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88 s10 low voltage at fuel pump

edited November -1 in Repair and Maintenance
I have a 1988 s10 4x4 with the 4.3L engine and I am reading low voltage at the fuel pump which i just replaced along with the fuel filter. I'm only reading 9 volts w/ ignition in on position and am getting no fuel out of fuel line. I've also checked voltage at relay and it's over 12 volts and I also tried replacing relay and had same result...Please help

Comments

  • edited July 2008
    Did you replace the fuel strainer (the "sock") on the fuel pump? Does the fuel pump run? A dirty strainer could be stopping fuel flow.

    Are you reading voltage within two seconds of the ignition switch being turned to RUN? Isn't voltage supposed to be present ONLY then (for two seconds) or during cranking of the engine?
    Power is supplied to the fuel pump, when the engine is running and there is oil pressure, by the oil pressure switch. If oil pressure is lost, and the engine is running, The oil pressure switch will open and cut power to the fuel pump.

    The fuel pump has to have a ground. Check any ground wire, or strap, connection on the gas tank and attached to the body. Loosen and retighten any grounding screws.
  • edited July 2008
    Yes, I replaced the strainer and I also replaced the sending unit which replaced the wiring harness as well. I was reading voltage during cranking, and it was only 3.7 volts. I'll try rechecking the ground. The oil pressure switch, do you think that would cause a low voltage reading?
  • edited July 2008
    I doubt it. The oil pressure switch flips the relay. The power should be running from the hot, through the relay, and straight to the fuel pump. I suspect a bad relay or bad wiring inbetween. But, since you replaced the relay, the problem must be in the wiring or the connectors.
  • edited July 2008
    There is a wiring problem between the relay and the fuel pump. A gray wire goes from the oil pressure switch, joins a gray wire from the fuel pump relay, and goes to the fuel pump. The connection could be faulty.

    Read ohms, on the oil pressure switch gray wire, to the fuel pump. It should be less than 1 ohm. Read the ohms from the relay, on the gray wire, to the fuel pump. It should be less than 1 ohms. If you get more ohms on the gray wire, when measured from the relay, but ok when measured from the oil pressure switch, that section of wire from relay to the connection (or, the connection, itself) is faulty (maybe corrosion).

    Measure the ohms through the fuel pump to its ground. The resistance shouldn't be more than the resistance of the fuel pump motor. If you have the specs sheet of the fuel pump, it will tell you the ohm value of the motor. If the resistance is higher, check connections and ground point.
  • edited July 2008
    As others have correctly stated, the problem is due to a faulty connection. You stated that you measured 12 volts at the relay but I wonder if you measured the correct lead to the pump. You may have been looking at the coil lead. The pump lead should be a larger wire. You also need to check the voltage on load side of the relay contacts, I assume that you did that. Hellokit stated that the wire running to the pump is gray. If the voltage is ok on both sides of the relay contacts then the trouble is on the wire running to the pump from the relay.
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