What little hair I got left I'm pulling out, HELP! Fuel delivery

[Superdav]

I have another thread about lights shorting out I finally took care of but now I got big problems.
When going over all the wiring after I found a melted piece of a fusible link I discovered the 2 and 3 injectors were wired together, checked it out and #3 doesn’t work, it’s not getting the ground signal.
So far..
Relay good, replaced it anyway
Fuse good
Injector itself good, swapped it out with one from other truck anyway (I have a 91 2wd with a scorched motor and found this 4wd I wanted)
Swapped ecu with one from my other truck
Checked all connections and couldn’t find anything noticeable
While working on it the fuel pump quit, replaced it with the one from my other truck (it was 2 years old) started right up, injector still dead and obvious from the idle, tapped and wiggled every connection I could find with it running, pump quit again and I put in a new one, injector dead said f-it went inside and let it sit for 4 days
Tonight I get home from work and it does not start, fuel pump is dead, not turning on, drained the fuel line and turned the key, still no fuel pump so I swap the ecu out again, nothing
After 3 fuel pumps and 2 ecu’s I think I can rule those out
I’m completely stumped and can’t sell it I’d lose too much money, right now it’s a $3400 yard ornament
Help?

[Superdav]

Forgot to add the last fuel pump I put in the sender/assembly from my other truck, took the ecu apart and couldn’t find anything burned, discolored or swollen and replaced filter
I know I have to check power to the pump and will do that my next day off I just got to find out which wire is what

[VStar650CL]

White/Blue (W/L) should be power, ground is Black (B). Check across the pins with a test light, if it's dim or out then check W/L to a chassis ground. If that's bright then the pump ground has an issue. If ground isn't the problem, the next most likely culprit is the Fuel Pump Relay.

[Superdav]

White/Blue (W/L) should be power, ground is Black (B). Check across the pins with a test light, if it's dim or out then check W/L to a chassis ground. If that's bright then the pump ground has an issue. If ground isn't the problem, the next most likely culprit is the Fuel Pump Relay.

Thank you will be back in a few days to report

[VStar650CL]

[FrenchPople]

Assuming you have the factory service manual with the wiring diagrams out, I would highly recommend checking your engine harness along with engine room harness. Chances are your problems going to be somewhere in there.

[Superdav]

Assuming you have the factory service manual with the wiring diagrams out, I would highly recommend checking your engine harness along with engine room harness. Chances are your problems going to be somewhere in there.

I do have it but don't know if I have that section printed, that part of the harness is exactly what I thought too, I tore all the tape off following the injector wires to the ecu and found nothing that looked damaged, really surprised me after finding the melted fusible link. I still don't doubt there's a problem there though and do plan to check again following those 2 wires, I pray I find a burnt wire somewhere cause this is about to give me brain problems

[Superdav]

Also I've been through 2 relays and I'll try a third and I guess I'll check that circuit too

[Superdav]

Bad connection in the relay box, fuel pump is running so now I’ve got a dead injector. I got continuity to the ecu pin and power to one side when truck is off. All injectors flicker a test light except number 3.
Ideas?

[Superdav]

Also other injectors at ecu have 12 volts except #3 (pin 103) how can there be no voltage at the ecu but 12 v at the injector plug? Am I on the right one?

[VStar650CL]

Injectors are basically just solenoids, and pretty much all Nissan injectors are driven on the ground side of the circuit. E.g., one side of the injector coil will connect to fused power, but the injector won't "fire" until a tran-sistor inside the ECU provides ground. So when checking the circuit, if you see 12V when you back-probe both sides of the injector connector but there's no 12V at the ECU, it means the wire is broken.

[VStar650CL]

PS - To understand what's going on, keep in mind that you're measuring the injector connector through a coil, so measuring with the connector loose versus back-probing it will show you two different things. With it loose only one side will show 12V and the other will show open circuit, because the ECU tran-sistor will be turned off. With a back-probe you'll see 12V on both sides, because with the tran-sistor off the coil acts like a pullup resistor. If you see 12V on only one side with a back-probe, it means either the injector coil is bad, the wire is shorted to ground, or the ECU tran-sistor is melted short circuit.

[Superdav]

PS - To understand what's going on, keep in mind that you're measuring the injector connector through a coil, so measuring with the connector loose versus back-probing it will show you two different things. With it loose only one side will show 12V and the other will show open circuit, because the ECU tran-sistor will be turned off. With a back-probe you'll see 12V on both sides, because with the tran-sistor off the coil acts like a pullup resistor. If you see 12V on only one side with a back-probe, it means either the injector coil is bad, the wire is shorted to ground, or the ECU tran-sistor is melted short circuit.

This is good news then, I was worried I killed an ECU

[VStar650CL]

These diagrams may help you and other readers. The schematic shows how the injectors are arranged electrically. To turn them on, the ECU activates the tran-sistor for a number of milliseconds appropriate to the RPM and throttle setting. The tran-sistor is off the rest of the time. You can think of the tran-sistor as a pushbutton switch, which the ECU simply pushes when it wants the injector to fire. That's what's shown in the other two diagrams. When the injector is off the button doesn't conduct, so if you're measuring voltage you see 12V on both sides of the injector connector. Only when the button makes contact does the ECU side of the connector go to 0V and cause current to flow through the coil. When you use a noid light, all you're really doing is substituting an LED for the coil.

One upshot of a system like this is that short circuits between the power and ECU wires are disastrous for the ECU. We often see that when rodents attack injector wires. When they short the two wires together, the tran-sistor sees a direct path to power and blows up as soon as the ECU tries to fire it. The fuse is usually too slow to save it, resulting in a blown ECU. It's also noteworthy that the FET tran-sistors can fail either short or open, since the "MO" in MOSFET stands for Metal Oxide. When the metal melts it can form a permanent short circuit to ground. If an injector seems to fire every time the connector is plugged in, that's usually the cause.

[Superdav]

Thank you all very much! and those diagrams are helpful
Ho lee sheet, ran a new wire and it works. Now I got to maneuver one through the firewall.
Up next getting the tach and speedometer working

[VStar650CL]

Up next getting the tach and speedometer working

I can help you with that too, but start a separate thread.