Good Morning,

Before I ask my question I wanted to take a moment to thank the senior members of this forum who contribute their expertise generously. I don't think I would be where I am in my C3 rebuild without your help! It feels like one way traffic right now but I hope to reach a point where my experiences allow me to contribute back to other member questions.

My weekend project was new gas tank, new sender unit, new fuel line rubber hoses and new fuel pump. All went well and car starts but fuel gauge is not working. It was working prior to the parts replacement.

Here is what I'm seeing using a multimeter:

1. Fuel tank is well grounded (measured between sender unit metal plate and frame rails)
2. Good ground between the ground wire on the sender unit and the vehicle ground (measured between ground wire and frame with ground wire removed from tank)
3. I'm seeing about 9.3 volts on the tan wire that runs to the top of the sender unit with the engine off and ignition in on position (measured between tan wire and sender unit plate with tan wire removed)
4. I'm seeing 30 ohms resistance between the electrode on the sender unit plate and the sender unit ground. (Measured with all wires disconnected). There are about 5 gallons in the 24 gallon capacity tank so 30 ohms seems reasonable??.
5. Fuel gauge shows completely empty when everything is connected and shows full if I pull the tan wire off the sender unit. With 5 gallons in the tank I'd expect the gauge to show between 1/5 and 1/4 full.

Any thoughts on additional trouble shooting steps greatly appreciated as always.

 

The sender unit wire is broken or internally separated somewhere along the line from the tank to the gauge. It is most likely at a connector on the sending unit (at the tank).
Without doing anything to the present wiring, use your ohmmeter to measure resistance from the sending unit connection fitting (where the wire is attached) to the [internal] wiring somewhere under the tank {at least 1 foot away from the sending unit connection}. Use a needle or sharp pin to puncture the insulation and make electrical contact with the internal wire. I'm betting that you will read INF. ohms and you need to repair the connection at the sending unit.

 

Go eyeball the Willcox troubleshooting page. Maybe the following will help. See here for my problem where there was really nothing wrong with mine, but I just had to do the proper sequence to get it working:
https://www.corvetteforum.com/forums...post1603259749

 

Thanks for the quick reply 7T1vette. I checked this out as you suggested and I can't see a connection problem. See the image below: I'm measuring from the terminal inside the cap on the tan wire that connects to the sender unit electrode to the frame ground and I have a steady 9 volts with the ignition in the on position. I feel if there was a connection problem associated with this wire I'd not have any voltage. I also see the
gauge go from empty to full when I connect and disconnect this tan wire from the sender electrode. Am starting to suspect that the new sender unit is bad.

 

I believe the resistance of the sending unit is between 0 and 90 ohms depending on how much is in the tank. Hook your meter to the sending unit again and use a bent coat hanger or other piece of wire to move the float arm up and down and observe how the resistance changes. You may have dead spots in the sweep of the variable resistor.

You can check the gauge in the other way. With the leads open I think it shows full, with the leads shorted it should show empty, and if you have any small resistors you can stick those between and the needle should show between. (Ex 45 ohms should be about half a tank)

 

Thanks, the checklist is helpful. Confirmed that the gauges are working as expected. Sadly I think my new sender unit is bad. Kicking myself for not testing it fully before completely refitting the tank.

 

Agreed, I would have a helper move float arm up and down, (reaching through filler cap opening with a wire hook) and read resistance with ohm meter. resistance should change smoothly and progressively as float arm goes up and down. Could just be that arm needs to be adjusted, (bent) as float may not be picking up arm with 5 gallons in tank.

 

I know the 15 gallon tanks where 0 - 90 ohm. Not sure if the 24 gallon tanks use the same range or not but nonetheless your approach makes sense. I've ordered a 0 - 200 ohm variable resistor (potentiometer) and will test using that. Thanks for the suggestion.

 

Never thought of that. I assumed they were pre-calibrated but makes sense that they might not be. I see you are in Queensland. I'm from Perth but live in Nashville now.

 

I'm from Wisconsin, but live in Queensland now. I've seen more than one new sending unit that has to have the arm rebent to get it "Dialed in" Correctly. Not hard to check through filler hole with a helper and a standard ohm meter.

 

Sender wire and connections do not appear to be the problem, based on your test results.

 

Thought I'd provide a conclusion to this thread. I hooked a 0 - 200 ohm potentiometer to the terminal post lead and ground of the fuel tank. The fuel gauge responded as expected between 0 and 90 ohms. 0 registered empty, 90 registered full. I then added another five gallons of gas to the tank bringing the total up to 10 gallons and reconnected wires. The fuel gauge registered about 1/8th of a tank. Moral of this story? Even with a brand new tank and sender unit the calibration of the sender unit float arm is miles off and needs to be bent down about 20 degrees. Kicking myself severely for not testing this before reassembling everything! Anyone who says you can pull the sender unit without dropping the tank and removing the three fuel lines is dreaming.

 

You CAN bend the float with it still in the tank!! You need a sturdy wooden dowel (1/2" to 1" diameter, about 3 feet long) and a sturdy piece of stiff wire (like coat-hanger wire) with a loop for your hand on one end and a "U" hook at the other. Depending on which way the float arm needs to be bent, use 'hanger'-wire to hook under the float arm and hold it where you want (within its range of movement) and use the dowel to apply pressure to cause the arm to bend as you need. If you want to raise the float, put the wire just behind the float and put pressure on the arm behind the wire. If you want to lower the float, hook wire under arm nearer to sender mechanism and put pressure on arm just behind the float.
Think it thru in advance of your calibration attempt, then work carefully so you get the change you need without damage to the sending mechanism or the float.

You can also 'test' the calibration without altering fuel level by using your dowel to hold the float down to the bottom of the tank AND by using the wire to lift the float to its approximate position for a full tank.

 

Hi 7T1Vette, I definitely appreciate the ideas and understand the principal of using one pole to hold down and another to bend but I think I'm missing something. I've tried just hooking the float arm with a bent wire to exercise the range motion and couldn't do it by feel alone - I couldn't snag the float arm blind. I bought a super slim LED flashlight and positioned it in the filler neck. This gave me enough light to see the arm and I suspect I could now try a bent wire but to do everything you suggest I'd need room for a rod, wire, flashlight and flat blade to hold the flap open so you can see in, which the 1-1/4" neck filler hole just doesn't allow for. What am I missing?

 

On my 68 I can remove the filler "neck" and reach in there and adjust the float as necessary.... it's a dang big hole! Unless you have really big arms .....

 

On mine the float, sender, filler neck and pick up tube is all one assembly (called the sender unit). You can remove the whole thing but it requires dropping the tank which is not that hard but a pain to do. I was lamenting that I have just installed a brand new tank and sender unit not realizing it would be uncalibrated. So, trying to find ways to calibrate without redoing the whole job.

 

Duct tape an LED mini-flashlight on the inside edge of the filler neck. Get the wire under the float arm (wherever you need it) and work the dowel down to the arm by 'feeling' the way by keeping contact with the wire. It can be done.
Removing the fill neck is an alternative way to get the job done. It provides more room. But it's a PITA to remove that rubber boot and all those screws. (You also need a new gasket when you're done....)
P.S. I highly recommend using Gorilla tape; that $#!t will stick to anything.