Fuel pump voltage issue (7V), occasional P0232 Code

I have a 98 Mazda B4000 with a 99 4.0L SOHC Explorer engine that was swapped by one of the two previous owners. I have become pretty proficient at doing some sensor rewiring and reading/intrepreting both vehicle wiring diagrams, but the fuel pump has me stumped. The fuel pump runs continuously with the KOEO, and if left that way for more than 5-6 seconds without starting the vehicle it will throw a pending P0232. It also runs without the relay and with the inertia switch unplugged. Before anyone suggests anything I finally figured out why, and after already having swapped my ECU, lock cylinder, and both PATS modules thinking it was the computer. Fortunately those items only ran me $45 at the local Pick n Pull. The newer computer is for a 99 SOHC while the original was for a 2000, so it actually shifts a bit better/more precise shift points.

I discovered that the previous owner had jumped the relay by feeding a 12V wire (hot while run and start only) a little after the inertia switch in the drivers side kick panel. The pump actually works like this and maintains good pressure, it just stays on (and throws a code) if the key is left in the ignition for too long without starting the truck. I have installed a kill switch on that 12V line so I can work on the truck with the KOEO and not have the pump run continuously. I have recently passed my state inspection with it like this, but I want to fix it in the long term.

What I have figured out so far is all of the wires connected to the relay have good continuity and resistance, but I have only visually inspected the grounds. I started tracing the voltage from the relay to the inertia switch and pump and the voltage always reads 12V for a second and then drops to 7V after the relay when the key is engaged (but not started). I am having trouble figuring out why the voltage is dropping. Two pins on the relay read 12V, one hot at all times (from fuse 9) and the other red wire (hot in run or start). The green/yellow wire leading to the computer (pin 15) and inertia switch only reads 7V after turning the key. The pump will not start/run with only 7V. The blue/orange wire coming from pin 80 reads only 0.03V directly at the ECU and at the relay. I am not sure what voltage this wire is supposed to read with KOEO, and I would suspect that this is possibly one of the culprits. Any suggestions on what to do/check next? I would greatly appreciate it seeing that electrical is not my strength. Fortunately my truck runs and drives great, but knowing that the fuel pump is not controlled by the relay (and inertia switch) is a bit unsettling.

I just wanted to add that I tried 5 different relays throughout the testing and always got 12V that dropped down to 7V within a second of turning the ignition...

When dealing with Relays you are dealing with 2 separate circuits.

In a relay you have the coil, this is literally a coil of wire wrapped around a metal core, when power is passed thru this wire the metal core becomes a magnet, on an automotive relay these are pins 85 and 86, there is no + or - direction, when one pin is + and the other - the magnet is on, most automotive relays are 12volt, but usually work fine down to 6volts, and the Coil part is very low amps, under 1amp for sure closer to .5 amps.
This is why relays are used, you can switch on and off high amp circuits using low amp wiring.

Other part of the relay is the load, these are the 2 pins that connect when the magnet is on and disconnect when magnet is off, there is no + or - either, it is just connecting two wires or disconnecting two wires, pins for these are 30 and 87, 30 is usually the "live pin" but doesn't have to be, 87 pin wire would run to the device being turned on and off by the relay, in this case the fuel pump.
On some relays there is a 5th pin, 87A, when relay is off, no magnet, pin 30 and pin 87A are connected, when Relay is on, magnet on, then pin 30 and pin 87A are disconnected and pin 30 and pin 87 are connected.

Pin 30 has a metal bar on a hinge, pins 87 and 87A have contacts.
A spring holds the metal bar so it is in contact with 87A, magnet off, when coil is powered, magnet on, metal bar is pulled down and disconnects from 87A and connects to 87.
If you have 12volts at pin 30, then you would have 12volts at pins 87A or 87, depending on relay's coil, powered or not powered, there is no loss on the load side, it is just metal to metal contacts.
A 12volt relay rated for 30amps, means the relay can be activated with 12volts, and its Load pins can handle up to 30amps

Ford powers the fuel pump relay's coil when key is on, that's the 12volts you saw at the relay socket when key is on, and goes away when key is off.
The other 12volts, hot all the time, is the load power, fuel pump power.
But coil is not Grounded, so no power will pass thru until it is grounded.
That's where the computer comes in, the computer grounds the fuel pump relay to turn it on, same as computer grounds the fuel injectors to open them.
There are two wires from the relay to the computer, the Coils Ground wire to control relay, and pin 87 wire.
Pin 87 sends power to the inertia switch AND to the computer, because computer monitors if 12volts is going to fuel pump, and if not then you can get P0232, this can mean computer doesn't see 12volts when it should or see 12volts when it shouldn't, i.e. " If the PCM(computer) sees an unexpected high voltage when the fuel pump isn't activated, P0232 may set."

Your drop to 7volts could be corroded connector at the fuel pump fuse(#9), or under relay socket.
It could be computer monitor circuit wiring.

It could even be the fuel pump, disconnect wire to pump at inertia switch, then test if voltage drops as before, it it holds at battery voltage then pump is drawing too much voltage.

And you are correct, the fuel pumps Ground could be loose or corroded, this ground is usually near the inertia switch, since thats where the pump and sender wires head back to the tank.

Thanks for the explanation RonD! I am still learning about electrical so that definitely gives me a better picture of how a relay works and how the fuel system is supposed to be wired. According to the wiring diagram the ground for the pump is G200, and the PCM is G108 and G123. Where are all of those located? I have seen diagrams from the Ford Service Manual posted on sites like JustAnswer that display these ground points, but not for my vehicle specifically. Any place I can view those diagrams? That AARC site I linked above is an awesome source of auto info. The usermame and password are both "tech". Any other grounds I should inspect/clean?

Sorry, don't know where those would be.

But in thinking about it, the fuel pump ground is probably OK, because fuel pump works when given 12volts as per original post.

So issue will be in the wiring from Fuse#9 to the inertia switch.