Sending Unit has me stumped!

[Ryanz]

I have a 1971 that we restored a year or 2 ago. New sending unit and gauge cluster. Everything worked until the end of last summer the gas gauge quit on me. After reading up I determined it was likely the brass float, so I removed and replaced with plastic float as it appeared to be taking on fuel. I've put it back in but must have a ground issue because it's reading over full (3 O'clock).

When I hook it up outside the tank, ground it manually, and move the float and it moves the gauge accordingly. But when I put it in the tank and hook it up - it stays at the over full position. Even when I try to run a ground to it I can't make it work properly. I've read all of the articles and I believe my ohms are correct. As I move it around inside the tank with it hooked up to a ground wire I can get it to work sometimes, but the second I secure it it pins over full. I even bought a new sending unit but have the exact same issue. Any suggestions would be welcome.

[dmjlambert]

The problem with restored truck and new paint is bolts won't provide ground when the metal the bolt touches is painted metal. Some people also paint the bolts. It is possible that is the problem, but don't know the story on how much stuff you have painted. What do you get if you connect a jumper cable from bare metal of the truck or neg battery terminal to the sending unit while installed? If that fixes it, you could run a small wire from a terminal on a bolt where you have scratched the metal to the sending unit. For an attaching point on the sender you could probably put the stripped part of the wire under a very small hole clamp.

[Ryanz]

Ya, the frustrating part is that it all worked for over a year after the restoration. So I don't understand the grounding issue now? And yes, it works outside the tank with a jumper wire, but not when inside the tank with a jumper wire. That's why I'm confused. Like if it works outside, that should rule out any gauge or wiring issues correct? It has to be a ground - but I can't recreate it inside the tank for some reason.

[Richard]

If you do not want to clean paint at bolt holes try star type lock washers on all ground bolts. Like these https://www.mcmaster.com/products/star-washers/

[Ryanz]

Originally Posted by Richard

If you do not want to clean paint at bolt holes try star type lock washers on all ground bolts. Like these https://www.mcmaster.com/products/star-washers/

Ya you bet. I did go and remove any paint from my frame and under body of the cab. The end that connects to my frame actually already has the sharp toothed washer. But I cleaned it as well. Unfortunately still no dice. As I said, the part that is baffling is that It worked for about a year 'after' the restore was complete. So I don't know why it would be a paint issue now but I guess weirder things have happened.

[pjmoreland]

Originally Posted by Ryanz

When I hook it up outside the tank, ground it manually, and move the float and it moves the gauge accordingly.

When you did this test, how did you ground the sender exactly? What part of the sender did you ground, and where on the truck did you get the ground?

[Ryanz]

Originally Posted by pjmoreland

When you did this test, how did you ground the sender exactly? What part of the sender did you ground, and where on the truck did you get the ground?

I just ran a wire from the top of the sender to a cab bolt that holds the door latch in. I also touched it to one of the fuel lines which also worked. This was all when testing outside of the tank.

[pjmoreland]

So the question is what changed between the sender being out of the tank vs. inside the tank.

1) Did you already try running a wire from the sender mounting flange to a known good ground?

2) I wonder if the tan sender wire has an intermittent break in it that works when the wire is flexed just right. You could try running a new wire from the "Fuel Ga" terminal on the fuse panel to your sender

What happens if you short the tan wire to the sender mounting flange? Does the gauge move to Empty (key in ON position)?

[Ryanz]

Originally Posted by pjmoreland

So the question is what changed between the sender being out of the tank vs. inside the tank.

1) Did you already try running a wire from the sender mounting flange to a known good ground?

2) I wonder if the tan sender wire has an intermittent break in it that works when the wire is flexed just right. You could try running a new wire from the "Fuel Ga" terminal on the fuse panel to your sender

What happens if you short the tan wire to the sender mounting flange? Does the gauge move to Empty (key in ON position)?

Needle moves to empty with the key on when shorting to mounting flange.

[Ryanz]

Originally Posted by Ryanz

Needle moves to empty with the key on when shorting to mounting flange.

And nothing changes when I run a jumper wire from fuse box directly to the sending unit.

