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1966 Ford Mustang Convertible 289
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Discussion Starter · #1 · (Edited)
hey all, i have been restoring my grandfathers 66 mustang and have used these forums for tips. first post!

problem : fuel gauge goes from E--> 3/4 F, never Full.

  • gauge pins to F when grounding the wire at the tank
  • gauge pins to 3/4 when grounding the wire with a 10 Ohm resistor
  • sending unit is showing ~13 Ohm with a full tank
  • gauge shows ~13 Ohm at the input wire
  • gauge pins to F if the constant voltage regulator is not grounded.

now that i've replaced and reset the sending unit, does anyone have tips on where I should go next? thanks!
 

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Here are the resistance in Ohms of the original 65 sending unit.
Maybe you can bend the rod of the sending unit and get close to these numbers on your dial will read correctly.
DISCONNECT BATTERY, and use ohm meter on the sending unit to measure.

Resistance in ohms----- Dial
78 ohms lower side of E
65 -----------------------------EMPTY
39 -----------------------------1/4
24 -----------------------------1/2
15 -----------------------------3/4
10 -------- --------------------FULL

I think the 65 and 66 used the same values. Maybe someone can confirm
Hope this helps,
Joe
 

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1966 Ford Mustang Convertible 289
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Discussion Starter · #4 ·
Here are the resistance in Ohms of the original 65 sending unit.
Maybe you can bend the rod of the sending unit and get close to these numbers on your dial will read correctly.
DISCONNECT BATTERY, and use ohm meter on the sending unit to measure.

Resistance in ohms----- Dial
78 ohms lower side of E
65 -----------------------------EMPTY
39 -----------------------------1/4
24 -----------------------------1/2
15 -----------------------------3/4
10 -------- --------------------FULL

I think the 65 and 66 used the same values. Maybe someone can confirm
Hope this helps,
Joe
hi joe, i think i ruled out the sending unit. theres something funky going on behind the dash. any ideas?
 

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The fuel, temperature and oil pressure gauges are all 6 volt. The circuits are fed by a constant voltage regulator that converts the alternator voltage to a constant 6 volts. If the CVR is putting out 5 volts, a full tank will read about 3/4.

To test, remove the connector at the gas tank and test the voltage at the harness terminal. If it's less than 6, change the CVR. If it reads about 6, then put a jumper across the terminal and the gauge should read full.
 

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1966 Ford Mustang Convertible 289
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Discussion Starter · #6 · (Edited)
update : bought a digital CVR, installed and still seeing 3/4 a tank. even with a 10Ohm resistor attached to ground. I hooked up a 5Ohm resistor and got to F. any other ideas? too bad I cant just remove resistance from the wire...
 

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What's the voltage of the CVR?
 

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i just checked, it’s 5.44V

oh wow, i didn’t realize it was adjustable. adjusted the voltage to 6.14 and we are reading full with a full tank!
There you go. (y)

The dash gauge is actually a little heater. The voltage heats up a resistor and the indicator moves on a bi-metal strip that deflects as it heats up. (This has the effect of dampening the indicator movement.)

Heat in a resistor is a function of the voltage squared, which explains why such a small voltage difference can have a large effect on movement of thee indicator. :cool:
 
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