1988 ford tank selector valve

The black reservoir/fuel tank selector unit you are referring to is the fuel tank switching valve, which is designed to switch between the two fuel tanks in your 1988 Ford Ranger XLT 4X4. This valve is responsible for directing fuel from the selected tank to the fuel pump and engine.
The second fuel tank selector unit you mentioned (part number 5sv2k) is likely a replacement part for the original fuel tank selector unit. It is not intended to replace the fuel tank switching valve.
The dual fuel tank system in your Ford Ranger requires both the fuel tank switching valve and the fuel tank selector unit to function properly. The fuel tank selector unit allows the driver to switch between the two fuel tanks, while the fuel tank switching valve directs fuel from the selected tank to the fuel pump and engine.
If the black fuel tank switching valve is leaking or malfunctioning, it should be replaced to ensure proper operation of your dual fuel tank system.

Have you checked the control switch that powers each tank ? Some models use a mechanical selector valve and others use an electrical switch to control which tank is being used. Both tanks run off of the same fuel pump relay.

The seal in the selector valve is bad on the front side.
Replace the valve or rebuild it.
Disconnect the battery ground cable before doing any work around gasoline.

There was a recall of sorts on the switching valve under the truck. You'll probably need to have it replaced. don-ohio

the selector is located between the two tanks and has 3 or 4 fuel lines connected to it as well as wires if its an electrical switch.some had a manual switch which is usually behind the front seat on the floor that switches the tanks from one to the other when turned.

You have checked the fuel pressure? Do you have a check engine light? As for the tank filling up the wrong one, that is caused from a bad tank select valve on the frame rail.

check inside of the frame drivers side by the rear wheel

Try checking the tank selector valve under the truck in the back by the tanks, it might be bad or not operating right, or have it checked.

Just in front of the side tank is the selector.Very common problem.The rear fuel tanks pump quits.

I’m happy to assist further over the phone at https://www.6ya.com/expert/joe_a3369546f8067589

There is no selector valve on a 1995 ford f150 300 engine. The selector switch on the dash sends power to the pump in the selected tank. The pump then sends fuel to the engine. If engine won't run on the selected switch, the pump in that tank needs replaced.