Question about 1984 Ford F 250

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Electrial fuel tank relay- no power to fuel relay

1984 ford f250 electric fuel pump and carberater fuel system. no power from switch to relay.

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There are a zillion cars out there today that use the oil pressure sending unit as a dual function switch, the first function to send oil pressure information to the dash gauge or idiot light and the second function being to run the fuel pump after the car starts. During the start cycle there is a fuel pump relay that takes over to run the fuel pump. After the engine is running that relay turns off and the current for the fuel pump is supplied via the oil pressure sending unit If that relay fails then the car won't start until the oil pressure is up to three or four PSI.
The logic is that in a bad accident the engine would stop, the oil pressure would go to zero and the electric fuel pump would stop pumping fuel through a possibly ruptured fuel line thus preventing a disastrous fire.
When this relay fails the engine will start after the starter motor has pumped up sufficient oil pressure to send electricity to the fuel pump and run it. When you turn off the engine the oil remains in the galleys for a short period of time, typically several minutes for an old worn engine, to a few hours for a new motor with good tight clearances. Therefore, the engine will start up again shortly after shutting it off but typically will be relative hard to start for longer time periods afterwards when the oil pump has to pump the thin warm oil back into the engine oil galleys. When the engine is cold and the oil is thicker it doesn't take too long to generate oil pressure and the car will start up relatively easily, however not as easily as it would if the fuel pump relay were working. The test is to listen carefully when turning on the ignition key to see if you hear the fuel pump running - it's in the gas tank so listen there for a humming noise. If you hear no humming but the car eventually starts and runs fine, then replace the fuel pump relay.

Posted on Jun 07, 2010


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There is a fuse for the fuel on the firewall inside the cab:

Fuel Pump The fuel delivery system uses either a high or low-pressure in-line or in-tank electric fuel pump, with some models equipped with both. It is a recirculating system that delivers fuel to a pressure regulating valve in the throttle body and returns excess fuel from the throttle body regulator back to the fuel tank. The electrical system uses two types of control relays, one controlled by a vacuum switch and the other controlled by the powertrain control module (PCM) to provide power to the fuel pump under various operating conditions.

An inertia switch is used as a safety device in the fuel system. The inertia switch is located in the cab, generally under the dashboard on the right side. It is designed to open the fuel pump power circuit in the event of a collision. The switch is reset by pushing each of 2 buttons on the switch simultaneously (some models use switches with only one reset button). The inertia switch should not be reset until the fuel system has been inspected for damage or leaks.
When the ignition switch is ON, it turns the EEC power relay ON. The EEC power relay provides power to the powertrain control module (PCM) and the control side of the fuel pump relay. Power for the fuel pump(s) is supplied through a fuse link or high current fuse attached to the starter solenoid (battery side). From the fuse link or high current fuse, current flows through the fuel pump relay and inertia switch to the fuel pump(s). The fuel pump relay is controlled by the PCM.
When the ignition switch is turned ON, the fuel pump(s) will operate. If the ignition switch is not turned to the START position the PCM will shut the fuel pump(s) OFF after 1 second. The PCM will operate the fuel pump(s) operate the fuel pump(s) when the ignition switch is turn to START position to provide fuel while cranking.
After the engine starts, the PCM will continue to operate the fuel pump(s) unless the engine stops, drops below 120 rpm or the inertia switch is tripped.

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