Struggling with fuel gauge

Hi guys. I'm having problems getting my fuel gauge working. I've troubleshot it as far as I could get and now I still can't isolate the cause.

First off, there is nothing stock about the situation, but everything is designed to work in place of stock, so I'll go over my configuration to start with: I have a 33 gallon tank for an 85-86 with a stock sender unit for an 85-86. I have an in-tank Aeromotive pump which I have wired with the 85-86 sender through the 85-86 hanger and the 85-86 3 prong plug. From the 3 prong plug I have the fuel pump wired into a relay and the sender wire hooked into the factory harness at the yellow wire that the original 84 sender was hooked into. From there the sender signal is sent to the dash via the factory harness where I have it hooked up to an NVU gauge set with a setting of 73-10 ohms to match the 85-86 sender.

If I disconnect the sender harness and test the sender by the tank, I get 74 ohms (which is what I would expect since I have a pretty full tank). If I go to the other side of the plug at the tank that leads to the relay and the gauge and I short the yellow wire to ground, the gauge reads "Full" when I turn the ignition switch on. So, I know the sender works on its own and the gauge works on its own. When I check the yellow wire behind the dash I get 66 ohms, which makes sense given that the 15 foot long length of wire has additional resistance.

Here's where it gets weird. When I plug everything back in and I turn the key to "on", the resistance on the yellow wire (tested directly behind the dash) goes to 0 ohm. So I know the sender is working, the gauge is working and the wiring between the two is intact. It seems like the fuel pump power or ground is somehow shorting the sender wire. Is that even possible? What can I do to isolate the problem?

Any help is appreciated.

Chad - I think your thinking and your sender are backwards.  The Bullnose senders have 10 ohms at Full and 73 ohms Empty.

And how are you testing the "resistance on the yellow wire (tested directly behind the dash)"?  IOW, where are you connecting the leads to the DVM, and what scale do you have it on?  If you have it on the resistance scale and the negative lead to ground and the positive lead to the yellow wire you should get a wonky reading.  That's because that is a powered circuit and you can't directly measure the resistance in a circuit under power.

But, let's skip that issue for a moment.  When you hook it all together and turn the key on what happens?  I think the gauge should read empty.

Thanks for chiming in Gary. Last things first, when I turn the car on the gauge reads empty. But, like I said, the tank is relatively full.

I am testing the yellow wire behind the dash by putting the positive probe on the yellow wire (behind a plug I made for all of the instruments) and the negative probe on a ground spot on the door sill hardware. I believe my unit was reading in whole ohms units but I’m not 100% sure because I don’t have a pic.

Am I doing it completely wrong?

An ohm meter works by sending a small current through the resistance and reading the voltage drop.  R=E/I.  But if you put it in a circuit with power the reading will be way wrong.  WAY wrong.  That's because there is already a voltage drop across the resistor before the meter sends its own current through.  So you can't test resistance accurately in a powered circuit - at least not with a standard DVM.

But if your gauge is reading empty then it may be reading correctly if you set it up for Empty at 73 ohms - which is what you have.

I know I bought an 85-86 sender and my fuel gauge is adjustable to accept either Ford standard and a couple of others. How do I test what reading the fuel level sender is actually sending to the gauge with my probes?

I think you know what it is sending - 73 ohms.  I just think you have the gauge in the wrong mode.  When you say "Ford standard" you need to specify the year as in '87 everything changed.  Empty is now something like 22 ohms and Full is 120 ohms or so.

I painted my fuel tank before I installed it. As far as I know, the entire tank is unintentionally isolated from ground. Could that affect the reading on my fuel gauge?

Yes, not being grounded would be a problem.  Check that with your DVM with one lead to the frame and the other on the tank.

I struggle with circuits. Had to take it twice in college (I was fortunate because the class was often referred to as e-mag, re-mag, three-mag).

Makes sense now how it's reading resistance with only one end of the wire; it's actually not. I get it now. Thanks, Gary.