Fuel Pump Issues

[mat82284]

Recently I'm experiencing a very weird issue that I have no idea what it could be other than two bad fuel pumps. Basally the car sat for a month because of a dead battery. I just swapped that battery out for a new one and the car wont start. Before the battery went dead the car ran just fine. I noticed the fuel pumps not turning on at all, so I decided to start testing. The very first thing i noticed is my 30amp fuse to my upgraded wiring was blown, so i replaced that. Tried to start it and it was blown again. I replaced it once more but this time only turned to the on position and the fuse blew again.

I soon found out that the negative and positive wires were creating connectivity with my multimeter, so i decided to drop the tank. After investigation I found that the upgraded wiring I did was fine, but the two GT Supercar pumps still wouldn't turn on. I then used my multimeter on the pump its self and touched the negative and positive together and got connectivity ( a beeping sound on the multimeter) I then checked the other pump and the same thing happened.

I don't know how to check to see if fuel pumps are bad, but i would assume that the negative and posative terminals on the fuel pumps shouldn't be connected to each other correct? I get a beeping sound showing connectivity between both points when connected.

After all that, I decided to check the wiring after the fpdm and noticed that no voltage was showing up at the pumps with the key in the on position. I then swapped fpdm's to a known working one and the same thing happened. I figured the voltage should at least show something instead of zero.

At this point It got real late and i stopped my search. I'm completely stumped, It kinda sounds like the fpdm isn't outputting or getting power, Then the fuel pumps have connectivity which completely confuses me. If that's normal for those pumps then thats good since I'd wonder how two pumps can go bad at the same time just from not being used for a month.

All help is appreciated.

 

[wmburns]

Ok...we may have some work to do on how to use a VOM.

Testing the motor windings with a continuity tester is not always going to give accurate results. Why? Because the winding resistance is fairly low to begin with. Recommend putting the meter on it's lowest scale. Zero the meter by touching the leads together. Measure the actual Ohms of resistance. A good pump will still have a fairly low resistance. Post.

Recommend that you test the power wiring for a ground fault. Measure the resistance to a known good ground. Should be high. If not, this may indicate wire chaffing and a short to ground.

Next, the FPDM is controlled by the PCM. Output from the FPDM would only occur while the PCM is calling for the FP to run. Which is the first few seconds after key on. So unless you are testing for voltage right at key on, you won't see it.

With the pumps disconnected, does the fuse hold?

Note, the gold standard for starting trouble shooting is +12 volts with the key on at the IFS switch. If voltage, the problem is downstream. If no voltage, the problem is upstream.

If you need a full set of wiring diagrams, I maybe able to help. PM if interested.

 

[mat82284]

Thanks for your advice.

I'll go ahead and check the ohms of the fuel pumps later tonight. Right now its about 98 outside so I'm planning on waiting till it cools down.

I didn't think about the voltage only showing for the first few seconds on the fpdm. That makes sense.

When the pumps are disconnected the fuse does hold. It doesn't pop if i discount the pumps. During all the testing last night I actually cut the stock wiring and used my 10 gauge wiring only to test. Basically I used a 10gauge ground wire that was grounded to the chasis, then the 10gauge power wire directly from the fpdm. The fpdm got a 10guage power wire from a relay directly from the battery.

This is the diagram that I used for the wiring upgrade. Instead of tapping into the original wires I soldered new 10gauge wire directly to the pumps. I made sure that there were not any ground shorts going through the hat. I also had the stock wiring connected to the upgraded wiring.

fpdm/fuel pump wiring upgrade

I'm still confused on how this could happen from just sitting a month. I'm wondering if the bad battery caused a short or something. I checked the fuel pump cut off switch and its good. I checked the engine bay fuel pump fuse and its good. I also tried connecting the stock wiring up without my upgraded wiring and it still didnt turn on.

I'm still wondering if the pumps are bad that its creating a full circuit causing this issue. I'll check the ohms tonight.

 

[mat82284]

Update, I checked each pumps resistance and it came up as 00.5 ohms on both pumps. I then tested for 12v at the inertia switch and there is 12v going to that.

I then checked for 12v at the posative of the pumps and i get zero volts, but maybe its because the voltage only stays on for a few seconds?

Also before any testing i replaced the fuse and it actually lasted the whole time without blowing turning on and off. The fuel pumps however do not come on at all.

I then checked every fuse in the hood and drivers side and every fuse is good. I tried another fpdm and still no go. I'm completely stumped, is this a bad ccrm? or something else? Or are the pumps bad?

 

[wmburns]

I looked over the wiring "mod" and don't understand what purpose it really solves. It appears for all practical purposes to move the CCRM to the trunk.

Learning moment: Ohm law.
Ohm's law - Wikipedia, the free encyclopedia

I=V/R I=current V=voltage(12), R=resistance (.5 in this case). This gives 12/.5=24 amps for both pumps. If dual pumps running in parallel, this gives 12 amp per pump.

So assuming the measurement is correct, the coil resistance is low but not a short.

If asked to guess, there is a ground fault in the wires running through the body. The insulation has chaffed where the wire passes over a metal part. Or where the wires run between the tank and body.

With the fuel pumps disconnected and key off, measure the resistance from the power wires to ground. Also measure the "extra" wire from the FPDM to the FP for a ground fault.

NOTE: this is important! It's possble that the fault is where the wire passes between the tank and the body. That means because the tank has been dropped, the fault may no longer be there. Inspect the insulation of the "extra" wires very closely.

The ground fault measurements should have been taken BEFORE taking everything apart. Important electrical troubleshooting leason.

 

[mat82284]

Thanks for the input again. I actually fixed it last night at 3am, but got to tired to post up.

The first issue was the relay. The ground that i had for it worked good, but the wire came out of the butt connector. Once that was fixed the fuses stooped popping. The other issue was actually the at the inertia switch. Apparently the wire wasn't properly touching the pin or something. I pulled the pin out, re-adjusted it and put it back in and now my pumps turn on and the car runs again.

This was a very strange issue, but i guess the wire took its toll after 10 years of the cars life lol. That inertia switch area is something i never mess with. I realized it was the wiring when i realized the black/red wire should have also had power.

Thanks for all your help, you pointed me in the right direction. I'm just happy its not the pumps, i looked online and the prices of the supercar gt pumps are near $200 for 1! I bought mine when they were $90 a piece. Now I'm wishing i had bought spares.