Fuel Gauge Issues

[agilbert81]

Hi. My truck is a 74 Chevy K10, 350 small block. The truck was given to me by my father-in-law. It has dual gas tanks. The issue that I have is when you start the truck, the needle on the gauge pins past full and stays there. I initially thought that is was the Fuel sending unit. So I dropped one tank and was going to replace it. Then it occured to me to ask my father-in-law if this issue only happened on one tank or both. He informed me that the issue was the same for both tanks. So to me that kinda ruled out the fuel sending unit. He was thinking that maybe it was s short in the dash. The instrument cluster has one harnes plug, and electic leads that run on an acitate sheet. I have another instrument cluster on a 78 chevy truck, is it compatible? So, my questions is, What would you do if you were me? I don't want to just go throw $120 at the truck and replace both fuel sending units if they dont need it.

[81_Chevy]

sounds like a ground problem, check your ground on the sending units. should be a wire that comes off of the tank and onto the frame somewhere

[bd]

Temporarily ground the sending unit wire and check whether the fuel gauge drops to Empty when you switch the ignition ON.  If it does, the gauge wiring is good to that point and you have an open downstream.

[agilbert81]

Thanks a bunch guys. Come to think of it. I remember the ground wire being disconnected but I wasn't sure if I did that on accident when I dropped the tank or not. If I didn't do it, that would make sense as to why it isn't working. I am going to try it when I get home tonight. Thanks guys

[agilbert81]

Another question, How do you find where there may be an open further down the wire?