1997 F53 Fuel Pump Relay Has No Power at Pin 87

[AppliedThinking]

Gulf Stream Class A Sun Sport. Super 460 1997 F53 chassis.
I did change out the fuel pump back in 2012 wit the 4pin pigtail harness.

Thinking I had a bad fuel pump. I did drop the tank and will replace the internal fuel pump, Vapor valve and inline fuel filter.
However I don't believe that is the cause with no fuel pressure.

Testing the fuel pump relay, I built a extension harness to all me to take measurements. PIN 30 has a good 12.7vdc reading. Apparently, when one turns on the ignition the fuel pump relay 'should" close providing 12vdc to PIN 87 Fuel Pump. I am not getting any power to relay PIN 87 which tells me the coil is not energized. I have tried this with both 87 connected to the chassis feed and also floating.
The connector at the fuel tank is disconnected and I "should" be seeing 12vdc to the red cond pins.
What is interesting, is that I can see 7vdc on the relay connection, pin 87, with the ignition on. Where is that coming from?

I did change out the inertia switch with a new one, just in case. No change with the fuel pump relay.

There is mention of an Engine Control Module somewhere under the driver side dash. Not sure if that will affect the power to the fuel pump relay? Both the Fuel Pump and Fuel pump coil fuses (both 20A) look good.

Not sure what else to check. I need to square away the electrical before anything else. At this point, I may need to hand over hand check the cable run going back to the fuel tank for signs of damage. With both ends of the power feed open, I should not be seeing any voltage.
Did I miss something?

V/R
JohnF

 

[Ronald1955]

The fuel pump circuit on a 1997 Ford F-53 motorhome chassis (equipped with the 7.5L 460 cu. in. V8 engine) consists of the fuel tank module assembly, fuel pump relay, inertia safety switch, fuse, and the Powertrain Control Module (PCM).

Circuit Components & Operation

• • Prime Signal: When you turn the ignition key to the "ON" position, the PCM grounds the fuel pump relay for about two seconds to prime the system with fuel pressure. [1]
  • RPM Signal: Once the engine starts cranking and running, the PCM keeps the relay energized based on the distributor/ignition RPM signal. If the engine stalls, the PCM cuts power to the pump for safety. [1]
  • Inertia Switch: A safety shut-off switch designed to cut power to the fuel pump in the event of a sudden collision or hard impact.

Troubleshooting Steps

• • Check the Prime Sound: Turn the key to "ON" (without cranking) and listen near the fuel tank or-fill neck for a faint two-second humming sound. If it does not hum, power is not reaching the pump during the prime cycle. [1]
  • Inspect Fuse and Relay: Locate the power distribution box under the hood to check the dedicated fuel pump fuse and relay. Swap or test the relay for continuity and corrosion. [1, 2, 3]
  • Test the Inertia Switch: Locate the inertia switch (typically tucked up under the dash or kick panel area inside the motorhome chassis area) and make sure it has not tripped or failed internally.
  • Verify Voltage: Backprobe the fuel pump connector or test at the relay harness. Low voltage (such as 7V instead of a full 12V) usually indicates high resistance, a bad ground, or a failing relay/wiring harness connection rather than a bad pump itself. [1]
  • Replacement Note: If replacing the in-tank pump assembly (such as OEM part F6PZ-9H307-D, be aware of Ford technical service bulletins for this era; some replacement pumps require a 4-pin to 3-pin jumper harness kit depending on the exact production date of your 1997 chassis. [1, 2]

Watch a general overview of how computer-controlled fuel pump electrical circuits operate and get diagnosed:

7m

 

[Ronald1955]

Hi John - The information I supplied was found on a google search.

 

[AppliedThinking]

Yes, thank you. I did drop the tank and pulled it out as it has been some time since I replaced the fuel pump anyway I also needed to get to the wiring harness lines behind the tank on the inside of the U frame.
I don't mean to cross post, but there is an exact thread I found at: F53 fuel pump voltage and grounds. This is very close to what I am seeing. I have also collected the wiring information for grounds and power. I did not know the inertia switch was after the fuel pump relay. Those ground terminations need love. I do have the same fuel pump assy as the gentleman who posted the reference thread. Lots of good info.
The 2in ratcheting straps worked well and I dropped it by myself on to a low profile plastic garage creeper. Navigated around the big foot jacks and out the driver side rear. Pumped out about 12gal of fuel prior to the move. Its a big tank 63in x 27in x14in 75gal or so. Was half full. More to pump out once I get the fuel caddy delivered.

The first thing I am going to tackle is the grounds. Too many years of corrosion. Will take apart the fuel pump assy and replace that. The vapor valve seal bottom half dropped into the tank (Ethanol). Most of the rubber hoses are also hard as rock.

I really appreciate the support here. I am sure that I will be on the road in the near future after I get it running and smogged.

Very Best Regards, JohnF

 

[Ronald1955]

If you do not have initial fuel pump prime, I would suspect you have corrosion at the frame rail harness connector. Can you supply voltage to pump and coach starts and runs. You could also have lost PCM signal. Good luck

 

[AppliedThinking]

Ronald1955, I believe you are spot on. I got the tank out and am still going to replace the fuel pump and strainers. The external strainer was kind of bent at an odd angle and had crud buildup. I will keep the pump as a emergency backup. I will be cutting an access hole in the RV floor, just in case. There is a void as well. All shielding will be kept on both layers. Good time to do it while the tank is out.
All I can think of is PTL that the 75gal tank was half full.
Received a new copper anti-seize compound and will start to overhaul the chassis grounds. I do have a map. I know the engine runs fine and fires right up with starting fluid. Not concerned with the PCM at this time. But you do make a good point regarding the initial PCM signal.
Finding parts has been a challenge. Delphi CFE0482 fuel pump has the correct connector. I did the 3 pin to 4 pin change out the last replacement. The LO146 fuel tank seal is out of stock everywhere. I did find it on RockAuto. Plenty of sources for the strainers. The vapor vent valve is another hard to find. Amazon for the Chinese after market. Same with the seal.
The only thing that I can do is chip away one thing at a time.
Very Best Regards, JohnF

 

[AppliedThinking]

Well, I found out that hard way. There is no cutting an access hole through the RV floor.
The two gray and black sewage tanks sit under the floor and on top of the fuel tank. Waste of time and now I have some patching to do. Doah!
Live and learn they say. Hope this helps someone not making the same mistake.

 

[Ronald1955]

Did you cut into the tanks?

 

[Ronald1955]

Well, I found out that hard way. There is no cutting an access hole through the RV floor.
The two gray and black sewage tanks sit under the floor and on top of the fuel tank. Waste of time and now I have some patching to do. Doah!
Live and learn they say. Hope this helps someone not making the same mistake.

Did you cut into the tanks?