My truck will only run on the back tank ive replaced both fuel pumps the fuel filter the tank selector switch and the fuel pump selonid

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.

check tank selector switch

mine was doing the same...switch was bad. a bit convoluted really but if your high pressure pump is running and not getting your tank pumps to go, unless you jump there at the switch harness prolly the switch.

I would drop the tank and replace the pump. These types of problems are fire hazard.

well, first, to find out if its the in tank fuel pump or the tank selector valve, the cheapest and probable easiest thing to do is find the selector valve under the truck and switch the incoming fuel lines. front tank to rear tank inlet, and vice versa. If the truck still doesnt run, you either have an electrical delivery problem to the in tank fuel pump or a bad fuel pump. If after switching the lines, it does run, then you need a new selector valve

You are right. There are three pumps. It sounds like the transfer pump in the front tank is bad. The in tank pumps just supply the high pressure pump to keep it primed. You need the front pump.

OK, guessing you know your truck. Without a pressure switch the pumps would run all the time the ignition is on. do they run ok without pumping? (pull one out of the tank). Do you have a relay that the tank selector switch activates? Most cars run the hot lead active, and the pumps run by applying the return lines. Your problem could be in the wiring from the pump back to the negative.

You have duel tanks with a fuel pump in each tank. You also have a fuel pump on the frame rail after the "tank selector". This device only holds a fuel filter that protects the main fuel pump. When you select front or rear tank on the dash, you are selecting which in tank fuel pump will operate. You can check the in tank fuel pump by listening for it to "humm" when the key is turned on..The in tank pumps only feed the main pump and cannot run the engine on their own. Hope this helps

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.

bad injection pump