Fuel pump issue
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Fleet Mechanic
Joined: Oct 2011
Posts: 1,891
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From: Birdsboro PA
Fuel pump issue
Okay guys, let me know if I am on the right track here. My mom has a 2001 expedition that has had an intermittent no start issue. My dad had a shop look at it and they determined it was being caused by the fuel pump not running. We originally thought it was caused by the aftermarket code alarm alarm system, but after digging into that I am pretty sure that is not the issue. The truck is intermittently not getting the ground to the fuel pump relay. I have read that the PCM is what controls that ground. If that is correct, I am leaning towards the PCM being the issue. The relay itself is good as I ran a temporary ground to the relay and the truck will fire right up. I am not getting power to the crash sensor either when this happens.
Let me know what you guys think.
Let me know what you guys think.
Logistics Pro
Joined: Jan 2001
Posts: 3,740
Likes: 37
From: Central NYS
. I have read that the PCM is what controls that ground. If that is correct, I am leaning towards the PCM being the issue. The relay itself is good as I ran a temporary ground to the relay and the truck will fire right up. I am not getting power to the crash sensor either when that happens
Your right. The PCM controls power to the fuel pump by controlling the control side of the fuel pump relay. The PCM grounds the control side of the fuel pump relay when you turn the ignition key to the "RUN" position. If it doesn't see a CKP sensor signal within a couple of seconds it "ungrounds" the control side of the FPR until it sees the CKP signal.
Before I would suspect the PCM, I would check the integrity of the LB/OG wire from the fuel pump relay back to the PCM connector. And I would also check the integrity of the CKP sensor circuit.
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Fleet Mechanic
Joined: Oct 2011
Posts: 1,891
Likes: 103
From: Birdsboro PA
When energized, that relay supplies power to the fuel pump by way of the crash sensor.
Your right. The PCM controls power to the fuel pump by controlling the control side of the fuel pump relay. The PCM grounds the control side of the fuel pump relay when you turn the ignition key to the "RUN" position. If it doesn't see a CKP sensor signal within a couple of seconds it "ungrounds" the control side of the FPR until it sees the CKP signal.
Before I would suspect the PCM, I would check the integrity of the LB/OG wire from the fuel pump relay back to the PCM connector. And I would also check the integrity of the CKP sensor circuit.
Your right. The PCM controls power to the fuel pump by controlling the control side of the fuel pump relay. The PCM grounds the control side of the fuel pump relay when you turn the ignition key to the "RUN" position. If it doesn't see a CKP sensor signal within a couple of seconds it "ungrounds" the control side of the FPR until it sees the CKP signal.
Before I would suspect the PCM, I would check the integrity of the LB/OG wire from the fuel pump relay back to the PCM connector. And I would also check the integrity of the CKP sensor circuit.
Hotshot
Joined: Jan 2004
Posts: 19,768
Likes: 1,082
From: St Louis
The PCM grounds the control side of the fuel pump relay when you turn the ignition key to the "RUN" position. If it doesn't see a CKP sensor signal within a couple of seconds it "ungrounds" the control side of the FPR until it sees the CKP signal.
When the key is first turned from OFF to RUN, the PCM enables the FP relay for about two seconds to pressurize the system, regardless of the CKP signal. Once the PCM sees the CKP signal during cranking, it then re-energizes the FP relay for normal fuel supply.
As noted, the first two seconds is independent of the CKP sensor. That voltage is only valid during that brief window so you need to be ready and observant to catch it. A test light at the inertia switch is my preferred way (unless I can actually HEAR the pump running).
Do not jump into the CKP sensor with out first checking the prerequisites.
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Fleet Mechanic
Joined: Oct 2011
Posts: 1,891
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From: Birdsboro PA
It is important to understand the start-up sequence.
When the key is first turned from OFF to RUN, the PCM enables the FP relay for about two seconds to pressurize the system, regardless of the CKP signal. Once the PCM sees the CKP signal during cranking, it then re-energizes the FP relay for normal fuel supply.
As noted, the first two seconds is independent of the CKP sensor. That voltage is only valid during that brief window so you need to be ready and observant to catch it. A test light at the inertia switch is my preferred way (unless I can actually HEAR the pump running).
Do not jump into the CKP sensor with out first checking the prerequisites.
When the key is first turned from OFF to RUN, the PCM enables the FP relay for about two seconds to pressurize the system, regardless of the CKP signal. Once the PCM sees the CKP signal during cranking, it then re-energizes the FP relay for normal fuel supply.
As noted, the first two seconds is independent of the CKP sensor. That voltage is only valid during that brief window so you need to be ready and observant to catch it. A test light at the inertia switch is my preferred way (unless I can actually HEAR the pump running).
Do not jump into the CKP sensor with out first checking the prerequisites.
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Fleet Mechanic
Joined: Oct 2011
Posts: 1,891
Likes: 103
From: Birdsboro PA
So I unplugged the pcm and checked continuity fron the pcm to the fp relay and it shiws around 80 ohms resistance. Is that correct? Seems high to me. I also checked the memory codes and it shows both p0230 and p0232, both related to the fuel pump circuit. P0230 is fp circuit low, and p0232 in secondary fp circuut high.
Hotshot
Joined: Jan 2004
Posts: 19,768
Likes: 1,082
From: St Louis
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Fleet Mechanic
Joined: Oct 2011
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From: Birdsboro PA
I am thinking im just gonna cut the wire at either end, and run a new one. It will be a pain to try and find a break in that big bundle.
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