Fuse block = 12 volts Pump Relay getting 12 volts DG/Y wire # 238 on wiring diagram only shows 6.58 volts at inertia switch
Sounds like a bad connection between the two points.
It may be easier to run a new wire than to tear the harness and connectors apart.
But what would be the cause of the power at the uel pump prime connector (line to truck to inertia switch) to only read 6.5 volts?
I didn't really understand your question - - if you have 12 volts at the fuel pump relay terminal and only 6.5 volts at the pump connector at the gas tank, something is using that voltage or there is a bad connection between the two points. If it is a bad connection it may be easier to run a new wire from the inertia switch to the pump.
Thank you for your reply. To clarify, on the fuel pump relay harness, I'm getting 12volts at pin 30. when I test pin 86 I get 0volts with the ignition switch off, and 12volts with the switch on. When I replace the Relay I get 12volts at the fuel pump prime outlet connected to pin 30 switch on or off and 6.5 volts at fuel pump prime outlet (DG/Y 238 wire) when the ignition switch is in the on position. this is the same voltage (6.5) that I get at the DG/Y wire at the Inertia switch in the trunk. If I run a new wire from the relay under the hood, where do I connect it in the circuit?
My colors don't match yours and I don't have a relay in front of me. But here is what I have. You should have a battery feed that is hot all the time, a hot with the key on, a ground from the computer, and the feed to the inertia switch. When the computer grounds the relay for 3 seconds the feed to the inertia switch should be the same as battery voltage. The inertia feed which is orange in my book goes to the inertia switch then to the pump. One confusing part is the pump prime outlet which is not on my diagram. I have red power from the EEC relay which is controlled by the key switch, orange to the inertia switch and then the pump, yellow from the battery power, and light blue - orange from the computer.
Again thank you for your time. I have been at a disadvantage in the fact I don't have the manual for my 1998 and have been using a manual for 1989-1993 Crownvic and Grand Marquis. The correct manual should be here by this afternoon. But, that being said hear is what I found in closewr inspection of the wiring on the vehicle. Pin 30 on the relay goes to the Fuel Pump Prime connector which is located near the relay cluster on the drivers side under the hood over the wheel well. It shows 12 volts all the time. Pin 87 goes to the other side of the Fuel Pump Prime connector, Pin # 8 on the PCM, and to the inertia switch in the trunk. When I test the connection at the Fuel Pump Prime connector I get 12volts as soon as I turn the ignition key, but then drops immediately to 6.5 - 6.9 volts. I get the exact same reading when I perform the test at the inertia switch connector in the trunk, 12v dropping to 6-5 - 6.9 volts. This wire also splits Pin # 8 on the PCM. Pin 86 on the Relay goes to the electronic distributor and splits elswhere including Pin 57 on the PCM labeled Power Output from EEC relay. Pin # 85 goes to the VIP Data Link and then to Pin 22 on the PCM. I cannot find a reason for the voltage drop once the key is turned. BTW, if I shoot Throttle Body cleaner into the throttle body, the vehicel will fire and attempt to run. Any suggestions as to where I should be looking? Fred
I don't have any suggestions until after you get the right wiring diagrams. Almost nothing you said in your last post matches the diagram I have. Bottom line is you should have 12 volts at the inertia switch UNLESS the pump is running. If the pump is running it would use most of the voltage in the circuit. And you should not have 12 volts at the inertia swtich beyond the first 3 seconds of key on time. The pink and black coming out of the switch is the last point before the connector for the pump in the tank.
I uplug the connector at the inertia switch to check the circuit, because the pump motor could be running but not pumping anything.
This test I performed at the inertia switch was with the connector unplugged.
How much voltage should there be once the initial 12 volts is present? Or should the voltage stay at 12 volts for the pump to function?
Stephen, Just received 1998 Grand Marquis Repair Manual. Will research and get back to you tomorrow . Fred
When you turn the key on, you should get 12 volts in the pump circuit for the 3 seconds the relay is energized by the computer.
