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My 87 4.0 fuel pump stopped working and I narowed it down to the fuel pump relay not getting any power supplied to it. I fixed it by just running a hot wire over from the battery. The pump still does work right and cycle on and off but what could have caused there to be no power to the relay? Is there an in-line fusible link? Thanks Josh
Check the ballast resistor. It's a big, white, ceramic resistor mounted to the driver-side fenderwall or firewall under the hood. It should NOT be cracked in 2 pieces... Resistance value should read approx 1.1 Ohms. A new one will run about 5 bucks if you need it. This circuit powers the pump.
I'm pretty sure the 87 had no ballast resistor. You'd still do well to check for one, but if there isn't one, you might need to go through the wiring with an ohmmeter.
Got the same problem with an '89 except mine is on the ground side. Are you having to jump the + side of the battery to the coil of the relay or the load side (straight to the pump)? There is a fusible link on for the relay that comes straight off the big post on the starter relay. It's labeled "Fusible Link G Orange" in all the diagrams. Basically with the key off and the relay unplugged (should be the second relay from the front of the vehicle) you should have 12v + to only one of the terminals where the relay plugs in.
The Fuel Pump relay coil, for the '88 4.0l, gets 12v from the ignition switch. The ECM supplies a ground to the relay to actuate it. When actuated, the relay supplies 12v to the fuel pump, to the EGR/EVAP solenoid, O2 Heater and A/C relay coils; the ECM supplies a ground to each of these relays to acuate them.
The Fuel Pump relay switches battery power that first goes through the fusible link, described by Micahroy, above.
The output from the relay (12v) can be seen at test point D1-8, located in the diagnostic connector (4.0L engine).
My field service manual indicates the "ballast" resistor may be added to '87 models if the owners complain of fuel pump noise. I believe the resistor was added by the factory for all '88 and later models; could have been added to mid-year '87s--as XJrob noted.
