73 Mustang Fuel Gauge

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Salhi_amer

Active member
Joined
Oct 20, 2014
Messages
37
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Location
Katy TX
My Car
1973 Mustang Mach 1
ok I thought buying a new sending unit fixed my fuel gauge problem but apparently it did not.

So I installed a new sending unit and still the gauge is not reading properly, on empty it read 1/2 a tank and on full it reads way past the full indicator, here is what happened, the car fuel gauge worked fine prior to engine rebuild. after rebuilding it stopped working along with the Tach, so after I brought it to a different mechanic, the 2 wires in the trunk for the fuel sending unit were cut and not even connected. now after they are connected is when this thing started happening, I have new voltage regulator, circuit board and everything works except the Tach & Fuel Gauge not reading correctly ( could these 2 be related )

Could it be a ground issue, its a brand new sending unit just installed ( my mechanic did test new unit is manually before installing it by moving it and it did read correctly on the gauge from E to F ) a

So I am thinking since previous sending unit did the same thing it may be a ground issue related to those 2 wires that were cut, could you guide me on how to test the ground and trace them in trunk or anyone would suggest anything.

 
The resistance at full range is typically 13 ohms; at empty it is typically 73 ohms. If your gauge is reading too high, there's not enough resistance in the circuitry (an unlikely scenario). Since it read correctly before installation, then that's not the problem. If your ground is bad, the gauge will read lower than normal. What's left is voltage related. If the voltage applied at the gauge itself is too high, you would be applying more voltage at the sending unit as well, so that would cause the gauge to read high.

Now then, the constant voltage regulator on the back of the dash cluster has as its input a resistor wire (different than the tach wire). If that is out of the circuit, the CVR will read higher than it should (the CVR actually averages input and no voltage over time; if higher voltage is applied to the CVR, the output likely will be higher). So...I'd start looking at the wiring at the dash cluster, the CVR, and what changes the doofus mechanic may have made by re-routing the CVR resistor wire through the tach.

 
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