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Voltage Drop Test Fuel Pump

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New Member
Hey everybody,
I don't have a lot of experience working with electronics, but I do work on my own car all the time. This time I noticed my fuel pump felt like its going weak (even though I replaced it 6 months ago), and I know that voltage problems could cause the pump to run weak. So I did a voltage drop test, and the positive side came out 0.02V, but the negative is 1.58V. I figured that there is something wrong with the cables or connectors, so I traced the wiring from the pump and found that it goes to the fuel pump controller in the trunk, before going to the battery. I disconnected the controller and run a voltage drop again at that connector for the controller, and both positive and negative were about 0.02V. I went ahead and bought a new controller thinking it will fix the problem, but it didn't. So I separated all the wires from the connector for the controller (2 ground, 2 power, and 2 thin wires for some gauges) and only connected both ground wires to the controller. Running the voltage drop at the fuel pump after this showed good results around 0.02v, but as soon as I plug the power wire to the controller I get 1.58v of drop. Just to test things, I swapped the 2 ground wires, and with the power wire connected that gave me only 0.44v of drop on the negative side? I am very confused and I thought I could find some help from experienced electricians. The car is 2002 Subaru Impreza WRX
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New Member
apply 12V to pump momentarily with a jumper.

it's probably seized.
Thanks for the link, a lot of information there. However, I believe that I was getting about 12.5v to the pump connector, and I am not even close to 400hp where they mentioned the problems arised.
But can you explain the jumper part, and something being seized more please? My elctrical knowledge is limited


New Member
what is your pump pressure?
I didn't get a chance to hook up a pressure gauge yet, but the car runs fine, I'm just way above my target AFRs on WOT pulls. And since I saw a voltage drop above 1.5v I figured that's what's needed to be fixed first? It's a Walbro 255 btw


Well-Known Member
Hi AFR....
What's your EGTs look like?
I presume you're running cold plugs.

Investigate fuel filter condition. If you have an intank pump check the 'bag' used as a fuel filter.
The controller could be throttling the fuel pump to manage AFR as well as the injector PWM.

MAF or MAP sensor may be having problems, verify a WOT pull without an air filter to see any change, if you get a change then that's indicative of MAP or MAF issues.

If you have a 5 volt output MAF/MAP sensor you can place a potentiometer across it and alter it's signal manually to get your AFRs adjusted....if that works you should replace the MAF/MAP.

O2 sensors may be going south causing the ECU to run best guess fueling.

Did you place a modified chip in the ECU?

Do you have laptop interface to the ECU for mapping turbo pressure vs RPM vs fueling?

I ran a hybrid weber 32/36 with a turbo pressure box manifold on the top with a throttle body injector feeding the 2nd throat of the carb on an 80's mazda 626, 2.0L, 5 fwd with a 2500lb copper clutch plate and a 1.5 ton truck master slave cylinder.. It was simply triggered by 5psi turbo pressure using a standard oil pressure sensor as the control. Ran rich at WOT but then....at WOT you ain't saving the environment, you're cooling the EGTs and your valves and running lower octane gas.

Eventually built an entire boost controller for it, from scratch....6 fuel maps, 3 open loop, 3 closed loop with traction launch control and AFR management with water injection interlocks. It had 19 menu items with up to 14 line items per menu and a manual . Then I had to upgrade suspension, brakes, radiator, oil cooling, deal with torque steering and pretty much ended up with a whole new machine at the end. Had one subaru part...the intercooler. Used to run with the Volvo S80 head to head, a dead match in performance. Had a one touch button for switching to hi performance boost mapping.
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New Member

I am data logging every drive with a laptop using Romraider. I will definitely map manifold pressure vs fueling vs rpm in trying to figure out my issue, but again, should I not get the voltage drop fixed first?

Unfortunately, my EGT sensor seems to have gone bad, as it always shows the same number, but I'm running a safe timing map and don't see any knock (FBKC, FLKC, or IAM). My WDCs are set to zero across the entire table, so I'm only boosting 5-10psi, (vs 14.7 factory). Doing this as a safety measure for now.

I also do have a 5 volt MAF sensor that I already rescaled using Romraider, and had great results with about 6 months ago, when I replaced the pump. The reason for that was actually when I did a WOT pull, I would hit 11:1 AFR and then it would slowly increase to 14:1 with out me lifting the pedal. So I figured there is a fuel delivery issue, and since the pump was six years old, I just got a new one, and it worked perfect after that. At that time I also cleaned a bag for the pump, that was pretty dirty. My next step will be the fuel filter, as I don't know when that was replaced last.

I was thinking to throw an old pump in there and see what results I get, just to test things out. Because again that pump could go to 11:1 (target) but couldn't stay there, when the current pump only goes to about 12.5:1.


Well-Known Member
So your AFR is creeping up , but u don't have EGT feedback to know if the post cat O2 sensor is reporting AFR correctly.
Well let's consider your options.
Do you have a common point ground for your ECUs etc? Or are they using the car body as the ground?
Verify the grounding of your fuel controller system...it might have degraded. You can run a direct #14 or #12 AWG hyperflex cable to a good common ground to improve this.

Have a look at this:

Also do some testing at night when it is cool to see if there is a difference. If so clean out the inside of the intercooler. MAFs are usually before the turbo/intercooler and won't pickup intercooler issues. At least on my SR20VET it is like that.

2) Clean the MAF sensor.
3) Check the hoses on the blow off valve, could be leaking intermittently.
4) Pull the O2 sensors and use some propane gas on them to verify expected voltage outputs
5) Get the EGT fixed so you can isolate the problem as NOT AFR inaccuracy.
6) Get an in cabin Fuel pressure gauge or one your system can datalog to see what's happening on the fuel rail, pressure loss etc.
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