Fuel pump voltage loss - Ford Mustang Forums : Corral.net Mustang Forum
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post #1 of 22 Old 11-22-2017, 11:29 PM Thread Starter
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Fuel pump voltage loss

I was doing some investigating and discovered I have at least a 1 volt loss at the fuel pump inertia switch. I have tested the pump ground to be sure it is not the ground. .01 loss from battery ground.
Car running battery voltage is 14.64 and is 13.23 on both sides of inertia switch
Car off (fuel pump jumped at diag. connector) 12.09 at battery and 10.65 at inertia switch.
Also tested at the fuel pump relay, output to fuel pump, no voltage loss.
So somewhere there's a corroded connector between the relay output and the fuel pump inertia switch.
I may just wire in a relay in the trunk and use the existing line to trigger it with a larger wire from the battery or ignition on switch.

I have checked every connector plug without pulling the carpet. i assume there must be one underneath?
What is the wiring path form the relay to the fuel pump inertia switch?

Thank you, Trent


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post #2 of 22 Old 11-23-2017, 01:00 AM
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Originally Posted by theYman View Post
I was doing some investigating and discovered I have at least a 1 volt loss at the fuel pump inertia switch. I have tested the pump ground to be sure it is not the ground. .01 loss from battery ground.
Car running battery voltage is 14.64 and is 13.23 on both sides of inertia switch
I assume you are grounding to the chassis in all tests?

That indicates the inertia switch is fine. You have 1.41 volts drop between battery and the switch.

Quote:
Car off (fuel pump jumped at diag. connector) 12.09 at battery and 10.65 at inertia switch.
Now you have 1.44 volts. That is within meter error of 1.41. So it confirms you have ~1.4 volts between battery and switch. That sounds normal for stock wiring and a bigger pump.


Quote:
Also tested at the fuel pump relay, output to fuel pump, no voltage loss.
So somewhere there's a corroded connector between the relay output and the fuel pump inertia switch.
You don't have a bad connection or a "corroded wire". You have normal voltage drop.


What pump are you running? Why are you so worried with ~13 volts at the pump?



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post #3 of 22 Old 11-23-2017, 08:06 AM Thread Starter
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After rethinking this, my only issue is the factory wiring is probably too small to support the Walbro 255 pump. I will trip the inertia switch and test with pump off. Betting I will see almost no voltage loss. I was reading a thread by Chris Richards @ Pro-m racing concerning this. He mentioned that a 255 pump w/ stock wiring could see a voltage drop. When the fuel pressure is set at idle with no load there can be enough voltage swing while driving that can cause pressure to increase/decrease. I am thinking it is why my car idles much better when the cooling fan is running. He also mentioned it can cause a deceleration surge, idle surge and stalling. I am going to wire in a relay today if the holiday allows. HAPPY THANKSGIVING

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post #4 of 22 Old 11-23-2017, 10:24 AM
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Putting in the relay is one of the best things to do in that department. And fairly easy. I run a Anderson PMS and I can see the injector pulse width change when the fan comes on like you. Has something to due with ECU voltage offset. Mine corrects, but I can feel the fan come on momentarily. Even though I have a fan controller, I just leave my bypass switch on. I figure blowing air thru a hot engine compartment don't hurt also.
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post #5 of 22 Old 11-23-2017, 02:23 PM
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Measure volt drop with both leads on same wire
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post #6 of 22 Old 11-23-2017, 03:01 PM Thread Starter
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Engine running 1.33 difference. Engine off .05 loss. I am installing a relay in the trunk . Evidently the fuel pump pulls more amps than the wire can maintain.

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post #7 of 22 Old 11-23-2017, 10:32 PM
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Engine running 1.33 difference. Engine off .05 loss. I am installing a relay in the trunk . Evidently the fuel pump pulls more amps than the wire can maintain.
Personally, I couldn't tolerate a 10% voltage drop across 6 feet of wire. Not that I'd worry about 13V at the pump, but rather, something is not up to snuff. If you had a 10% drop feeding, let's say, your A/C compressor at home, it would get hotter than hell, more so than normal. If drop between the inertia switch and pump is minimal, why not clip off the existing wire at the pump relay, and run a new wire, preferably large enough to handle 10+ amps. over that distance, whatever it is. 10 amps will cause "feelable" heat buildup in a 10-foot length of #18 wire, which is maybe what's there. My '04 Explorer has #14 wire feeding the pump; that I buy!

(Did a little looking). #18 copper wire, resistance/10 feet = 0.06 ohm. 10 amps flowing through that 10 feet of wire drops only 0.6 V. suggests something else is amiss. I use 10 amps. because I once measured pump current draw on a stock 5.0 pump and found 5-6 amps.
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post #8 of 22 Old 11-24-2017, 12:16 AM
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The first problem is he has long standard supply wiring to the pump and a larger pump. The Walbro draws about 15 amps, depending on pressure demand, the particular pump he gets, and the pump voltage.

The second problem is he is getting bad advice. There is no reason at all to measure voltage drops with the pump off. Measuring with the pump off is useless.

The wiring for the pump runs through the trunk stuff, forward through harnesses and connectors, through a relay, and eventually through the engine compartment harness up to a start relay fuse link. That 14 ga path COMMONLY has about 1/2 volt drop with a stock pump. That drop is perfectly fine with a stock pump. He now is drawing nearly triple the current. With triple current, he can expect triple voltage drop, and that is about what he measured.


