Very odd fuel pump power problem
Thread Starter
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Tuned
Joined: Nov 2004
Posts: 444
Likes: 1
From: Nelson, BC, Canada
Very odd fuel pump power problem
Bear with me here...
Truck wants to start, grunts like it wants to go, then dies. Repeatable.
Checked wiring harness to pump... shows 11.2v, YET... KO... pump does NOT RUN
Disconnected OEM hardness, used another harness to pump motor, connected directly to 12v battery, runs and whirs no problem.
Scenario 1: With fuel pump connected to OEM harness with 11.2v, pump DOES NOT RUN, truck will try and start but dies.
Scenario 2: With fuel pump connected to bypass harness to standalone 12v battery, pump DOES RUN, truck starts and runs, no problem.
Scenario 3: Back to scenario 1 as a test, same results as scenario 1.
I have a spare pump here that I plugged in to the OEM harness with 11.2v and it too DOES NOT RUN.
I CAN hear the relay in the CJB click. Also, if the relay was not tripping I should not have power to the pump
My thoughts... bad relay in the CJB which gives voltage but not enough current??? Contacts fried and too small contact patch?
At a loss here fellas. Looking forward to your awesome wisdom and experience.
Truck wants to start, grunts like it wants to go, then dies. Repeatable.
Checked wiring harness to pump... shows 11.2v, YET... KO... pump does NOT RUN
Disconnected OEM hardness, used another harness to pump motor, connected directly to 12v battery, runs and whirs no problem.
Scenario 1: With fuel pump connected to OEM harness with 11.2v, pump DOES NOT RUN, truck will try and start but dies.
Scenario 2: With fuel pump connected to bypass harness to standalone 12v battery, pump DOES RUN, truck starts and runs, no problem.
Scenario 3: Back to scenario 1 as a test, same results as scenario 1.
I have a spare pump here that I plugged in to the OEM harness with 11.2v and it too DOES NOT RUN.
I CAN hear the relay in the CJB click. Also, if the relay was not tripping I should not have power to the pump
My thoughts... bad relay in the CJB which gives voltage but not enough current??? Contacts fried and too small contact patch?
At a loss here fellas. Looking forward to your awesome wisdom and experience.
Last edited by kalsc; Jan 21, 2023 at 06:31 PM. Reason: corrected spelling
Hotshot
Joined: Jan 2004
Posts: 19,762
Likes: 1,079
From: St Louis
Are you measuring that 11.2 volts with the pump disconnected or connected, and fuel pump relay energized or not? Power-on pump priming cycle is only about 3 seconds long so you have to make that measurement during that brief timing window with the pump connected in order to get a valid reading.
Thread Starter
|
Tuned
Joined: Nov 2004
Posts: 444
Likes: 1
From: Nelson, BC, Canada
Voltage measured with pump disconnected and at various time lengths... initial key on, then again a decent amount of time after.
Simplified.... KO: relay clicks (although quite noisily, almost "dirty" sounding), but pump DOES not come on, even for priming. I even took the cap off the fuel bowl on the engine to see if diesel is being pumped to it (overflow), nope.
However... when I direct wire the fuel pump to a 12v battery, it whirs and pumps no problem, upper fuel bowl definitely receives fuel. And with the cap back on, truck starts and runs no problem.
Side note... I have ensured the "inertia switch" on the passenger side kick panel is not tripped perse.
Came across this in another thread... quite interesting... which is why I am suspect of the relay... which seems to be very common problem.
https://www.ford-trucks.com/forums/1...31-solved.html
Simplified.... KO: relay clicks (although quite noisily, almost "dirty" sounding), but pump DOES not come on, even for priming. I even took the cap off the fuel bowl on the engine to see if diesel is being pumped to it (overflow), nope.
However... when I direct wire the fuel pump to a 12v battery, it whirs and pumps no problem, upper fuel bowl definitely receives fuel. And with the cap back on, truck starts and runs no problem.
Side note... I have ensured the "inertia switch" on the passenger side kick panel is not tripped perse.
Came across this in another thread... quite interesting... which is why I am suspect of the relay... which seems to be very common problem.
https://www.ford-trucks.com/forums/1...31-solved.html
Hotshot
Joined: Jan 2004
Posts: 19,762
Likes: 1,079
From: St Louis
Instead of guessing....
You need to check the fuel pump drive power with the pump connected so that it's properly loaded down. Measuring at the inertia switch is the easiest as it's generally most accessible.
You need to check the fuel pump drive power with the pump connected so that it's properly loaded down. Measuring at the inertia switch is the easiest as it's generally most accessible.
Thread Starter
|
Tuned
Joined: Nov 2004
Posts: 444
Likes: 1
From: Nelson, BC, Canada
Agreed... re:guessing
Learning something here.
I did not know that voltage relative to the fuel pump could be taken (tested) at the inertia switch wires? Great.
That being said... procedure wise...
How best to check the voltage at the inertia switch? Unplug it and measure across the leads? Leave it plugged in and check voltage across the leads? This of course with truck in KO...
Thanks again for the insight.
Learning something here.
I did not know that voltage relative to the fuel pump could be taken (tested) at the inertia switch wires? Great.
That being said... procedure wise...
How best to check the voltage at the inertia switch? Unplug it and measure across the leads? Leave it plugged in and check voltage across the leads? This of course with truck in KO...
Thanks again for the insight.
