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Fuel pump relay - what's the voltage? Got stranded this morning.

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    #1

    Fuel pump relay - what's the voltage? Got stranded this morning.

    I've had the fuel pump relay go out a few times. This morning it happened again, or so I thought. I cranked the engine and it didn't start.

    Pulled the relay and plugged in my trusty jumper wire to hot-wire the pump. I didn't hear it come on. Uh-oh. Tried starting anyway to no avail.

    Pulled a spark plug wire and verified that I had spark. Got a ride and called a tow truck.

    Once I got it home I hopped in and turned the key. Fuel pump on, engine starts. FML. Tried it again, no pump, no start. Tried several times with intermittent failure/success.

    Pulled the relay and checked voltage at the switching terminals in the harness plug. With the key on, i'm getting about 1.25 volts. With key turned to "start", it jumps to about 3.6V. Shouldn't it get more than that?
     Would this possibly indicate a bad ignition switch?

    I connected the relay itself to 12V and it switches every time. Out of curiosity, I connected it to a 1.5V AA Duracell battery and it did nothing.

    I plugged the jumper back into the harness plug. Now it makes the pump come on EVERY TIME. So I guess there is a chance I didn't plug the jumper in properly, prior to the tow. If that is the case maybe my fuel pump is OK.
    Tags: None
    #2
    Change the PCM relay if you are satisfied the fuel pump relay is okay. Power goes through the PCM relay first, (and on to the pcm and fuel pump) if it is bad, not much will work. And yes, you;ll have spark as that is all standalone until the engine starts.

    Alex.

    Comment

      #3
      +1 check the ECA relay (powers all the other relays in the car that are not hot at all times).
      Attached Files

      Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
      rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

      Originally posted by gadget73
      ... and it should all work like magic and unicorns and stuff.

      Originally posted by dmccaig
      Overhead, some poor bastards are flying in airplanes.

      Comment

        #4
        So is the PCM relay the same as the ECA relay?

        Would that cause my switching voltage at the fuel pump relay to be so low?

        Comment

          #5
          I once had a fuel pump relay that worked or didn't work as it pleased. I never knew when it would leave me stranded.
          It was an aftermarket relay not a Motorcraft. Since replacing it with one from Ford, I've had no more problems.

          Comment

            #6
            I'm pretty sure my fuel pump relay is fine. I was hoping someone could confirm whether the switching voltage supplied to the fuel pump relay is supposed to be so low. It seems to me like it should get a normal 12V.

            Comment

              #7
              The older cars had a stupid key that made interchanging the FP and ECM relays impractical unless you had a knife handy. The replacement relays are universal. The 90+ cars use standard Bosch relays. Swap it with the AC relay, since you need both fuel and ECM power to drive. The AC you can live without for short bits in the winter.

              Can't recall if the power side of the fuel pump relay comes through the ECM relay or not. I know the trigger voltage for the FP relay does. If the power side comes through the ECM relay, a bad ECM relay will definitely cause you problems at the fuel pump relay socket. If its off the starter solenoid via it's own fuse link, look there for problems. I have no idea what year or model vehicle you have, and I'm pretty sure it changed at some point over the years.
              86 Lincoln Town Car (Galactica).
              5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

              91 Lincoln Mark VII LSC grandpa spec white and cranberry

              1984 Lincoln Continental TurboDiesel - rolls coal

              Originally posted by phayzer5
              I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

              Comment

                #8
                Originally posted by pollock View Post
                So is the PCM relay the same as the ECA relay?
                yes. ECA on the diagram. Feeds the PCM/ECM and all other key on relays in the car.

                Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
                rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

                Originally posted by gadget73
                ... and it should all work like magic and unicorns and stuff.

                Originally posted by dmccaig
                Overhead, some poor bastards are flying in airplanes.

                Comment

                  #9
                  Originally posted by gadget73 View Post
                  The older cars had a stupid key that made interchanging the FP and ECM relays impractical unless you had a knife handy. The replacement relays are universal. The 90+ cars use standard Bosch relays. Swap it with the AC relay, since you need both fuel and ECM power to drive. The AC you can live without for short bits in the winter.
                  Napa lists fuel and pcm with separate p/n's. One (probably ecm) has a damper diode in it.

                  Alex.

                  Comment

                    #10
                    Probably actually the FP that has the diode, since that one is actually wired to an ECM output. The ECM relay is driven by the key switch, which could GAF about such things. Truth be told, all relays ought to have diodes on them. I get misfires out of my car from the headlight relay harness that I never put flyback diodes on. Going from high to low beams once in a while will give a distinct hitch.
                    86 Lincoln Town Car (Galactica).
                    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

                    91 Lincoln Mark VII LSC grandpa spec white and cranberry

                    1984 Lincoln Continental TurboDiesel - rolls coal

                    Originally posted by phayzer5
                    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

                    Comment

                      #11
                      Replacing the ECA relay was the ticket. Once I did that, switching voltage at the fuel pump relay was back up to 10-12 volts.

                      Thanks for the help.

                      Comment

                        #12

                        Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
                        rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

                        Originally posted by gadget73
                        ... and it should all work like magic and unicorns and stuff.

                        Originally posted by dmccaig
                        Overhead, some poor bastards are flying in airplanes.

                        Comment

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