Truck died recently, so i thought it was the TPS or the IAC. Wasnt them though because this is an electrical issue or something. I got towed home and magically it was working again once it was unloaded, but it was intermittent. Turning the key to ignition, you heard the double click of the relay which temporarily sends 12v to the fuel pump and i also heard the whine of the fuel pump. But then sometimes there is no double click of the relay, which is odd. Also, even if there is the double clicking of the relay, the fuel pump as of right now wont turn on anymore even though the 12v is present. Yes, i tested the direct fuel pump connection and 12v is present. Relays and fuses are all fine; i swapped them all around with good ones bc they're all the same. I even jumped the relays to bypass them entirely, and the fuel pump still wont turn on even though 12v is present. So finally i simply ran 12v directly from battery all the way to the pump and....it turns on as expected (its brand new pump). Why wont that temporary 12v (when you turn key to ignition) make the fuel pump whine like it should, but, a direct jump from the battery will?

Basically all relays fuses and whatnnot are good, and 12v is in the wiring all the way to the fuel pump (i checked with multi meter). I tested the fuel pump directly and it works fine. I also attached another brand new fuel pump, and it wont start. So yes, fuel pump is fine, relays fine, fuse fine, the momentary 12v is in the wiring all the way to the fuel pump connector when i turn to ignition only, CPS is brand new...etc. Its gotta be an electrical issue, bc the fuel pump wont turn on even if the relays click on and off when i turn the key to ignition. To reiterate: yes i tested the entire wiring up to the fuel pump and 12v is there as always.

What am i left with. The PCM? I will do the ignition switch next because that's easy. Otherwise i cant think of anything else. The PCM talks to the relays, and then the fuel pump should turn on? It makes no senses, because even when the relays click on and off, there is no temporary whine of the fuel pump. It was an intermittent issue a few days ago, the fuel pump turned on, but now it does nothing at all. Yeah idk, makes no sense. But since the PCM tells the relays what do to, maybe its a bad PCM?? What else?

 

OEM Fuel pump?

 

As mentioned in my OP, fuel pump works fine regardless if it is OEM or not. I actually have another brand new fuel pump that I already was able to connect underneath the truck just to verify if it was the fuel pump or not. As also mentioned in my op, I have already jumped both fuel pump directly and the motors start up just fine. The fuel pump is not the issue here.

Something has to be electrical related here because sometimes the relays click on and off when you put it into ignition like they're supposed to, other times they don't click on at all. But now it's to the point where whether the relay clicks on and off or not, the fuel pump doesn't start up. It did a couple days ago but now it doesn't. In fact even if I bypass the relay and 12 volts is in the connector, the fuel pump still doesn't start up. We're running a few more tests with the wiring but that's where I'm at the moment including everything I said in my OP

 

Is the circuit falling on its face due to a flakey connection? Rig up test lamps (bulbs) of varying wattages and see what happens.

 

I had somewhat of a similar issue with a Dodge Dakota recently and finally fixed it by replacing the PCM after exhausting all other testing/options.

What year is your truck (along with the engine type and transmission used) and what is the part number of your PCM?

 

You tested power to the pump but what about a good ground? Did you test for power with the connector hooked up to the fuel pump? If there is a bad connection you may show 12V with no load and next to nothing with a load. Same for ground.

 

EDIT: This didnt work

Okay so i'll update here incase anybody in future runs across thread. So its been confusing. As mentioned, i tested all electrical and it was fine (ground was solid, and 12v was solid). Used multimeter to see resistance and also voltage, even in the literal end of the connector. So naturally, i had kept moving on and trouble shooting other stuff to no avail. I came back to the connector because it just wasn't making sense how i was seeing 12v, but yet the pump wont turn on, even when i manually wiggled the connector around on the fuel pump. Finally i decided to "extend" the connector by taking it off, and running a wire from the positive side and negative side, directly to the fuel pump. And...it worked when i turned on the ignition! So i then took dielectric grease as one last attempt before buying a used PCM, and smeared it on the connector before plugging it back in. Truck started right up with every attempt. I have no long term results yet, but, it appears that for some odd reason, the connector isnt making good contact prior to that grease being added. It seemed to get worse when the weather was hot, and, after driving for awhile. I will cut off the connector and solder on a newly-greased one.

Again no long term results yet, but if i dont update the thread, then it can be assumed this was the culprit. It was just confusing since the electrical readings were solid, so i didnt think at first to "extend" it directly to the pump.

 

Yep appeared so! Solid readings on multi meter and i even plugged in 12v lights and it was all fine. So it didnt make sense. Finally i "extended" the positive and negative of the connector, directly to the fuel pump and it worked. So appears some how the connection was just not solid, even with shaking it around to make sure it was snug. Adding grease to the connector made it start right up, but, i dont trust it long term so i will add a new connector

 

Over time, the connections corrode and expand, so the pins don't make good contact. Sometimes, just unplugging/plugging it back in several times will solve the issue. Other times, need to redo the pins on one side, or simply replace the female connector....

 

Yep, techs will often perform drag tests by inserting a thin blade to see if it slides in/out with resistance. Obviously don't use too thick a tester or you just spread the pins and potentially create the very issue you're trying to find