1967 mustang gas guage problem.Guage,wiring,or sending unit?
I have a 1967 Mustang convertible.The gas guage is reading E all of the time.I would like to fix it but I don't know where to start.I have around ten gallons of gas in the tank.When I turn the ignition on the guage will move from below the E line to the E line. That is it. I went under the tank and banged on the sending unit(hoping the float was stuck from years of non use).Now here is where I have the problem.I am no electrician so bear with me.I pulled the wire off the sending unit on the tank.I took the alligator clip on my tester and connected it to the sending unit. I touched the other end to the inside of wire connector making sure I hit metal.The light on the tester didn't go on.So I took the alligator clip and grounded it to a bolt on my frame and my test light light up and it was flashing on and off.I read in some other article that that it was normal for the tester to flash.While I was touching the wire to the bolt my son was watching the gas guage and the needle didn't move.So I would think the wiring and the guage is good?Does that mean the sending unit on the tank is bad? If anyone has a clue let me know. Thanks,Ken
Hello. First, you want to disconnect that wire from the sending unit and ground it out somewhere with the key in the on position. Your gauge should peg to full like that. You'll need a wire that you can plug into the sending unit wire, and is long enough for you to be able to see the gas gauge while you're holding the other end of the wire on a suitable ground. If the gauge does indeed peg like that, which I'm pretty confident that it will, since the gauhe flinhes a little when you turn the key on, then the problem is inside the gas tank. What has probably happened is that the float has developed a little hole in it and can longer float. You will have to remove the sending unit from the gas tank, check and see if the float has done that, and replace it. It's far more likely that the problem is just that you need a new float ( about 6 bucks) instead of a new sending unit. ( about 160 bucks for a new Ford sending unit) Here's the diagram of that system. This is actually for a 65/66 car, but the 67s are the same. Hope that helps. The Care and Feeding of Ponies: Gauges don't work?
Each of the gauge/sender sets can behave just a little bit different but I agree with Veronica that the fact that your needle moves just a little bit says the gauge in the dash is probably OK.
The float in the tank might have a hole in it, or not. I have one that sat for 25 years and the float was fine but the gauge didn't work. My best guess it that sticky stuff from old gas had gummed up the sender so that the float couldn't move it. Apparently when it finally did pull loose the fingers that touch the wiring in the sender got bent so the gauge was grossly in error. I finally got it all fixed but it took a while. Before that I had nearly decided to buy one of the $160 Ford units instead of the cheaper ones. The guy selling them said the expensive Ford one don't always work either which made me work harder to fix mine.
These SU's lose their ground over time from rust and corrosion.
The Pulsing signal of 5 volts combined with a 'ground out' provide the gauging.
I recommend removing the S/U from the tank and performing tests beside the car with jumper wires.
You can move the arm manually watching the gauge rise or fall.
When testing ....Just make sure the Sending unit jumper wire runs right to the battery ground cable.
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I Pulled the connector off the sending unit and attached some long wire to the connector so I could see my gas guage move. I made the mistake of putting my light tester on it. I did that so I could make sure the wire didn't come loose and watch the light flash when I grounded it to my door handle. It did move a little but only a little past the E line. So I was going to come in here and write you guys another letter saying that it didn't work either. I took the light tester off the wire and put the wire directly on the door handle and sure enough the guage pegged to full. It worked! I didn't think the light tester would make a difference but I guess it does. Thank you for helping me. I will probably see if I can clean any rust off the sending unit and if that doesn't work I will replace it unless anyone has any other ideas. Thanks Veronica,mil1ion,and ivy66gt for all the help. Thanks again, Ken
Before you decide to buy a new one fiddling with the old one like I described in the other thread won't cost you much of anything and might get it working again. Or then it might not. At the worst you might ruin it but then if you were going to buy a new one, who cares.
All of the Mustang's gauges are the same except for the AMP gauge. They're just labeled differently. They are calibrated to read empty or zero at 70 Ohms of resistance. The full or maximum reading will be at 10 Ohms. All of the senders are calibrated for this 70-10 Ohm range. The fuel sender at 1/2 tank should read 40 Ohms.
All Ford gauges, except the AMP gauge are 70-10 Ohm calibrated up to the model year 1987 for Mustang and between 1987 and 1989 for other Ford products, where the calibration went to 16 Ohms empty or zero and 158 Ohms full or max.
You can check one gauge against another (Except AMP) by changing the sender wire. The amount of needle movement should be the same if both gauges are working correctly.
Everything The Greek has said is correct except for the resistance value quoted for 1/2 scale deflection. 40 Ohms would be correct if the gauges were linear WRT resistance. Mathematically they are not linear but logarithmic. That's mostly because the gauges are thermal and thermal energy is related to the 1/R, not R. What does that mean?
~Ohms >> Reading
150 >> No movement - same as if the wire were broken or disconnected
73 >> Off scale 1/2 needle width below E
68 >> E
50 >> 1/8
26 >> 1/2
15 >> 7/8
12 >> F
10 >> Off scale a needle's width above F
Since the gauges are all a little different these numbers are not exact, but close. Shorting the sender wire to ground is a way to test the gauge but since its Zero Ohms to ground it will be overheating the gauge if you keep the short there for very long.
Even with an empty tank the needle will move just a bit when the key is turned on so that it is at the 73 Ohm position. That tells you the wire is not broken. With the 16 gallon tank and an original Ford float properly adjusted the first 1.5 gallons does not change the gauge reading; Ford's way of getting you to fill up early so you won't run out of gas. Likewise the last gallon does not change the slightly more than F reading; makes you feel good that the needle didn't drop after the first 10 miles of driving.
I had the same problem with my 67 Coupe a few years ago. After replacing the sending unit it was all OK. Easy fix, but you have to drain the gas tank. Hope this helps.
Brad
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I didn't want to get too deeply into those technical details since its actually a little more complicated yet. Not having dissected a gauge head I don't know the exact numbers for it so there is a little uncertaintly. Assume the gauge head has a resistance, G, while the gauge sender is R Ohms. The current, I, is the same in both and would be V/(R+G). The power dissipated in G would then be G*I^2 or (G*V^2)/(R+G)^2
Assuming all the others things are constant, power in the gauge head is sort of inversely related to R squared which doubles the slope of the logarithmic plot. The gauge resistance may change as it heats up which may alter that a bit but its still a very non-linear function. For a temperature or oil pressure gauge, the plot of R vs Temp or PSI is nearly a straight line when the R scale is logarithmic. To avoid such problems with assumptions the numbers I gave above are actual gauge measurements taken while adding gas a gallon at a time to my tank.
1967 mustang gas guage problem.Guage,wiring,or sending unit? 
First, you want to disconnect that wire from the sending unit and ground it out somewhere with the key in the on position. Your gauge should peg to full like that. You'll need a wire that you can plug into the sending unit wire, and is long enough for you to be able to see the gas gauge while you're holding the other end of the wire on a suitable ground. If the gauge does indeed peg like that, which I'm pretty confident that it will, since the gauhe flinhes a little when you turn the key on, then the problem is inside the gas tank. What has probably happened is that the float has developed a little hole in it and can longer float. You will have to remove the sending unit from the gas tank, check and see if the float has done that, and replace it. It's far more likely that the problem is just that you need a new float ( about 6 bucks) instead of a new sending unit. ( about 160 bucks for a new Ford sending unit) Here's the diagram of that system. This is actually for a 65/66 car, but the 67s are the same. Hope that helps. 