1992 -2010 Mercury Grand Marquis LS 4.6L engine fuel pump wiring diagram as follows control circuit pin 86 from PCM relayto pin 86 control circuit FP relay, pin 85 from control circuit FP relay to pin 90 on PCM module, pin 30 FP relay from engine compartment fuse box 20a, pin 87 from FP relay to pin 40 on PCM module and to fuel pump prime connector and to inertia switch in trunk. inertia switch to fuel pump PK/BK. Fuel pump to ground BK. With inertia switch connector off, circuit tests at 12v immediately when key turned, then drops to 6.5volts. Should this voltage remain at 6.5v for the pump or stay at 12volts?
can email or fax the two one page diagrams to you if you would like to see them
Looks like the 6.5 volt current is a reference from the PCM. When the fuel pump relay is on the pink-black to the inertia switch should be battery voltage. The PCM grounds the fuel pump relay for 3 seconds then only if it sees oil pressure or RPM to know the engine is running. So what you have is probably normal. We normally bypass the relay with a jumper wire to power the pump as long as the key is on to test for fuel pump pressure. Its possible the PCM relay is faulty and not powering the fuel pump relay past the first 3 seconds. That is the power on the red wire going into the pump relay.
Some models have the computer "look" for oil pressure and some use RPM from the crank sensor to know the engine is running so that the fuel pump relay continues to get a ground from the PCM. When the car won't start and the fuel pump circuit is suspected, we bypass the relay to see if the engine is going to run off the pump, then go looking for why the pump is not getting power when it should be.
The pump only gets power for 3 seconds without the engine running to prevent a fire during a wreck.
So if the check valve in the pump is faulty and the pressure drops to zero when the engine is off, you have to cycle the key on and off several times to get the pressure back up enough to get the engine started.
When you say to bypass the relay with a jumper wire are you sayiong to perform this on the PCM relay or the Fuel Pump relay?
I was referring to the fuel pump relay, but you may need to do it to both to get constant power to the pump.
Thanks skychief2001 for your help. I changed the pump and screen this morning and hicle runs like a charm. I also changed the fuel filter last week as well. Thanks for your responses.
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SOURCE: where is the fuel pump relay switch on a 1992
This relay is almost always in the engine compartment relay box. If you don't see it there, please get back to me and I'll look it up.
SOURCE: 1999 Mercury Grand Marquis. with KOEO, voltage to
With my pump failure, I read the exact same 6.5 v at the inertia switch in my trunk with the key in the "on" position and engine not running or cranking. When cranking, the voltage to the pump jumped to +12V and if the engine did run, the voltage should stay at +12V.
The Ford manual states +12v should be at the motor for some number of seconds in the key "on" position then drop off if the engine is not cranking or does not start. In my pump failure case, I never saw 12V at the inertia switch in just the key "on" position, only +6.5V as the +12v fuel relay under the hood never engaged and/or stayed latched on.
In my situation, I measured an open circuit at the pump motor winding and suspect the +6.5V I was reading came from the Powertrain Control Module. I am also guessing if my pump winding was not open, the +6v voltage would have been +12v for a number of seconds for fuel priming as the fuel pump relay is temporarily energized, then drop to zero (relay off) if the engine does not continue to crank or run.
With my pump motor winding open, the Power Control Module may not have been loaded properly to function (reading +6.5 floating volts instead) in the key "on" position and therefore never climbed to the full +12V. There obviously was no fuel pressure at my schrader valve on the fuel rail when cranking which indicated an open wiring circuit (splice, ground or connector) or open pump winding. The 12v I was reading when cranking at least indicated the fuel pump relay was working.
With my new pump installed, I listened for the pump running in just the key "on" position. I believe I barely heard it running after the repair as it was running very quietly which indicates the +12v was there which supports my theory.
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