Fuel gauge or sending unit problem

[other_shoe]

I'm having problems with getting a proper (or any) reading on my fuel gauge.

Last summer I paid my semi-local Mustang shop to fix the sending unit, which at the time showed empty with about five gallons remaining and full at anything over ten gallons. It worked wonderfully for about a month and then started showing empty no matter the fuel level. Because the shop is only semi-local, I couldn't make time to get the car out there to get them to fix it, but I finally dug into it myself yesterday.

First, I grounded the wire coming from the sending unit in order to check the gauge, and the gauge pegged full.

Second, I drained the tank and took out the sending unit. At this point I discovered the reason for the abrupt failure: the float was no longer attached to the sending unit.

Third, after attempting to coax the float out with a coat hanger, I pulled the tank in order to use gravity to get the float out. Upon doing so I discovered that my shop had left the filter from the old sending unit rattling/floating around the tank.

At this point I made a clear mistake. Instead of testing the variable resistance on the sending unit, I assumed that the problem was only the detached float and proceeded to reattach it (hopefully more securely) and reassemble the tank.

Unfortunately, when I put four gallons back in the tank, it still showed empty. So, after checking to make sure the tank was grounded (continuity and no ohms between tank and the body), I drained the tank again and pulled the sending unit, again.

The multimeter showed 80 ohms with the lever all the way down and 6 at its apogee. I'm guessing this is about right, but I don't know.

So I again installed the unit and tested to make sure it was grounded, and then I put about four gallons of fuel in the tank. And nothing on the gauge.

With those four gallons, the multimeter is showing about 65 ohms off of the sending unit, which I would think would be enough to register on the gauge.

What am I missing, Stangnet?

 

[j69302]

What year is your car?

There is a Instrument Voltage Regulator that feeds power to all of the gauges other than the ammeter/volts. It should be on the back of you instrument cluster. It cuts the 12 volts to about 6 volts. This regulator can give erroneous readings on the gauges. And in my experience, it does not necessarily affect all gauges at the same time(Even though you would think so). Most common for mine was the oil pressure gauge would cut in and out.

I am wondering if it is outputting enough to peg the gauge with it shorted, but not enough with the sending unit mid range somewhere.

Try shorting the gauge again, and if it pegs full, check and see what the voltage is at the gauge. Should be about 6 volts. If it is not, I would replace the instrument voltage regulator. You can get a reproduction one, or a aftermarket solid state one which should last forever.

Make sure to measure voltage on the correct side of the gauge.

 

[other_shoe]

It is a 67. And thank you. I'll try this.

 

[other_shoe]

After checking everything again (and a good thing too because there was a fuel leak at the sending unit), I determined that the IVR is probably not supplying enough voltage or at least being erratic. I also suspect that I need a better ground for the tank. But I went ahead and put another eight gallons in the tank and now it reads just shy of half full (with approximately 3/4 of a tank). I can live with that until I get a new IVR since the key thing is not running out.