1988 fuel gauge not working

[matt12305]

As the title states..fuel gauge is not working. It's a 1988 XLT 2.9l. it has dual fuel tanks. Last year I replaced the front tank with the bigger 17gal(I couldn't find a 14.5gal), I also replaced the sending unit and the high pressure pump. I put around 4 gallons in the gas tank and my fuel gauge has not moved at all. I suspect it could be damaged wiring. What's the best way to check the wiring for it? Just follow the wire all the way down or is there a way to use a multimeter to check?
Please advise

On a side note, I am one part away..wait no 2 parts away to getting this thing running smoothly. Needs new motor mounts, and a new map sensor (got code 22 from obd1) after that, some new tires and maybe coolant flush and it'll be good to go.

 

[franklin2]

Best way to check the wiring and the gauge is to unplug the plug at the sending unit, and with the switch set to the correct fuel tank, take a short piece of wire and jump the blue/yellow wire to the black wire on the front tank, or the yellow/lightblue wire to the black wire on the rear tank. The fuel gauge should swing full scale empty or full. Then take the jumper off, the fuel gauge should swing full scale the opposite way. If it passes this test the wiring and the fuel gauge in the dash is working, and there is something wrong with the sending unit.

I looked and with dual tanks you also have a switching valve on the frame. Besides switching the fluid lines for the fuel, this valve also switches the sending unit wiring. So there is a possibility this valve could have a fault also.

According to what I was looking at, you have a low pressure electric pump in each tank, and then a single high pressure pump on the frame rail after the switching valve.