[Ryanz]

Originally Posted by Ryanz

And nothing changes when I run a jumper wire from fuse box directly to the sending unit.

And unfortunately nothing changes when I run a good known ground to the flange once it’s in the tank. Gauge consistently reads 3 o’clock unless grounding out the tan wire or the power terminal itself, then it moves to empty.

[dmjlambert]

What do you mean by grounding out the power terminal itself?

[Ryanz]

Originally Posted by dmjlambert

What do you mean by grounding out the power terminal itself?

Just meaning if I touch ground to the terminal where the tan wire connects (while connected) it pushes the needle to empty.

[pjmoreland]

Disconnect the tan wire from the sender and use a multimeter to measure the resistance between the sender terminal and the sender mounting flange. What reading do you get? How much fuel is in your tank?

[Ryanz]

Originally Posted by pjmoreland

Disconnect the tan wire from the sender and use a multimeter to measure the resistance between the sender terminal and the sender mounting flange. What reading do you get? How much fuel is in your tank?

I get about 70 ohms, tank is 3/4 full

[pjmoreland]

That sounds good. So strange. Do you get the same 70-Ohm reading if you measure between the sender terminal and the bolt on your door striker plate?

[Ryanz]

Originally Posted by pjmoreland

That sounds good. So strange. Do you get the same 70-Ohm reading if you measure between the sender terminal and the bolt on your door striker plate?

Yes sir.

[pjmoreland]

With the tan sender wire hooked back up to the sender, and the ignition ON, can you measure the voltage at the sender terminal and at the "Fuel Ga" terminal at the fuse panel? Don't unplug the connectors when taking these two measurements.

The voltage measurements should be virtually the same in these two locations, as long as the tan sending wire is good. Also, the voltage level should give an indication of whether there is an appropriate balance of resistances in the gauge winding that is responsible for pulling the needle toward Empty vs. the sender.

[Ryanz]

Originally Posted by pjmoreland

With the tan sender wire hooked back up to the sender, and the ignition ON, can you measure the voltage at the sender terminal and at the "Fuel Ga" terminal at the fuse panel? Don't unplug the connectors when taking these two measurements.

The voltage measurements should be virtually the same in these two locations, as long as the tan sending wire is good. Also, the voltage level should give an indication of whether there is an appropriate balance of resistances in the gauge winding that is responsible for pulling the needle toward Empty vs. the sender.

Alright - I think we have maybe found our problem. With my voltage tester set on 10v the reading goes to 125 at both the sender and the fuse box. Would that indicate my resister is fried?

[pjmoreland]

Is that 0.125V?

[pjmoreland]

I just measured the FULL winding resistance on a gauge I have, and it was 104 Ohms. If I assume your battery voltage is 12.5V and then do a voltage divider calculation with the 104-Ohm gauge winding and the 70-Ohm sender, then your measurement at the tan wire should have been approximately 6V. A reading lower than that indicates either lower than expected resistance at the sender or higher than expected resistance at the gauge.

[Ryanz]

Originally Posted by pjmoreland

I just measured the Empty winding resistance on a gauge I have, and it was 104 Ohms. If I assume your battery voltage is 12.5V and then do a voltage divider calculation with the 104-Ohm gauge winding and the 70-Ohm sender, then your measurement at the tan wire should have been approximately 5V. A reading lower than that indicates either lower than expected resistance at the sender or higher than expected resistance at the gauge.

Ya, I’m guessing this is my issue. I’m going to try one from my other gauge cluster. Can you remind me if the little notches in the resistor go to the top or bottom? I want to be sure I have it right. I’ll do a search on the forum. I swapped them once already thinking that may be the problem but perhaps I had it on upside down.

[pjmoreland]

Send some photos of the notches, if you can.

Is your gauge a reproduction unit? I don't have any direct experience with reproduction gauges, but I have read that some of them are built to work without an external resistor. Having an extra resistor on there that isn't needed will make the gauge read more full than normal.

[pjmoreland]

The gauge works correctly when your sender is out of the tank. I have a hard time thinking there is something wrong with the gauge based on that.

[pjmoreland]

Could you upload a photo of the entire front of your multimeter getting the 125 reading?