Proper voltage range for fuel pump

[CFI]

I probed my fuel pump harness while the engine was running, and measured 11.6 - 11.9 volts. Is this acceptable?

[Bitsyncmaster]

I tested an original type pump and it still provided 75 PSI down to 7 volts. Did not test the new GM types but I would think 11 volts should be OK. What I do is test the + voltage at the RPM relay with the meter ground on the battery (neg). Then test the + voltage on the pump and the difference is the + voltage drop in the harness and connectors. Then measure your neg pump voltage still referancing the battery neg. and that is the neg drop of the harness and inertia switch.

[CFI]

Thanks. I should have mentioned that it is a GM style fuel pump. I haven’t had any problems but with so much time on my hands I figured I’d run some of the tests I’ve been meaning to do. My concern is running the pump for many consecutive hours at a voltage that’s too low.

[Bitsyncmaster]

I think it's normal to drop 1 volt or a little more. When I was testing with the old pump I found that two pin connector at the fuel pump was dropping almost 1 volt. You may need a small pin to punch through the wire insulation to narrow down what voltage drops you have.

[Ron]

FWIW- The specs I've seen for late GM style pumps said, "1 Volt total...", or, "0.50V max per side". So, I'd check the negative side too...

[CFI]

Thanks guys. I’ll take some more measurements today.

[cis6409]

I was getting 9-10 volts on my original style pump, so renewed and cleaned the wiring at the 5 (iirc) different connections from the fuse to pump.

after that was able to hear the pump run faster and quieter. Got the voltage up to 12.2v.

The biggest voltage drop I noticed was on the power side of the circuit rather than the ground side but that was just on my car. Have not checked another car for comparison.

I was gonna try and get some weather pack connectors for the mini loom to pump at the least, to see if that could help.

Shane

[Bitsyncmaster]

I was getting 9-10 volts on my original style pump, so renewed and cleaned the wiring at the 5 different connections from the fuse to pump.

after that was able to hear the pump run faster and quieter. Got the voltage up to 12.2v.

The biggest voltage drop I noticed was on the power side of the circuit rather than the ground side.

+1
I seem to remember the wiring at the pump was a pretty small gauge. Most of the wiring in our car is 19 AWG which is a weird number but it the British wiring. I did some rewiring using 14 AWG and since I have my battery disconnect to work if the inertia switch trips I could just run the pump ground to the frame.

[CFI]

More tests today using multi meter

Engine off, all electrical off:

Battery alone: 12.5 volts
Negative on battery, positive at RPM power wire= 12.5 volts
Negative on battery, positive on the fuel pump ground wire= .78 (I think I had key on for this one)

Engine on:

Positive on fuel pump positive wire, negative on battery= 13.0
Positive on fuel pump ground wire, negative on battery= .82

Positive on fuel pump positive wire, negative on fuel pump ground wire= varies between 11.82 and 12.02

So it seems like approx 1 volt loss on ground side.

Edit: alternator charging at 14.2 volts, measured on positive bolt in engine bay

[Ron]

With the pump running:
Put your negative lead on the battery (-) and positive lead on the pump's ground wire. Record.
Put your positive lead on the battery (+) and negative lead on the pump's feed wire. Record.
Add together.

EDIT: The .78V you got earlier would indicate a problem on the ground side. (According to Chevy manuals anyway;-)