That tells me his wiring is normal because the 1.4 volts drop he sees under load is reasonably expected. If he had a bad plug or some other issue he would have a heck of lot more than 1.4 volts drop. It would also be overheating the pin and would not last long. Since no plugs are melting, nothing has caught fire, and the drop is pretty much normal his choice is to pull a FUSED number 10 or 12 from the start relay battery and alternator common back to the trunk where a second relay is mounted, or to live with 13 volts run voltage on the pump. The 13 volts he measured is actually fine unless he has no headroom at all on the pump flow. If he needs 55 gallons an hour he will be in trouble.

His system is normal. If the 1.4 volts bothers him, then he needs a dedicated direct 12-14 ga fused wire and a trunk relay with the coil triggered from his OEM system.
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post #9 of 22 Old 11-24-2017, 11:33 PM Thread Starter
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Today I rewired the pump using a relay in the trunk. I had some 8 gauge wire left over from an amp install, so I utilized it. Now there is almost no voltage loss while the engine is running. I have 14.65 at the battery and 14.61 at the fuel pump. I have not checked the fuel pressure yet but I am betting it has increased maybe 10% since that is the amount of voltage increase.

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post #10 of 22 Old 11-26-2017, 03:53 AM
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Originally Posted by theYman View Post
Today I rewired the pump using a relay in the trunk. I had some 8 gauge wire left over from an amp install, so I utilized it. Now there is almost no voltage loss while the engine is running. I have 14.65 at the battery and 14.61 at the fuel pump. I have not checked the fuel pressure yet but I am betting it has increased maybe 10% since that is the amount of voltage increase.
If the regulator is working properly the pressure will not change one bit. If it changes, it means you have a regulator system problem.

Even with the old voltage and wire you had more than enough voltage to maintain flow and pressure until you got near the capacity of the pump. The only place the smaller wiring would have hurt you is up around the 40-45GPH fuel flow. About the worse case for that would be 500 horsepower. Below that level you would have had adequate fuel with your old wire as far as the pump flow rate goes.

I hope you fused that wire, especially since it is 8 gauge. You should fuse that wire as close to the pump draw as is reliable. Certainly no larger than a 25-30 amp fuse or breaker up at the BATTERY end.

Personally I would use an automatic reset breaker. My pump wiring is protected by a 50 amp breaker **at the battery** connection end, but I have a really big dual pump supporting ~1500 HP or so. Up underneath the Lexan battery cover on the positive post you can see the 50A breaker.

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post #11 of 22 Old 11-26-2017, 07:05 AM Thread Starter
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I used a mega fuse holder with a 30 amp installed. Its 6 inches from the battery.

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post #12 of 22 Old 11-26-2017, 11:10 AM Thread Starter
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I disagree with fuel preasure comment. I am using the BWD adjustable FPR for a lightning. It opearates on spring pressure against fuel head pressure. If pump output increase the spring will overcome less and the pressure will be higher. My warm idle preasure is now 44 with vacuum disconnected. I had to adust it to get 39.
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post #13 of 22 Old 11-27-2017, 09:29 AM
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If your pump volume decrease was noticed at idle, you have far less volume than a simple volt drop you indicated can account for
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post #14 of 22 Old 11-27-2017, 06:46 PM
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I disagree with fuel preasure comment. I am using the BWD adjustable FPR for a lightning. It opearates on spring pressure against fuel head pressure. If pump output increase the spring will overcome less and the pressure will be higher. My warm idle preasure is now 44 with vacuum disconnected. I had to adust it to get 39.
If your regulator really changes rail pressure with a pump pressure change, your fuel return system is a mess.

The only way possible a pump pressure change should change regulator pressure is if the regulator is junk, the return line does not have adequate flow, or your plumbing from rail to regulator is poor.

If that happened, you really need to fix your system because the low flow pressure will be unstable as hell.

I can pull one pump off line (I have two pumps in parallel) and my regulated rail pressure does not even change one pound. You must either have a bad regulator or a flow issue on the regulator side.


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post #15 of 22 Old 11-27-2017, 10:59 PM Thread Starter
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If your pump volume decrease was noticed at idle, you have far less volume than a simple volt drop you indicated can account for
I did not have a decrease at idle. Fuel pressure was set at 39. After the modification the pressure rose to 44.

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post #16 of 22 Old 11-28-2017, 12:18 AM
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if you set it at 39, it should have stayed there

that is what the reg does.
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post #17 of 22 Old 11-28-2017, 02:08 AM
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I did not have a decrease at idle. Fuel pressure was set at 39. After the modification the pressure rose to 44.
Any idle pressure change at low fuel use after a wiring change it means you have a regulator or plumbing problem.

Doesn't matter if pressure goes up or down, any change any direction is a clear indicator your regulator system or plumbing system on the return side has major problems someplace.


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post #18 of 22 Old 11-28-2017, 10:07 AM Thread Starter
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I don't agree and here's why. Let's say someone upgrades their stock fuel pump. By your theory the stock regulator should still be able to maintain 39 psi. even though the pressure has increased substantially. The higher voltage spins the pump faster vs upgraded fuel pump, still the same result more pressure to regulate.

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post #19 of 22 Old 11-28-2017, 11:26 AM
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You set the spring pressure

You need to understand how the reg works

If the pump does increase vol the reg opens further, to maintain setpoint

That is why it is there

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post #20 of 22 Old 11-28-2017, 07:21 PM Thread Starter
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I am using this FPR https://www.oreillyauto.com/detail/b...3/ford/mustang Maybe I need to upgrade. I am going to check fpr again tomorrow.

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post #21 of 22 Old 11-28-2017, 07:28 PM
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get a factory non adjustable

that is an upgrade

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post #22 of 22 Old 11-28-2017, 07:53 PM
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...Maybe I need to upgrade...
This is an upgrade.
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