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1986 caprice classic 305 fuel guage jumps past full when i turn the key even when car is started

on my 86 caprice classic. the gas Gauge jumps to full when i turn the key but its only like a gallon of gas in there is it the sending unit the ground or something else and is there any way to test it thanks

The gauge is actually an ammeter that is reading current flow to ground…The sending unit inside the tank varies the resistance to ground as the float moves…Sounds like the wire from the gauge (hot) is shorted to ground causing full meter deflection. Step one is dropping the tank enough to disconnect the gauge wire. The gauge should now read empty, zero current flow…This indicates the sending unit is bad (shorted out). If disconnecting the wire makes no difference in the gauge reading, there is a short somewhere along the wire back to the gauge or the gauge itself is shorted out…

ok thanks ill try this in the morning

Actually on these old chevys they do it just the opposite. The higher the resistance, the higher the gauge reads, so chances are you have an open circuit from the gauge to the sender. Grounding the pink wire at the tank should cause the gauge to read empty. If that happens then you have a bad sender which is the most common cause for your problem. Specifications are 90 ohms full, 0 ohms empty.

only thing i did to it so farr is unplug the wire and it went up futher and plugged it back up and it went down a little.

Pete is right, I had it backwards…Ground the wire and the gauge should drop to zero. If so, replace the in-tank sender…

i grounded the wire to the guage and it went to empty so ill b dropping the tank soon thanks and lookin in to a sending unit thanks

i plugged it up to my other car and it started doing the same thing so that dies confirm that it is the sending unit thanks again