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Thread Starter
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Tuned
Joined: Nov 2004
Posts: 444
Likes: 1
From: Nelson, BC, Canada
***UPDATE***
I checked voltage across the inertia switch leads, with KO (run), voltage DOES show and is the same as what I get at the pump leads
***UPDATE***
Had read in another forum where even though voltage does show, there may not be enough current due to either a bad ground or bad relay. I have since cleaned up grounds that I can find in the engine bay.
Bad relay?
I checked voltage across the inertia switch leads, with KO (run), voltage DOES show and is the same as what I get at the pump leads
***UPDATE***
Had read in another forum where even though voltage does show, there may not be enough current due to either a bad ground or bad relay. I have since cleaned up grounds that I can find in the engine bay.
Bad relay?
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Hotshot
Joined: Jan 2004
Posts: 19,762
Likes: 1,079
From: St Louis
Bad relay?
Because that relay isn't easily replaced, it's usual simplest to scab on an external relay to take over its job. Lot less expensive, too.
Thread Starter
|
Tuned
Joined: Nov 2004
Posts: 444
Likes: 1
From: Nelson, BC, Canada
OK... fair... thanks for the help!! 
Posted this higher up as well.... this fella did an amazing job of a relay fix!
https://www.ford-trucks.com/forums/1...31-solved.html
Posted this higher up as well.... this fella did an amazing job of a relay fix!
https://www.ford-trucks.com/forums/1...31-solved.html
Fleet Mechanic
Joined: Nov 2015
Posts: 1,487
Likes: 217
Bear with me here...
Truck wants to start, grunts like it wants to go, then dies. Repeatable.
Checked wiring harness to pump... shows 11.2v, YET... KO... pump does NOT RUN
Disconnected OEM hardness, used another harness to pump motor, connected directly to 12v battery, runs and whirs no problem.
Scenario 1: With fuel pump connected to OEM harness with 11.2v, pump DOES NOT RUN, truck will try and start but dies.
Scenario 2: With fuel pump connected to bypass harness to standalone 12v battery, pump DOES RUN, truck starts and runs, no problem.
Scenario 3: Back to scenario 1 as a test, same results as scenario 1.
I have a spare pump here that I plugged in to the OEM harness with 11.2v and it too DOES NOT RUN.
I CAN hear the relay in the CJB click. Also, if the relay was not tripping I should not have power to the pump
My thoughts... bad relay in the CJB which gives voltage but not enough current??? Contacts fried and too small contact patch?
At a loss here fellas. Looking forward to your awesome wisdom and experience.
Truck wants to start, grunts like it wants to go, then dies. Repeatable.
Checked wiring harness to pump... shows 11.2v, YET... KO... pump does NOT RUN
Disconnected OEM hardness, used another harness to pump motor, connected directly to 12v battery, runs and whirs no problem.
Scenario 1: With fuel pump connected to OEM harness with 11.2v, pump DOES NOT RUN, truck will try and start but dies.
Scenario 2: With fuel pump connected to bypass harness to standalone 12v battery, pump DOES RUN, truck starts and runs, no problem.
Scenario 3: Back to scenario 1 as a test, same results as scenario 1.
I have a spare pump here that I plugged in to the OEM harness with 11.2v and it too DOES NOT RUN.
I CAN hear the relay in the CJB click. Also, if the relay was not tripping I should not have power to the pump
My thoughts... bad relay in the CJB which gives voltage but not enough current??? Contacts fried and too small contact patch?
At a loss here fellas. Looking forward to your awesome wisdom and experience.
Also….Check the fuses. I know it sounds funny but the fuses sometimes get this high resistance and start losing the connection at the fuse box contacts ….pull the fuel pump and the fuel pump relay fuses and look for black spots on the spade tabs or arc like black soot.
Hotshot
Joined: Jan 2004
Posts: 19,762
Likes: 1,079
From: St Louis
i would pull the fuel pump relay and measure voltage in abd out of the relay. If it’s dropping swap it with another relay temporarily..
Fleet Mechanic
Joined: Nov 2015
Posts: 1,487
Likes: 217
Removing and soldering a new relay on that board is actually pretty simple. You’ll need a solder sucker, a solder gun with a fine tip and temp control and lead free flux solder. I used to replace relays in boards all the time in the field for AC controls. I used to have a bunch of those Omron solder on relays. They get used in everything.
you already know you have voltage drop. Start tracing the circuit
Hotshot
Joined: Jan 2004
Posts: 19,762
Likes: 1,079
From: St Louis
Actually, we don't know that we have voltage drop. We do know that the voltage that is measured on the fuel pump power line is NOT going to expected battery voltage when the relay is supposed to be energized. If the tests were done as I prescribed and were accurately reported, it can be concluded that the relay is not energizing at all as the FP power is not changing when it's supposed to.
It is quite typical on gas F150s for there to be 6-7 volts on that line with the fuel pump connected and when the key is on and the relay is not energized. I've not previously seen such a measurement on this era for diesel so I don't know what reading is to be expected. This "phantom" power is actually coming from the PCM's input port that is supposed to monitor for fuel pump voltage.
Yes, the relay can be replaced, but it's not an exercise for someone who isn't comfortable using the needed tools. It isn't simply an unplug and plug back in exercise.
It is quite typical on gas F150s for there to be 6-7 volts on that line with the fuel pump connected and when the key is on and the relay is not energized. I've not previously seen such a measurement on this era for diesel so I don't know what reading is to be expected. This "phantom" power is actually coming from the PCM's input port that is supposed to monitor for fuel pump voltage.
Yes, the relay can be replaced, but it's not an exercise for someone who isn't comfortable using the needed tools. It isn't simply an unplug and plug back in exercise.