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;-)

[CFI]

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;-)

Ok I did those tests.

Engine running:
Negative to battery, positive to pump ground= .82
Positive to battery, negative to pump feed= 1.27 to 1.5

[Bitsyncmaster]

Ok I did those tests.

Engine running:
Negative to battery, positive to pump ground= .82
Positive to battery, negative to pump feed= 1.27 to 1.5

So your dropping a little over 2 volts in all. If you do my modification where you cut the ground wire on the inertia switch and run that switch pin to frame ground you will gain about 0.3 to 0.4 volts (less drop on the ground). That will also bump your volt gauge up the same amount when the engine is running.

[Ron]

Ok, you have a ~2.2V drop at the pump, with everything in the circuit connected. So it's getting ~14.2V (good) minus 2.2V = 12.0V, which meets the newer pump's voltage requirement.

Should of asked to begin with but are you just curious or having a problem with it??

EDIT: +1 Bitsy

[CFI]

So your dropping a little over 2 volts in all. If you do my modification where you cut the ground wire on the inertia switch and run that switch pin to frame ground you will gain about 0.3 to 0.4 volts (less drop on the ground). That will also bump your volt gauge up the same amount when the engine is running.

Will do, thanks.

[CFI]

Ok, you have a ~2.2V drop at the pump, with everything in the circuit connected. So it's getting ~14.2V (good) minus 2.2V = 12.0V, which meets the newer pump's voltage requirement.

Should of asked to begin with but are you just curious or having a problem with it??

EDIT: +1 Bitsy

Thanks. Just bored and running some tests I’ve been curious about.

[David T]

My guess on why the pumps burn out (the original style) is that either the screens get plugged up and/or the pick-up hose gets kinked or collapses. Running the pump dry (without fuel moving in and out) causes it to run hot and with no lubrication, the gears inside get worn quickly. The pump gets very noisy and starts to draw a lot more current. The excessive current causes the windings inside the motor to overheat and short out, killing the pump. While that happens you also wind up melting fuse #7 out of the fuse block. My cure is to install a spring inside the pick-up hose to prevent it from kinking or collapsing from the suction.

[CFI]

My cure is to install a spring inside the pick-up hose to prevent it from kinking or collapsing from the suction.

I did that on my old setup after the pump got noisy on a long drive. It worked great.

[ritztoys]

If you do my modification where you cut the ground wire on the inertia switch and run that switch pin to frame ground you will gain about 0.3 to 0.4 volts (less drop on the ground). That will also bump your volt gauge up the same amount when the engine is running.

I did exactly this mod as suggested by Dave last fall, I was very happy with the results and the voltage to the gauge was much better.

[cis6409]

Just as interesting side note I forgot to mention, that I only found there was an issue with the pump voltage when I was doing a power run on a dyno test.

When this test was being conducted, the dyno guy had to stop the power run when it got to near the top end higher rpm. The fueling dropped way off the charts. He immediately stopped the power run and explained a couple of issues that on his experience what that usually pointed to. After checking the other suggestions they all checked out, it left only the the pump.
The fuel pump was not able to match the required demand at the higher engine loads. He then asked me if the pump was old, and I said no it was only 6 months old at the time. He suggested checking the power supply.

After cleaning up the connections and increasing the voltage the power run was able to be completed safely the next time.

Shane

[David T]

Interesting that it showed up in that manner. Not unexpected, just unusual to find it that way. Typically the pump gets really noisy on a hot day after running for a while with less than 1/2 tank of fuel.There was a story a while back of a group of Deloreans on a cruise and they got to a ferry and had to wait a while. Almost all of them had noisy fuel pumps. It was hot and they had less than 1/2 tank of fuel. Once the pump gets noisy you can figure it is going to die soon. You have to figure if you don't provide enough electric power to the fuel pump, it can't pump enough fuel to the motor. What kind of HP and Torque did you see on the dyno?

[cis6409]

125bhp and 148lb/ft, on a manual. The overall max figures were not my top priority due to the inaccuracies of different dyno type machines. As long as they were in the ball park of factory specs I was happy enough.

The fuelling and torque curves were more important for me at the time for tuning purposes and seeing if the engine was 'happy' at all engine rpms and loads.

[cis6409]

It was an interesting experiment as I always wanted to test the engine on a dyno to see what It would be like, without it I wouldn't have known my voltage was at the pump was bad so I learned something :)

Once I finish my rebuild of the car, I plan to go back again and fine tune the set up.

Shane

[David T]

Dyno testing does expose any weaknesses and helps you to get the most out of what you have. With enough testing you can fiddle with things to optimize the drivetrain.