Hold on, don't toss any more parts at this problem, lets think about this some more.
In post #1 you said the fuel pump & filter were replaced, then in post #26 you said the fuel pressure regulator was replaced, then in post #38 you said that replacing the fuel pump Power Relay made a Big difference.

So, building on the fuel pump power relay making a Big difference & you saying above in post #43 that one on cycle got you only 8psi & 4 more on/off cycles of the ignition key finally got you 36psi & it's holding there but is wimpy on power, maybe a under load voltage drop to the fuel pump causing a fuel delivery/Volume feed problem, belongs on the suspect list. At the fuel rail, the fuel pump should deliver about 1/2pt in 15 seconds pump run time. On your OBD-1 system DLC, there is a separate wire that'll allow you to run the fuel pump continuously for the volume test.

Before doing the volume test, back up & have a look at the power relay socket contacts for corrosion, or loose fit.
If ok, thump to trip the in cabin pump inertia cut-off switch & reset it, to see if it may have high resistance/under load voltage dropping problems in it's contacts.
You could monitor pump under load feed voltage at the inertia switch to see if it or the wiring to it, or the power relay contacts are a problem. This will check most but not all of the fuel pump wiring run for under load voltage drop.

If all that seems ok, a kink, or clog in the fuel pump feed, or return lines can cause wimpy pump fuel pressure, or volume feed at the rail Schrader valve test port, so check the feed & return fuel line routing from the tank to fuel rail for restrictions.

More thoughts for consideration, let us know what you find.

 

Sounds like it's time for a desperation WAG. 1989 was the first 2.9 Ranger to have only the in tank fuel pump.
1988 & older had an in tank pump & a frame mount pump. The check valve was in the frame mount pump.

Since your fuel pressure is totaly whacked, make sure the parts store sold you a pump for a 1990.

 

.04 ohm at inertia switch
12 volts at interia switch connectors

11.8 volts and 76 ohms at terminals 1-2 on fuel pump relay

Through terminal 3 and 4 I was getting 12 volts at .62 amps

I think I know where the problem is







I never thought to look at the connection at all and completely skipped this. I forgot K.I.S.S

 

Ok, good find & feedback, looks like those corroded connections & wire insulation breaks belong high up on your suspect list. Let us know how it goes after all are tidy & tight.

 

If 12.0 volts open circuit is the highest you can read, say across the battery posts, it indicates the battery soc is at about 50%, so use your smart battery charger to put the battery on a slow charge rate, of say 4-6 amps, so it'll get a good deep recharge while your working on the relay socket contacts.

 

I replaced the relay socket but still nothing.

If the pump has the ability to build up enough pressure however doesn't then there could be a number of issues between mechanical and electrical.

In the sense of mechanical, I could have a bad pump, FPR, clogged filter, or kinked line. I know that everything listed is brand new and in good condition.

So then we go to electrical. I know there is a computer, the relay and connector, the inertia switch and connector, and the connector at the pump. I checked the inertia switch and its working properly with nearly nothing for resistance. The fuel pump relay and connector are brand new. So all that's left to check would be the terminals at the fuel pump and the computer its self.

Am I wrong?

Yes the pump I bought was for 1990.

Quick thought: Would it matter if the fuel pump installed was for extended cab or standard cab?

 

Ok, good idea to install a new connector & contacts, that old one looked to be plenty corroded.
Consider performing another fuel pump running under load voltage drop test at the output side of the inertia switch, to confirm Murphy isn't messing with your repair & trouble shooting efforts. This will check most of the fuel pump wiring run, but not the wiring run from the output side of the inertia switch to the tank, or the tank electrical connection. How did the tank end connector contacts look when you replaced the pump?
Also perform another KOEO & KOER fuel pressure test & a flow/volume over time test & post the numbers.
I'm not sure if there was a different reach pump used on your year/model. Did you order, get & install the specified part?

 

I'll do the voltage check across the inertia switch in the morning.
When cycling the key on I head the pump run which it does But should the pump prime after cranking?

Do you think the pump is running long enough? It only runs for 1.2 seconds at most

The pump i installed is for the truck but I was wondering.

 

Also, did you clean the wire strands good, so all the corrosion & oxide was removed before crimping the new contacts on?

 

I actually soldered the contacts and cut the wires well before the oxidation

 

Yes at key on the pump runs for only a couple of seconds & cuts off, then when we crank the engine the computer looks for oil pressure & crank sensor signals that tell it the engine has oil pressure & it's safe to run the pump, so it looks at the crank sensor to see if the engine is running & if so, it turns the pump power relay back on to run the fuel pump.
The fuel pump needs full under load voltage to do it's thing & the fuel line to & through the filter to the fuel rail pressure regulator & return line back to the tank need to be unobstructed.
So, perform the pressure & flow rate over time tests, to make sure volume is up to par.

 

Roger on also soldering the wires for good contact.

 

How do I go about the volume test?
When testing the fuel pump under a load do I test across the inertia switch or from the output side to ground?

You say the pumps only supposed to run for a couple of seconds but the things barely running for a second. Is there a possibility the pumps not running long enough to prime?

 

1 second exactly is how long the pump is supposed to run. That 1 second should be enough time to build full fuel pressure.

I'd be checking the pump voltage at the pump itself.

 

Review post 46 on the fuel pump volume test. It'll tattle on any lingering problems.
Yes test the voltage drop from the output/fuel pump side of the inertia switch to ground. This doesn't test All of the pump wiring, but does test all that you've found faulty & repaired.
As suggested, a voltage drop test at the fuel pump would be all telling, but more difficult to do, so that's why I suggested re-doing the KOEO, KOER fuel pressure & volume tests, they're easier to get at & do & if ok, tells you the pump connections are likely ok.
Again, how did the pump wiring, connector & contacts look when you replaced the fuel pump?