Having issues with my new build. Fuel pressure dropping, and apparently a voltage loss as well. My entire fuel system is Racetronix, from twin 340 LPH pumps to all lines, regulator, rails, double pump hotwire harness and intermediate harness. Started street tuning the combo and figured out that I am dropping fuel pressure starting around 5000 RPM. Sometimes a little sooner, sometimes later.

Tested the wiring this morning and I am getting a 1.1V drop from the alternator stud to the pump bulkhead on both the red and blue power wires. I have tested EVERY location along the routing path. Voltages are as follows: 14.4V @ alternator, 14.3V @ fuse ( on the side going to the back of the car ), 13.5V at the power going into the relay, 13.3V at the power going into the 7-way connector, and 13.3V at the bulkhead connector. Dropping 1.1V over a relatively short length of wire is troublesome.

The wiring is routed along the same path as my fuel lines, along the driver's side of the car. Exits from the top of the tank on the passenger side, drops down to where the relays are mounted and the ground wire is attached to the body. Goes across above the axle, turns and follows the fuel lines up front, then diverges at the firewall and goes behind the cylinder head and intake and travels over the top of the intake next to the fuel rail. Hotwires are then attached to the output on the alternator, and ground for the pumps is attached to a stud that grounds to the block.

I cannot figure out what is causing this drop in voltage, there's a 0.8V drop in the length of wire running from the fuses to the relays in the rear. That's excessive for 10AWG wiring. Does anyone have any ideas of how to fix this? I'm attaching a screenshot of a datalog so the fuel issue can be seen:

 

Hard to tell without actually looking at it and testing but I would recommend the basics, unplug your 10 awg wire and measure resistance from beginning to end. Try a 8 or 6 awg wire. Is heat an issue? Try looking for heat spots. Not much but hope this helps 👍

 

were you using the same ground reference on your meter?

i would start with replacing the crimped ends and see if that helps. if not, the whole run of wire needs to be replaced.

this is assuming you already checked all connections are properly seated/secured

 

Wasn't using the same ground reference, I didn't have long enough leads. I tested at the wire, not the crimps, so the crimps are good. It really seems to be in the length of wire from the fuse to the relay, but it's also BOTH my hotwires. Seems unlikely that each run of wire would have the same defect. The harness is brand new as well, would seem odd to have a problem like this in brand new wiring, unless the copper is inferior. I've checked all connections, everything is solid.

 

There is some really shitty wire coming out of XHINA, Copper flashed aluminum strands. Total garbage.

Not saying that IS your problem, but Real, genuine Belden copper 10 Gauge will carry 30 amps and not heat up.
Also, make sure your crimp terminals are tin plated COPPER and not nickle plated STEEL. The STEEL ring terminals are sometimes used in high voltage industrial wiring, because they can handle heat better, but NOT EVER useful for 12 volt DC wiring, where the conductivity of COPPER is essential.
.
Also: crimp, solder and heat shrink all connections, if you are able.

 

You are not doing an actual voltage drop test, you are just checking the voltage. There's a difference.

To do an actual "voltage drop" test, you are checking two points on the same circuit and the circuit has to be loaded(current flowing) This can be done on the positive or negative side of a circuit.

All a DMM does is show the differental in voltage.

When you put the postive lead on the positive battery post and the negative lead on negative battery post and the meter shows 14.7volts, what it actually means is there is 14.7volts at the postive side and 0 volts on the negative. That's checking voltage.

When doing a voltage drop test the lower the number the better, the higher the number is bad.

A high number indicates that between those points, there is excessive resistance that is "dropping" the voltage. This can be from undersized wiring that can't flow enough current, broken wire strands inside the wire, corrosion or bad crimp on a terminal. That's why using ohms to check if a circuit is good is absolutely pointless because there is no current flowing in the circuit. The only thing a ohm test is good for is to test for an open circuit.


Examples of a voltage drop on your fuel pump circuit would be, remember the fuel pump has to be running

One lead on the big terminal on the alternator and one lead on the fuse, .02
One lead on the fuse and one lead on the 30 terminal of the relay, .02
One lead on the 87 terminal of the relay one lead on the power wire on the fuel pump, 2 volts.

This would indicate there's a problem in the wiring between the 87 terminal of the relay and the fuel pump.

Another thing you can do is test the entire circuit. What I mean is leave one probe on the alternator, start at the fuel pump and back track.
This will show the voltage drop of the entire circuit starting out and as you work back towards the power source, you will find where the problem is.

I hope some of this made sense. I always find it hard to explain what a voltage drop test actually is.

 

Thank you for that! I have discovered I have other issues, mainly that I'm overspinning my alternator causing a drop in voltage which causes a fuel pressure drop. Have a new alternator pulley being delivered today so I can remedy that problem. Once that's installed, I'm going to see about checking for a voltage drop as you described.

 

I blew up, literally, my Powermaster alternator because I didn't calculate how fast it was spinning. 10k rpm over the recommended limit...it will do it, but not very many times.

Needless to say, I emailed them and asked for their recommendation on the second one..