Electric fuel pump draw

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terry

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Hi folks
what type of voltage draw should I see on my volt gauge when I turn on my electric fuel pump? my volt gauge will drop from about 12.5 volts to 11.5 volts. I have change fuel pumps from holley to a Mallory and I have change out the volt gauge for another. same draw. seems like my volt gauge may read as low as 11 volts but when I measure the voltage at the battery with car running it reads 12.7.
 
"The thing is" there may be enough load on the circuit that the volt gauge is hooked to, that you are pulling that point down that much.

So first question is "to what is the voltmeter connected?"

And, it would determine what all is on that line. EG if it's coming off the ignition switch, you at least have the ignition system, and the alternator field / voltage regulator, and maybe some other things

Is the fuel pump running through a relay?

If you know the brand / model of the pump, the manufacturer's website "might" show the amp draw of the pump
 
thanks 67dart
I do not have a relay connected to the pump. its been such a long time since I put in the volt gauge I will need to look and see what else may be on that circuit.
 
You really should run a pump off a relay. And, if/ when you do, you can easily hide a second switch in series in the coil circuit, to act as an anti--theft
 
generally pumps will draw so much AMPs with no pressure. then with more pressure the amps will go up. but should have a limit that it will not go past even with full load. i have worked on industrial oil pumps that put out 5,000 to 6,000 PSI. and as much as 50 - 60 gal per min. but there are some higher.
 
You probably wired the fuel pump to the IGN1 circuit, which runs thru the key switch. The 1 V drop is probably due to the fuse and switch. I would guess once running the fuel pump might draw ~5 A, but depends on the pump. The voltage drop would cause 5 W of heat. If in the key switch that might be enough heat to damage it.

As stated, much better to power the fuel pump off a relay. That gives you the option of adding an oil pressure cut-off switch for more safety in a crash, plus an anti-theft switch.
 
Thanks for all your input. once I looked at the volt meter I had it wired to the fuse box radio fuse. I changed the wire to my brake light fuse and now the volt gauge reads almost 13 volts at idle. I feel this is what it did read before this issue came up.
I like the idea of a relay but I do not need it as a theft device. I do have a master on/off switch at the rear bumper.
How would I wire in a relay?
 
So you have a rear mount battery? the pump is also in the rear? If so I'd mount a "Bosch style" relay back there near the battery. Run an applicable fuse, say, 20A off the battery to the relay contact, and then the load contact off to the pump.

"Break" your original pump wire coming back to the pump and feed that to the relay coil


Bosh relay:

30 would come off battery, through fuse, to 30

87 would run off to feed pump

85 relay coil ground

86 relay trigger would hook to your old pump supply wire coming from up front If you want a crash switch or anti theft, you can wire it in series with this wire. There are others, but Rangers for years have a simple available crash / roll switch, a good safety measure

RelayWiringGuide.jpg
 
yes I do have the battery in the trunk and my pump is on the inside frame rail by the rear end. I do have a 7 amp fuse in line with the fuel pump hot wire but what is the added reason for the relay?
 
Well it takes the load off exactly what we are talking about........why your voltage is low at some points in the harness. The ignition circuit was never designed for much "or any" added loads, and in fact I've written about this 'a bunch.'.........Voltage drop in the ignition harness causes the VR to run OVER voltage
 
ahhhhhhhhhhhhhhh now its clear as to the reason for the relay. i'll add one in when I change out the holley pump back to the Mallory 140. thanks for all your help and info
 
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