GM is stupid, let me say that much to begin with. Secondly I need to rewire the system to get rid of this voltage drop issue I'm having which is causing a lack of fuel pressure, thats the only thing left it can be since I've replaced every other component in the fuel system. Anyway I'm looking over the schematic and trying to figure out WTF the engineers were snorting when they wired this thing. IMO the fuel pump feed wire from the relay to the pump should have nothing more then a circuit protector of some sort in it, not a trip to the PCM, a fuse, through a relay, through a switch, and to the pump. I'm thinking the oil pressure cut out needs to be moved to the switch side of the relay rather then the load side, and both the relay supplied power and relay power out to the pump need to be upgraded. The biggest head scratcher is trying to understand the PCM side of it. One wire that comes from F12 at the PCM triggers the relay, and another wire at F6 apparently supplies the voltage to the whole system near as I can tell. I believe the wire at F6 can be done away with and this is where the good 12 gauge relay supply would come in at to take its place. I'm assuming the other wire at F12 is the signal wire for the PCM's on and off command of the fuel pump? The only other wire I'm in question about is at the relay, is red and described as a Fuel Pump Prime Conn. What would that be?

 

wouldnt you essentially be rewiring the relay? which can only be wired a certain way for that function? voltage drop? if the relay is bad, just change it to a new relay...

seems like you are trying to analyze the gm circuitry of the pump, when its really not a better way to set it up... its set up for safety man...

 

I managed to confuse the crap out of myself. Tried what I originally had in mind and it doesn't work right. Rethought it and if I just run a heavy gauge wire to the pump which I've already done with 12 volt fused power, I then put a relay inline on this wire to switch it off and on, and trigger the relay with the grey wire that originally powered the pump. This way anytime the grey wire has power, it is switching the relay on and power from the heavy gauge wire runs the pump. I tend to overcomlicate things sometimes. This way only one wire needs to be cut and otherwise all the stock wiring stays the same.

 

http://www.racetronix.com/RX-FL98-FPKG-2.html

 

There are a couple of items that you need to understand about the way the fuel pump circuit is wired. Then it will make sense.

The fuel pump relay is in parallel with the oil pressure switch. Either one being active will apply power to the FP. The oil pessure switch is a backup in the case of a relay failure.

The ECM/PCM activates the FP replay for 2 seconds at key-on. The PCM will then activate the FP relay whenever it sees DRP's (distributor reference pulses).

As oil pressure builds in the engine the oil pressure switch closes. This is in parallel with the FP relay. And will actually turn on the FP if the relay fails and the engine is cranked long enough (have long crank times, check the FP relay).

Now, that 'other' wire going to the ECM/PCM, it is a sense line. And is important. On this wire the ECM/PCM is reading the fuel pump voltage. From this the ECM/PCM does a diagnostic check of the FP relay. Code 54 on ECMs.

On the '8746 ECM along with others, this sense line is also used to compensate the injector PW for voltage. Some ECMs/PCMs are calibration selectable as to whether they use the FP voltage or the ignition voltage for this purpose.

The oil pressure switch wiring is redundent, and may be removed. The sense line back to the ECM/PCM may not be required. But you will need to check the code for that answer.

See, told you it would make sense (sometimes it's clear as mud).

RBob.

 

You can leave the sense and signal wires from the PCM in place and just run the heavier gauge fused wire to the relay and from the relay to the pump. Everything else can remain the same. Also make sure that you match the wire gauge on the ground side as well. If you decide to change the wiring within the tank, make sure you use wires and connectors rated for continuous immersion in gas and that there are no solder or butt joints within the tank itself.

 

I ended up running a 12 gauge wire from the Aux A post at the underhood fuse block which is constant and fused 12 volt power down along the frame rail to a relay and then on into and directly to the pump. I then cut the stock grey fuel pump wire in the harness at the frame rail which was a mighty 22 gauge, maybe smaller wire, and used it to trigger the relay. I taped the relay up as good as I could and tied it to frame rail and it shouldn't be effected by the elements but time will tell. Basically only one wire had to be cut this way and its still PCM controlled. I am however still seeing a 3-4 psi drop in fuel pressure from stone cold to operating temp and the drive was no more then 2 miles from the shop to my parents house. Maybe thats normal. Fast355 said liquid fill gauges will change the reading a little bit with heat because the fluid expands and messes with the sensing machanism of the gauge. Who knows.