Having problem with 1998 Mercury Grand Marquis LS Fuel Pump. Only getting 6.58 volts at the pump. Suggestions?

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? 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. 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? 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. 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 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 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. 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. 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? 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 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. 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? 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 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. 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. 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. 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. 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? 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?