Hello, 66 F100, 352, 3-spd. Fuel gauge is not working. Replaced VR with unit from NPD, no luck. VR is outputting 8V. Ordered adjustable VR (Amazon) and wired it up. Have good 5V now. Gauge still does not work. Grounded fuel sender wire, gauge goes slowly full scale. VR and sender grounds are good. Pulled sender and checked with ohm meter. Has good resistance sweep. Plugged sender in with it still removed from tank and still nothing at fuel gauge while moving float arm. Question is, what voltage do I need at the gauge? Is my amazon VR putting out correct voltage?? Hope I didn’t miss anything. Anyone ever have this problem?
Thanks in advance.

 

When the sender was removed, how did you ground it when you moved the float? Did you check it installed and grounded? How is it not working?

 

66v8, I grounded it with same ground used when installed.

 

You should have 5 volts at the gauge. It sounds like the sender is bad. If you bypassed the sending unit and shorted the harness to the tank, and it went to full, then that only leaves the sending unit.

 

Using ohm meter, I have a good resistance sweep when sender is removed from tank. Orange wire from sending unit to gauge is good as well. That’s why I’m confused.

 

Here is the way to test the gauges in these old trucks without taking anything apart. These gauges are King-Seeley type gauges and all of them work the same, Fuel, Temperature an Oil just the faces are different to depict what they are measuring. They are measuring resistance to ground at a nominal 5 volts.
You need two resistors one 10 ohm and one 73 ohm. 73 ohm resistors are a little hard to find a 75 ohm will work just fine.
These gauges are calibrated at 80 ohm empty and 10 ohm full.
This test will allow you to determine where your problem is without taking the whole truck apart. And it assumes that your wiring and grounds are in good shape.
Remove the sender wire an connect it to the 10 ohm resistor to ground, turn the ignition switch on and the gauge should go exactly to the full mark. Connect the 73 ohm resistor and it should read at the empty mark.
If the gauge passes this test you know the sender is at fault. To test the fuel sender you have to remove it from the tank. With an ohm meter connected to it, it should read 8-12 ohms full and 60-86 ohms empty. The fuel sender in my truck reads 13 ohms full and 73 ohms empty close enough.
If your gauge doesn't pass this test the way to tell if you have a bad gauge or a bad ICVR is to run the same test on the Temp gauge an the Oil gauge if you have one. If that gauge fails the test also you know you have a bad ICVR. If it passes the test you know you have a bad gauge.
I would never short a sender wire directly to ground because you could damage the gauge. Especially if the ICVR is bad an allowing a full 12V in the circuit.
Checking the actual average voltage output of an electro mechanical ICVR is nearly impossible unless you have an oscilloscope. About all you can tell with a volt meter is if it's working or not. The resistor test is about the only way to tell if the gauges an ICVR are happy together.
Also there is no way to accurately test the temp sender or oil sender if they fail just replace them.

 

Crop Duster, I did what you said. Great advice by the way. So..... I definitely had a bad VR. I have a good 5volts now. My tank is full, everything connected, needle goes 3/4. 75ohm resister in, goes full scale. 10ohm, full scale. Sender wire disconnected altogether, 3/4. What the heck?? Just to make sure, which terminal does the 5V’s connect to? There are no markings. If I were to reverse them, seems like it would go negative. Idk. Is my gauge shorted?? Is there a way to check the gauge? Thanks again!

 

One thing to be aware of in this video. At least doesn't hurt to be aware of the adjustments.

 

Is there a way to check the gauge? Thanks again!


Yes. Disconnect both wires from the gauge and connect an ohm meter set on the 1 ohm scale. If it is an analog meter be sure to zero the needle. If it is digital and cannot be set to zero connect the leads together and note the reading so you can subtract that from the gauge reading.They should read between 10 and 14 ohms. If it is above or below that reading after the ohm meter has been connected to it for at least 1 minute the gauge is bad. These gauges are extremely sensitive to any voltage at all. These two pictures are the same gauge the first is a cold gauge the second is the same gauge after being connected to the ohm meter for 2 minutes. As you can see that tiny amount of voltage moved the needle. What I'm saying is if you are going to adjust the gauge as in the video be sure you have your mind completely wrapped around how these gauges work.

 

Tested gauge. 12.7 ohms after 2 minutes.

 

Am I the only one who had wonky audio in that video?