66 gas gauge - Ford Mustang Forum
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post #1 of 11 (permalink) Old 07-11-2019 Thread Starter
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66 gas gauge

When I fill the tank the gauge only goes to 3/4 full , then with use it drops . Is it the gauge or the tank float ?

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post #2 of 11 (permalink) Old 07-11-2019
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Originally Posted by mustangmarty2 View Post
When I fill the tank the gauge only goes to 3/4 full , then with use it drops . Is it the gauge or the tank float ?
My experience exactly. The gauge always read low by 1/4 tank. I added this piece of electronics to make the correction and find it works well.
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1966 Mustang 2+2, A code, AOD AT, PS, PDB, AC, Pony deluxe interior
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post #3 of 11 (permalink) Old 07-11-2019
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I rewired my car with a 65 repro harness, replaced fuel sending unit, gauge and CVR. At the end I'm still at 3/4 with a full tank. I gave up and on fixing this issue and learned to live with it.


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post #4 of 11 (permalink) Old 07-11-2019
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You will need to remove the sender from the tank and test the resistance with an Ohm meter. It should read 10 Ohms at empty and close to 60 Ohms at full.
Here is a video explaining it, although from a 68, the senders are exactly the same in operation.

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15/5/1964 260 Manual Coupe.
23/3/1965 289 Manual Fastback.
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post #5 of 11 (permalink) Old 07-12-2019
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Could be sender, gauge, CVR or a combination of all the above. If you have replaced everything then its more likely at least 2 of those 3 are the problem. That doesn't include the wiring which could also be bad but that isn't usually the problem.

Tony has the right idea except for a first try you don't need to remove the sender from the tank. That's not too difficult, its getting it back in without a leak that is harder. You may need a new, $1 rubber seal every time you take out the sender.

There are two wires on the back of your gauge. One is voltage from the CVR and the other one goes directly to the sender in your tank. ID the one that goes to your sender and measure the resistance through that wire to ground. If its close to 10 Ohms when the tank is full then your sender is just fine and more than likely your CVR is not putting out the correct voltage. Many of the repros sold today are...shall we say...inadequate. There was a thread here just recently about a 'new' CVR with the wrong voltage. 5 VDC won't cut it unless you don't want your gauges to ever have full-scale deflection. 6 VDC is a little too much, but close, and works a lot better than 5. From the factory the needle went way beyond F with the tank actually full.

On the other end of the scale there are varying definitions of 'Empty'. The official low end reading would be around 70-80 Ohms although the gauge needle starts to move, not by much, just barely, at around 100 Ohms. If you are concerned with the 1/2 tank mark that should be near 25 Ohms; the scale is highly non-linear WRT Ohms. You can check out your gauge by substituting resistors to ground instead of connecting up the tank sender: 80, 25 and 10 should do it for E, 1/2 and F. That will tell you how well your gauge COULD work with correct sender readings.

Its also very likely if you have a repro sender that the float arm isn't bent correctly. If Full and Empty readings indicate that is your problem then get familiar with taking out the sender and bending the float arm. With the sender out check that it goes over a range of 10 to 80 Ohms (or something close) when you move the arm to its limits. If not, you have a bad sender. There is a recess in the top of the tank for the float to go when the tank is full and a similar depression in the bottom. The repro sender I had to use is not capable of the same accuracy as an original but its not too bad after a few tries at float bending. It should come off of E with 2-3 gallons in the tank and reach F 1-2 gallons before the tank is full.

I drained the tank completely (bone dry) and then added gas back one gallon at a time while measuring the sender resistance. Be careful or all that gas sitting in cans waiting to be added can blow up your garage. Its a long and boring project but necessary if you want the best gauge accuracy. Its a whole lot better than guessing at how much is still in the tank.
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post #6 of 11 (permalink) Old 07-12-2019 Thread Starter
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Wow I guess it is a common problem . Thank you all
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post #7 of 11 (permalink) Old 07-13-2019
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Very common if the original sender isn't still being used. Unfortunately, I don't know of any source for NOS senders today. When still available there was a good reason why those sold for several times the price of a repro.
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post #8 of 11 (permalink) Old 4 Weeks Ago
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The accuracy of the fuel and other gauges is determined by both the gauge and sender. All can be calibrated. What follows also applies to the oil pressure and temp in general.

For the fuel gauge, first, remove the tank. Roll the tank over while observing the float. The float should almost touch the top of the tank when inverted, and almost touch the bottom of the tank when the tank is in normal position. Making both of these thing occur may require altering the length of the float rod.

The important thing is to have the sender almost touch the bottom of the tank when empty. Having empty accurate is very important, full is not so important.

Then the gauge can be calibrated. There are full and empty adjusters on the back of the gauge. I drilled holes in the gauge shell so I can adjust the gauge with the gauge assembly assembled.

Connect both the gauge and sender to their normal wiring, but in such a way that you have access to them. Swing the sender to full, and adjust the gauge to full. Then empty, then full again, then empty again,,,. After about three iterations you will pick up a pattern of how full and empty effect each other so you can get both ends accurate. Again, empty is the most important.
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post #9 of 11 (permalink) Old 4 Weeks Ago
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You should know when it's full, the filler will shut off.
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post #10 of 11 (permalink) Old 3 Weeks Ago
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Quote:
Originally Posted by GT'sGT View Post
You should know when it's full, the filler will shut off.
True and you can hear the gas bubbling up just before the pump cuts out.

15/5/1964 260 Manual Coupe.
23/3/1965 289 Manual Fastback.
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post #11 of 11 (permalink) Old 3 Weeks Ago
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The two screws on the back of the gauge also affect the linearlity between E and F. Mark the position of each one before you move them. After trying to 'calibrate' using those screws I decided the gauge had worked better across the dial's range before I started tinkering. I put the screws back to their original positions. The gauge is thermal instead of electric so its not easy to calibrate it. A bimetallic strip heated by electricity deforms to move the needle.
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