Gas gauge shows empty, Tracing problem on a 1975 in-cab tank

I have a '75 short box with the gas tank in the cab behind the seat. Just bought the truck last Fall and I've been working on it on and off since. I have had the gauges out and checked things and they all looked good so I put them back in. They all work except "maybe" the gas gauge. It read empty so today I took the truck out for the first time and drove straight to the gas station. I went to fill it up and it wouldn't take any fuel. So I went right back to the shop and looked inside with a light and the tank was full! The first thing I thought was, "who sells a truck with a full gas tank"?!?! Second thing I thought was, "is it the gauge, is it the sending unit, is it the wiring"...

So, just thought I would ask here if anyone has spent time tracing this problem down and where I should start.

Thanks!

Ground the sending unit wire at the tank end. Then turn the key on. If the gauge starts to rise shut the key off. Also try running a ground wire from one of the screws on the sending unit to the body and see if that helps.If the gauge moves then the sending unit is bad. If the gauge doesn't move and the rest of the gauges work then it's either the wiring from the gauge to the tank or a bad gauge.If the rest of the gauges don't work then it is probably the voltage limiter plugged into the back of the cluster.

I had a problem with mine and it turned out to be the printed circuit board. You couldn't see it but when I jumped the wire from the sending unit straight to the gauge terminal it worked fine. It's the blue with white tracer wire I believe.


Can you answer a question for me? Does your truck have glue on door weatherstips?

beautiful truck

Nothing to add to the troubleshooting tips. On my PU the sender and wiring was good and gauge bad. I installed an aftermarket fuel gauge until I am ready to tear into the cluster.

A general suggestion. Drive it! (and burn up that old gas). A 5 gallon can in the bed just in case.

Nice truck.

My bets are on the sending unit itself, also the wiring on the old 70's Dodge pickups were never very good when they were new plus now that they are 40 some years old they've usually had alot of good old redneck engineering to add to the mix. LOL

I have an old gauge that was working just faded needle so I replaced it. You can have it if you want it and don't mind waiting on shipping.

It really is!

Ok, I grounded the sending unit wire at the sender and turned the key on and the gauge went up. Here is a picture of the sender at the tank. Is there supposed to be a grounding strap on the fuel line over the rubber connecting hose like on the car tanks???

I've never seen a strap on the trucks, and none of my cars, and the gauges have worked fine.

You could try a wire wrapped around both metal fuel line ends and see if it works but I don't remember a jumper on the trucks. I'm old so don't trust my memory. It looks like it wouldn't be any problem to pull the sending unit and see what shape it's in.

Does it need this?

Yes, there is glue on them. Most of the weatherstripping is brittle and I am going to replace it here eventually.

My 74 doesn't have any ground wires or straps on the sender. You could just jump it but I suspect you need a sender. Check on ebay I know I've seen them.

One quick thought is to make sure the float has not become heavy, from internal leak. Had a friends new repro sender do that, not cool.

If the float is bad you can use one from ford.

Alrighty! I pulled the sending unit out today and to my surprise it was in near new condition. BUT, the float was leaking and full of gas and there's the problem. I took a float off a spare sender on the shelf and put it all back together and in the tank. Now the gas gauge shows 3/4 of a tank when I know it's full. So, that's better than it was before when it was showing empty all of the time. Not sure if there are any adjustments I can make to the sender to read full at full. If there are then let me know otherwise I'll live with what I got for now. Thanks for the tips everyone!

Carefully bend the float arm down a little to get it to go farther up when in the tank.