Is this the gauge or sending unit?

With truck running oil gauge is pegged at the high end of the gauge. With engine off, key out the gauge reads nothing (all the way to the left). With engine off but key in 'run' position if I unplug the wire to the sending unit on the block the gauge drops to nothing, when I ground the wire the gauge pegs, it goes all the way up.

1987 F150 I6.

Thanks for any advice.

 

my guess would be the sending unit. sounds like it is stopped up. or maybe you just have one heck of an oil pump(just kidding)
I would try the sending unit first anyhow. they are only about $10 dollars

 

Are any of the other gauges malfunctioning ?

 

Alt & temp gauge seem fine, gas gauge does the way past full when tank filled and stays there until the tank is about 1/4 empty then swings back to read more accurately (just the back tank, the front tank gauge reads fine).

 

 

It was the sending unit, now it reads at the 'o' in 'normal'.

Thanks