I have a weird issue going on with my fuel sending unit, and I am sure I am overlooking something simple.

This is in my 1947 Ford Truck, but the entire system (tank, sending unit, engine) are from an 82 f150 with a 351w. The gauge is an aftermarket gauge but is compatible with my sender (according to manufacturer). The tank is the 19 gallon tank that is located behind the rear axle. The tank and the sending unit I have are linked below.

First problem was that the gauge read empty when the tank was full. This was the very first time I had started the engine and the gas tank had never been anything other than full for me to compare it to. I immediately thought that the wiring must be backwards on the sending unit, so I switched it. The tank, of course, read full. Yay, problem solved!...No, not solved. The gauge ALWAYS reads full. I was down to less than half a tank and it still read full.

So I figure I must have something wrong with the sending unit. I dropped the tank, spilled gasoline everywhere trying to drain it, and investigated. When I pulled the sending unit, everything looked fine. I plugged it in to the wiring harness to see if it worked when I moved the float manually. And it worked perfectly. Full read full, half read half, and empty read empty. This was weird. So I put the unit back into the empty tank, and it immediately reads full.

I reversed the wiring back to how I originally had it, and the sending unit worked perfectly outside of the tank. Full read as full, half read half, and empty read empty. So both ways I had it wired, it worked perfectly outside of the tank.

When I put it back into the tank, the gauge read empty, as it should since there was no fuel. I filled the tank about 1/4-3/8 full with fuel and the gauge still reads empty at all times.

Mind you, when I originally pulled the tank, it still had a lot of fuel in it and I could see that the float is floating as intended when there is fuel in it.

Is it somehow grounding itself? I have the ground wire cleanly and securely fastened, so I don't see how that could be the case. But if it is, I am not sure how to stop it. Is it a bad sending unit? Its brand new, and seems to work perfectly outside of the tank.

Any thoughts/advise are appreciated.

Fuel Tank (Mine is original, but this is the size/shape):

https://www.rockauto.com/en/moreinfo...121308&jsn=473

Fuel Tank Sending Unit (this is the exact unit I have):

https://www.rockauto.com/en/moreinfo...121308&jsn=481

 

Bad sending unit.I would suspect. Meter out the sending unit to make sure...

 

Okay I will give that a shot. I did it before installation and it seemed to be okay, but that was a few years ago. Its been a looonnnnggg build.

If its a bad unit, why would it seem to work outside of the tank and not work when its in the tank?

 

It may be grounding through the tank once the ring is cinched down.

 

 

I think it is faulty and something is making contact once it is cinched down with the ring. Install and meter it out to double check...

 

You have two wires going to the sending unit correct? One of them is ground. The other goes to the gauge in the dash.

Take it out again, get a voltmeter or testlight, put one lead to a good ground on the truck, probe both wires coming from the truck. With the key on, engine off, one of the wires should have voltage on it. The other should not, that is the ground wire. If that is not what you are getting, something in the wiring is not correct in the truck.

Another test you can do, if you do determine which wire on the sending unit is ground, take a wire and bolt it to the truck ground, and then take this wire and ground out the sending unit while you are holding it out of the truck and working it manually. You said it worked fine working it manually, but if as soon as you ground the sending unit metal with the ground wire from the truck and it starts acting funny like it does when it's in the tank, then you have verified it is a wiring problem with the grounding of the tank.

Post a link to the aftermarket gauge you have.

 

 

EOTZ-9275-A .. Fuel Tank Sending Unit - Use with Aft Axle STEEL Fuel Tank / Obsolete ~ 88 available NOS

1980/84 F100/350.

 

As Matt said once cinched down with the ring.
It may be flexing the sender base and making it short out.
I don't know if there is a way to test this as I don't think you can move the float when installed.
Someone did say with the unit out of the tank to run a jumper from the sending unit to the truck and test it.
I know you tested it when out before but you did not have a jumper to ground.
Dave ----

ps: do you have a build thread on your project? If would like to see it post up a link to it.

 

I have never had good luck with Spectra Premium products - their in-tank pumps & sending units.

 

I installed the new sending unit with the same exact results.

However, I am only getting 8.5v to the sending unit. I probed the fuel sending unit wire coming out of the firewall and its 8.5v there as well. So it looks like I am having an issue with power to the sending unit. I can't figure out how the wiring is bad, though.


***edit*** I am getting 12v to the gauge, and only 8.5 out of the gauge to the sender. I assume that I have a bad gauge? If that is the case, why does it work properly when out of the tank but not when its inside the tank?

 

First lets look at that 8.5v
How are you measuring it or I should say what type of meter, digital or analog?
If you check for power on that wire with a test light what does the test light do?

Did I read you are using an after market gauge? Who makes it and how do they want you to wire it?
Now my testing and I think like others is with a stock Ford setup.
There is a IVR/ICVR (Instrument Voltage Regulator / Instrument Cluster Voltage Regulator) that gets 12v input and about 6v out put. This does it by turning on & off the power so the output is 6vandifyou use a test light it will flash on & off. I don't think you can use a digital to see this on/off, analog the needle will bonce.

Now that output then goes out to the fuel / temp / oil gauges then from the gauges to the senders for each.
If you were to check the sender wires with a test light it will blink. The gauges are a "meter" to measure the resistance of the sender.
So I wonder if the aftermarket gauge is not compatible with the sender even if they say it is?

I do have aftermarket gauges (speedHut.com) in another project and part of the fuel gauge set up to the stock sender was not as easy as plug it in.
They would need the sender to set it up right but I gave them the factory full / empty ohm and hope the half tank is close but I mainly want the empty level to be right on if I can.
What also helped was they did a sender like mine a few months before so they kind of went by it also so here's hoping it will work.
Dave ----

 

Here is the gauge I have:

https://www.jegs.com/i/Equus/390/7361/10002/-1?CAWELAID=1710683239&CAGPSPN=pla&CAAGID=157690684 31&CATCI=aud-224375369591la-176557698071&CATARGETID=230006180003463448&cadevic e=m&gclid=EAIaIQobChMI9ePxytnO3QIVkmB-Ch0cPwppEAQYAiABEgLcaPD_BwE

Here is the install instructions that came with the gauge:

https://www.jegs.com/InstallationIns...0/390-8462.pdf

I've got a dedicated switched 12v coming from the wiring harness, and then I have a ground from the gauge. Lastly, I have a wire coming from the gauge to the sender.

I am using a digital voltmeter. It reads 12v into the gauge, and 8.5 out to sender. Same 8.5 at sender. I just went out and put a test light on it. It lights up normally at the 12v into the gauge, but barely lights at all at the terminal going to sender or at sender. It doesn't blink, it just comes on VERY dimly.

I know that equus gauges aren't top quality, but I have used them in the past with good success. I guess there is a decent chance that the sender and the gauge aren't compatible, but the OHM's are consistent with what I have read that they should be. I would think they would know what the factory resistance should be. Plus, I am still struggling to understand why it works when the sending unit is out of the tank. It works perfectly. But if the sending unit is inside the tank, it stays on empty. But it has done this with two different sending units.

 

The EVTM shows an 8-to-9-ohm resistance wire in line before the ICVR.

Point being, the factory gauges were never designed to work on 12V.