Ok, my new to me 89 beater has the dual tank option. I'm still not sure why they used two tanks instead of one big one but that is not my point. The truck was originally a fleet vehicle that ran on propane or some form of natural gas. When the previous owner purchased the truck he had the motor and trans rebuilt converted back to gas and installed a new forward tanks and sending unit. The rear tank has never been used. The newer front tank has had a sending unit fail, I think. The gauge is pegged at E. Since I am using this truck as a daily beater I would like to have the gauge working. It looks like my options are to either have a shop drop the tank and install a new sending unit, or I could do it myself with my dad and save some $$$. I'm not sure how much a shop would charge for this work and I don't want to drop tons on a beater truck. Any suggestions? I see sending units anywhere from $40-$100 at Autozone. I drive about 300mi a week to and from work so filling up after about 100 or so is getting old. I'm also considering replacing the rear tank, since it is probably full of rust due to almost 20 years of non-use. I have no idea how much this will cost so any suggestions would be nice. This is one of the must fix items on my list.

Oh yeah, BTW, how do I know if I have an external pump or in tank?

 

If they put it back to factory then you have a Low-Pressure Boost Pump in each tank and a Hi-Pressure pump on the frame rail along with a selector valve and filter.

As for the gauge you need to find out where the problem is.
Unplug the plug at the tank, turn on the key and see what the gauge points to. Then ground the gauge pin in the plug and the gauge should peg at the other end of the scale. Should be a Dark Blue wire with a Yellow stripe.

 

and that means the gauge is good? I'm having the same problem

 

Yes it means the gauge is OK.

 

I grounded the wire that you mentioned and no movement at the gauge. I switched to my rear tank and Wam! my gauge works! but when I switch it to the front tank the gauge goes down to empty 9 even though I filled up the tank) ...I guess it can be the sending unit or that wire that you mentioned must be open somewhere.
any tips?

 

A grounded wire should be a passed empty fuel gauge reading.
A open wire (not connected) should be a passed full fuel gauge reading.
145 ohms = full
22.5 ohms = empty