The wiring shorted out due to the "custom wiring" job done by the previous owners. There are 5 red wires coming into the cab (3 go to the computer harness, 1 to the relay, and 1 more to a connector that isn't hooked up to anything), and they were stripped and all crimped/welded together with what appears to have been a hot pair of pliers. The wire to the relay shorted and smoked, and the snapped in 2. I rewired the wires seperately and put a new wire to the relay. The factory relay and the new relay both click when I turn the ignition on, but the pumps didn't turn on. After hours of trying other things, I was running a wire directly from the battery to the fuel pump wires to see if they would even turn on or not. The High pressure pump turns on just fine, but the in-tank pump makes a really quiet noise like it is dying, and the truck still didn't turn on after both pumps appeared to be on. Also, when checking the wires with a test light and ignition on. None of the 5 red wires had power, the yellow wire coming off of the tank for the fuel gauge made the test light flash, and of the grounds had no power (at least something is working right!)

What sound exactly does the in-tank pump make when it turns on? The high pressure pump makes a louder, whining sound. I am just trying to figure out if the pump is bad before I try and siphon out the full tank of gas (truck ran for about 30 seconds after completely filling up) to drop the tank. I have a repair manual and it doesn't tell a whole lot about it.

I'd really appreciate any help I can get!!!

 

Not sure what you are wanting but here is a diagram of your fuel system.



/

 

That is a much better wiring diagram than I have found, thanks!

I am curious as to what what noise, if any, the in-tank pump should make when it has power so I can tell if it is gone, going out, or if there is something else wrong.

 

Some run so quiet you can hardly hear them and some are very load right out of the box.
Unhook the hose coming from the tank and put it in a bucket. Run the pump and see if it pumps OK. It will only have about 4-5 psi but make sure it is a steady flow.
Note you will have to ground the TAN-LT GREEN striped wire at the self test EEC connector and turn on the key to run the pump. Do not run it to long or the frame mounted hi pressure pump can over heat with no gas going through it.

 

The '88s LP pump runs very quiet, but I've heard loud ones. Does the truck run badly?

Adrianspeeder

 

When it ran, not really. There was a hesitation and it would almost die/would die when you went from reverse/park to drive sometimes. That's about it.

I ran a hot wire straight from the battery to test the in tank fuel pump lines. The yellow one (which should be fuel level) I believe turns on the high pressure pump (louder whining from up front). The orange one turns the in tank pump on, and it sounds like it is gurgling in the tank. The other I'm assuming is the ground. Was checking the fuse panel with a test light and not one fuse lit the test light up (checked with ignition "on" and in Acc.). Now the radio doesn't turn on??? The only problem with that is the manual shows the panel for a carb'd engine, and not the 5.0 FI, and I cannot find a picture of it anywhere.

The fuel pump shut off switch, or inertia switch. How do I tell if it has been tripped? I can push the little white button over and over. It doesn't stay down (not sure if it should), it just pops right back up after I push it down.

 

When it pops, it pops much further out.

You should have a plug going into your inertia switch with 2 yellow wires. If you doubt this part, unplug it and bypass it while testing...

 

The reason I asked was because I have no idea how it works on this thing, but there is a piece of material (sound deadening I presume) that it hits, and if I move it the button pops up further.

I'm guessing it is fine, main thing right now is to figure out why the fuse panel and possibly the ECU doesn't have power. I have it narrowed down (I think) to a loss of power somewhere along the line. I have fixed bad/fried connections, but nothing. The only thing left that I can think of to do besides getting a new harness and rewiring the entire truck, is to run a wire from the power lines of the pumps and the hot wire for the computer to a switch and then to a 12V source so I know they will get power.

 

If the pump on the frame runs then your inertia switch is OK.

Maybe this one will help


/

 

I got it running today. I had to take about a 2 week break on the project. I connected those 2 fuel pump power wires together into one. Ran that wire to the other red wires in the cab. Found out one of those red wires was shorting by hooking a lead to the battery and touching each wire. Connected all of the other red wires along with the fuel pump wire to a switch. Turn the ignition on, flip the switch, and it starts up. Not the optimal way, but it works and is running stronger than before (so far anyways). It isn't cutting out when switching gears like it used to either.

Thanks for all of the help guys!!!