump Relay.....On engine bay fuse/relay block. relay # 2


https://www.2carpros.com/forum/automotive_pictures/266999_fuse_14.jpg



Fuel Pump Actuation
When ignition switch is turned to START position, PCM operates fuel pump relay to provide fuel for starting engine. PCM senses engine speed and shuts off fuel pump by opening ground circuit to fuel pump relay when engine stops or speed drops to less than 120 RPM. When ignition switch is in ON position, EEC power relay is energized (contacts closed). Power is provided to fuel pump relay and timer in PCM. Fuel pump receives power through fuel pump relay contacts.

When dual tanks are used, each tank is equipped with a low pressure pump. Such a system has a total of 3 pumps: 2 low pressure and one high pressure.
High Pressure Fuel Pump
The high pressure fuel pump is positioned inside fuel tank. A reservoir is built onto pump and sender assembly.
In a 2-tank system, sender assembly handles switching of high pressure fuel through internal valves. Should one tank overfill during use (return line returns fuel to wrong tank), pump and sender unit in overfilling tank need replacement.
The high pressure fuel pump is capable of pumping over 33 gallons (125L) of fuel per hour at a working pressure of 39.2 psi (2.75 kg/cm2 ). This pump also has internal pressure relief and discharge check valves.
Mechanical Selector Valve
A driver-operated selector switch controls selector valve for switching fuel supply from one tank to another. This valve switches fuel supply and return lines from one tank to another in response to fuel pressure from in-tank pumps acting on its actuating diaphragm.
The diaphragm switches tank connection when less than 2 psi of fuel pressure is acting on upper side of front tank and lower side of rear tank. Valve function depends upon proper operation of in-tank low pressure pumps. In all dual-tank vehicles, excess fuel not used by engine is returned to same tank from which it was pumped.*****
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Wednesday, December 10th, 2008 AT 9:31 AM
Tiny
RICKB
  • MEMBER
  • 1994 FORD F-150
I have a ford f-150 1994 300 6cyl 4.9L fuel injected twin tank truck, yester day while driving it just shut off, I hot wired the fuel pumps they work fine, I replaced the fuel relay, truck still won't start, 12 volts going into the relay but only 6 volts comming out, pumps need 12 volts to work, do you know whats wrong?
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Saturday, June 6th, 2020 AT 1:08 PM (Merged)
Tiny
ASEMASTER6371
  • EXPERT
Disconnect pump and see what the voltage is. The pump may be bad pulling down the voltage.

Roy
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Saturday, June 6th, 2020 AT 1:08 PM (Merged)
Tiny
RICKB
  • MEMBER
With that done, there is only 6 volts comming out of the fuel relay
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Saturday, June 6th, 2020 AT 1:08 PM (Merged)
Tiny
ASEMASTER6371
  • EXPERT
It may be a bad relay but before replacing it, disconnect the inertia switch and measure as well.

Roy
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Saturday, June 6th, 2020 AT 1:08 PM (Merged)
Tiny
RLM18
  • MEMBER
  • 1994 FORD F-150
  • 6 CYL
  • 2WD
  • MANUAL
  • 60,000 MILES
The truck started running rough and now is hard to start. Checked out plugs and wires. Noticed the fuel pump does not shut off when turning the key on. Tried swapping around relays but I do not which is which. I may have mixed them up. There are 3. Two are the same. I do not have a manual. Which relay goes where? What else could cause the fuel pump to not shut off? Thank you.
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Saturday, June 6th, 2020 AT 1:08 PM (Merged)
Tiny
JDL
  • EXPERT
Hi, thanks for the donation. Relay number 2, should be for the fuel pump. If you got the relays mixed-up, read on the relays, if the diagams-- amp rating--model numbers are all the same, it won't matter which relay goes for the fuel pump as long as it is functional. If you plugged a faulty relay into a different circuit, it could effect that circuit.

I'd have to test the circuits at the fuel pump relay, there is a power relay that sends voltage to coil side of fuel pump relay, I'd check that also. The pcm grounds the coil side of fuel pump relay in orfer to energize.


https://www.2carpros.com/forum/automotive_pictures/170934_ford_f150_underhood_fuse_box_1.jpg

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Saturday, June 6th, 2020 AT 1:08 PM (Merged)
Tiny
PASEFLOORS@GMAIL.COM
  • MEMBER
  • 1994 FORD F-150
  • 126,211 MILES
Hello I have a 1994 f-150 with a 4.9L and dual tanks. I shut off the truck and it would not start. Both fuel pumps would not come on. The fuel pumps will sometimes both work then when I shut off the truck and then they both wont hum when theyre priming. Ive changed the maxifuse the relay and the inertia switch. I can hear the relay clicking on when I first turn the key on. The only thing I havent changed is the tank selector switch but I took it out and made sure it had a secure fit. And when its working I can switch to either tank with no problem, then both will die. Any help you can offer would be greatly appreciated. Thanks again. Joe
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Saturday, June 6th, 2020 AT 1:08 PM (Merged)
Tiny
ASEMASTER6371
  • EXPERT
The switching valve at the bottom is the only thing you have not changed. Check it and see if it is working

Roy
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Saturday, June 6th, 2020 AT 1:08 PM (Merged)
Tiny
PASEFLOORS@GMAIL.COM
  • MEMBER
Thank you for taking the time to help me out. Im not familiar with the switching valve. How would I check it and where is it located? Thanks again
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Saturday, June 6th, 2020 AT 1:08 PM (Merged)
Tiny
ASEMASTER6371
  • EXPERT
Under the truck. Find the fuel lines and they all come together at one point and the switch will allow one tank at a time to supply fuel.

You need a voltmeter to verify voltage and switching

Roy
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Saturday, June 6th, 2020 AT 1:08 PM (Merged)
Tiny
PASEFLOORS@GMAIL.COM
  • MEMBER
Thanks Roy, this is my first time to a mechanic forum and you where very helpful. This valve would have to be the problem I will test it in the morning. Thanks Joe
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Saturday, June 6th, 2020 AT 1:08 PM (Merged)
Tiny
ASEMASTER6371
  • EXPERT
Good luck

Roy
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Saturday, June 6th, 2020 AT 1:08 PM (Merged)
Tiny
DAVE PHILLIPS
  • MEMBER
  • 1994 FORD F-150
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 300,000 MILES
I have power at fuel pump relay but no power to fuel pumps
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Saturday, June 6th, 2020 AT 1:08 PM (Merged)
Tiny
MERLIN2021
  • EXPERT
It's either the wiring to the pumps, or the PCM is not recieving a signal from the cam sensor.
FUEL CONTROL Precise fuel metering is accomplished with EEC-IV system. The PCM continually monitors engine operating conditions based on information received from various sensors and switches. In response to information received, PCM calculates optimum air/fuel mixture in relation to present engine operating conditions and affects required metering adjustments through output actuators control. Fuel Injection The PCM controls fuel injectors to meter pulse width or time each injector is energized. Each injector receives battery voltage through ignition switch circuit. The PCM-controlled ground circuit completes circuit to energize injector. The PCM receives inputs from engine sensors to compute fuel flow necessary to maintain air/fuel mixture ratio throughout entire engine operational range. Each cylinder has a solenoid-operated injector which sprays fuel toward back of each intake valve. Injector bodies consist of solenoid-actuated pintle and needle valve assembly. Check for spark and let me know if it has spark or not, at the plugs of course!
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Saturday, June 6th, 2020 AT 1:08 PM (Merged)

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