I'm chasing a possible charging issue on my $6E car and I noticed that the fuel pump voltage is about 0.5 V lower than the battery (charging) voltage. Is this normal? I'm curious if the ECM uses this voltage reference for the injector pulse width correction tables, because I also seem to have a rich condition that I'm having trouble eliminating. I found the following post from 2001, but it wasn't clear which voltage is used for the PW correction: https://www.thirdgen.org/forums/diy-...orrection.html

I've measured the voltage drop at both the ECM and the fuel pump relay, which leads me to believe there might be a problem with splice S163, shown here. The voltage at Fuse No. 2 is good and the drop only happens when there's a load on the circuit (car running). Charging voltage is 14 V on the nose.

 

I have noticed slightly lower voltage at the pump vs battery on my car when it was using stock fuel system. I upgraded to the racetronix walbro kit with hotwire and it made a big difference in voltage at pump. I saw closer to 13.5 volts after i did the hotwire swap. It just plugs fuel relay power source into the alternator output so you see closer to charge voltage than battery voltage. Helps gain more output from the pump but only really needed if you are on the verge of maxing out a single pump which is closer to 550 whp, depending on setup.

I dont think fuel pump voltage itself will cause fueling issues at the ecm. Less volts may hurt pump flow output but as long as its still delivering more than the engine can use, it will maintain fuel pressure. Check to see if the pressure drops at all.
Battery voltage will however change injector output due to the offset tables. They should be calibrated so voltage drop still delivers enough fuel to maintain air fuel ratio, unless you have non stock injectors and dont have injector offset data, then perhaps a charge issue will change fueling. Could go lean could go rich depending on offset table values

 

The resistance of the wire from the relay to under the tank will drop your voltage from 0.5-1 volts. I measured it myself when putting my stealth fuel pump in. There was some debate as to running larger wires and as a result of the voltage drop some do recommend it but only to increase the overall output of your pump. If you look up the specs for the amp draw and wire length you'll see its within spec but not ideal due to voltage drop. I didn't run new wire because it is unlikely that that will be the limit of my fuel pump... if I push my fuel pump to like 90% its time to add an external pump for me. Long story short its totally normal And should be much closer to 0.5 volts dropped than to 1 volt. Mine measured 0.5 but others reported seeing up to 1 volt. I noticed now that I put in the ebl that it also shows the same drop but I do not know how this is taken into account or even at all. I can tell you that with my march under drive pullies I do not charge at idle and see 11.5 v at the pump per ebl. By a little more than 800 rpm I see 13v. I do not see any fuel pressure changes with that change in voltage but it seems to me it would effect the max gph the pump can put out so if you are on the verge of maxing your pump upgrading the wire or adding a booster pump/pump upgrade would be in order.

 

Some cars higher at battery; others the opposite. A quick look at the $6e code seems to indicate that pump voltage will be used as the default as long as battery voltage is >= 10v and voltage at the pump is >= 2v. But it really does not matter as long as both are reasonable values because regardless of which voltage is used, that value will be used to lookup the needed correction from the 'Injector PW Correction value .vs. Voltage' table.

 

I appreciate the comments, guys. I'm glad to know that the voltage drop I'm seeing is normal, so to speak. I probed the orange supply wire and found that most of the voltage drop occurs between the fender fuse and the splice I mentioned in my first post. If I clip a test lead between the battery and the splice, the fuel pump voltage goes up by about 0.3 V. At this point, I don't think a separate feed line is necessary because I'm not maxing out the pump.

Also, thanks for confirming that the injector pulse width correction table uses the fuel pump voltage.