I bought a 1990 bronco with the 5.0 that’s been sitting for years, I installed a new fuel tank and pump and nothing works. I check voltage and I’m only getting 8 volts to the fuel pump, Checked voltage at inertia switch and I got 13 volts when I first turn the key but than it drops after 1 second to 8 volts, The big wire on the fuel pump relay has 13 volts continuously so I figure that’s the feed, my tester don’t fit in the other wires that come out of the relay, Is this drop in voltage normal or is there something else wrong?

 

If the large wire at the fuel pump relay you're referring to is the Yellow wire, then yes, that is the feed wire for the fuel pump relay. On vehicles with electric fuel pumps it's not uncommon for the pump to only run long enough to pressurize the fuel rail, then shut off until the engine is running. But your 1990 might also have a fuel return line in which case it might also let the pump run constantly when the key is turned to the Run position. You'd probably need to probe the Tan/Lt Green wire at the relay to see if the PCM is requesting the relay engage for only 1 second. The 8 volt reading is a bit confusing though. Are you sure you have a good ground with your tester?

Do you have a fuel pressure gauge, or an auto parts store nearly that might loan a fuel pressure tester with the Ford adapter to see if you are getting pressure at the rail? If not, you can also see if the engine will start off of a squirt of starting fluid / throttle body cleaner / etc. If it starts and runs for a short period of time this way, that tells you that you have a fuel-related issue. If it doesn't start this way, then you get to keep looking to confirm you have spark, you have compression, and you have spark when you're supposed to (such as making sure the distributor wasn't installed 180 degrees from where it should be).

-Rod

 

Your listed voltages and power up sequence are nothing out of the ordinary. When you cycle the key from Off to Run the computer (PCM) energizes the fuel pump relay for ~1-2 seconds. After that time period the fuel pump relay drops out. The ~8VDC reading is the fuel pump monitor circuit inside the computer. Perfectly normal readings based on your statement in the original post.

 

The 1-2 second priming is normal. All of these EFI trucks have a return line. If the pump is constantly running with the key in RUN and the engine off that usually means that the computer is bad.

 

I had the fuel pump wired wrong, I replaced the pump, tank and the ring that the fuel pump mounts to, I went to a newer ring because it was the only one I could get and wired it wrong, Now I’m getting the pump to run once I turn the key on for a second or two. I need to find the Schrader valve to test fuel pressure

 

How to check fuel pressure on 1987-1997 gasoline Ford trucks

courtesy of Prototypemech