1970 Mach 1 Fuel Gauge Issues

[70 Mach]

Hello All,
Last year I purchased a 1970 Mach 1, believe it or not but at my wife's request since I sold my original one to buy her an engagement ring 44 years ago. As my original one from back in the 70's the fuel gauge in the dash didn't work. I ordered a new sender from CJ Pony and installed it, during this operation I noticed that the one I removed looked to be new also, the tank in the car also looked to be a recent replacement. Once completed I check the gauge and it still didn't work. I grounded the stem of the sending unit to the frame without any change. I decided to send it to my local garage to see if they could figure it out what was wrong. I told them that I hadn't tried grounding the sender wire to see if the gauge worked, which they did, and the original worked fine. They drained the tank pulled out the sending unit and tested it outside of the tank and it worked fine, actually they both worked. They reinstalled it back into the tank and it didn't work. they looked into the tank as best as they could and actually raised the float some and got a reading. (they did check to make sure the float wasn't leaking gas into the brass ball).

Has anyone come across this problem before? I would hate to purchase another tank and end up with the same problem.

Thank you!

 

[Noobz347]

Try running [your own] set of wires from the sender to the gauge, [bypassing] the stock wiring. Run the grounds for the sender [and] the gauge, directly to the battery ground post for this test.

Feed power to the sender directly from battery power.


If this works, you have a wiring problem or bad ground at either the sender, gauge, or both. Replace the sending wire from the sender to the gauge and clean or replace the grounds.

If it [doesn't] work then it is likely that resistor in the gauge is bad. At that point, you would need to repair or replace the the gauge.

 

[AeroCoupe]

Have you checked the IVR? There is also a resistor that is inline with the IVR that can go bad.

 

[70 Mach]

Have you checked the IVR? There is also a resistor that is inline with the IVR that can go bad.

Thanks for responding, not sure what the IVR resistor is or where it's located. This is a 1970 with the original 351C. I checked the tank to make sure it was grounded along with adding a separate ground from the sender. We grounded the sensor wire at the tank for a second and the fuel gauge spiked. Next, we removed the sending unit from the tank with the additional ground attached and lifted the float 1/2 way and the fuel gaged registered 1/2 tank, lifted it completely up and the gauge read full. We checked the new sensor and the old one to see if fuel was leaking into the float ball, which it wasn't. Reinstalled the sensor back into the tank and the gauge only reads about 1/8 of a tank with 10 gallons of fuel in it. We couldn't see very well if something was obstructing the float or not. I know that the tank was a replacement tank installed before I purchased the car and now it's do I buy a new tank and risk the same outcome or not? Thanks!

 

[AeroCoupe]

IVR Location:


Cannot recommend this site enough for the 1969 & 1970 cars:

Quick dash ground/voltage regulator question

I recently replaced my dash lamps with LED, new circuit board and new voltage regulator. Gauges worked before. LEDs work - does that mean I have a good ground for everything else? Gauges do not work - Is this most likely new voltage regulator. Thinking of taking it out and putting old one back in...

1969stang.com

 

[Mcmahst]

I’m wondering if you have the sending unit installed correctly. Installed correctly, the stem should be clocked in the 6:30ish position. Just another thing to try when you run out of ideas. Good luck.

 

[GOvert]

If this is a one wire sending unit, use a hose clamp and right next to where the rubber hose is attached to it, clamp a short length of wire to the metal stem and ground that wire to the body. The sending unit may not be getting a good ground where it is attached in the tank.

 

[Mach1 Driver]

The fuel gauge works, so the IVR is fine and all that is left is the wire to the tank and the sender. The sender works outside of the tank. I guess its possible that they put the wrong fuel tank in the car, but unlikely. You can do this yourself- the hardest part is getting all the gas out of the tank. A stock tank has a drain plug.
Here is how to adjust (bend) the float arm so the gauge reads correctly, and is courtesy of Woodchuck on VMF:
This is easiest to do with the tank removed. Attach a small neodymium magnet to the float with masking tape, etc. Place the sender in the tank and stick the float to the top of the tank. Hook up your ohmmeter and look for 0-10 ohms from the wire connection on the sender, and the other to the body of the tank. Adjust the float arm until you get that value. Remove the magnet and check the resistance with the float on the bottom of the tank. Look for 73-78 ohms. The closest you can get to 10 and 73 ohms, the better. At those 2 values the fuel gauge should be almost pegged on "F" or "E".

Below is a schematic (not a wire diagram) of the gauge circuit: