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I am working on a 1987 E 350 motor home it has a 460 with a holley 4 barrel carb and a electric pusher pump in the tank. The problem is the gauge only goes up to just below the empty mark, the tank is full of gas and the sending unit was supposedly changed. I changed to a spare gauge cluster and the same thing.I pulled the plug off of the sending unit and checked voltage and it was bounsing all over the place, so I put a test lite on it and it was pulsing ,what the heck is goig on????HELP P.S. I also had the same pulsing at the plug end under the hood with the connection together.
 

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Sometimes the instrument panel has it's own voltage regulator. How do the other gauges look, running a little lower on thise too? It does not have to be a linear amount of loss on the other guages. If it is faulty, then you are not suppling a constant good voltage to your gauge.

I thought my gas guage is faulty too. I fill it up, the needle points full, then just driving around town once, it points to empty.... then I realize.... it's my driving habbits... (laughs)
 

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Actually, if I recall the voltage should be pulsing. Check the ground at the sending unit and make sure it is providing a good solid ground. I would put a jumper wire on it just to be sure grounding it to a good clean ground on the frame.
If that isn't the problem I would obtain a new sending unit or one that is know to work accurately. Just plug it into the wires that you remove from the one in the tank ensuring you have a good ground and move the float up and down through its range. Have someone check the fuel gauge and it should be moving up and down.
You can start with this and see what happens.
Bad grounds are nasty
 

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Supposed to pulse, ground the wire against the frame and see if it goes to full. If it does, its the sender, or a bad ground at the tank. My guess is the sender
 
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