Topic: Fuel gauge reading low

[DaveF]

Recently, I was having an issue with the fuel gauge On my ?68 XR-7. It wouldn?t read above 3/4 of a tank, no matter how much gas was in it. I figured that the gauge voltage might be a little low, so I checked the voltage, and to my surprise it was set at about 6volts (instead of the normal 5 volts) I reduced the voltage to 5 volts (I have a variable voltage regulator for my gauges) Now the gauge only reads between a half and 3/4 of a tank. I am using an electronic gauge testing device that is designed to give accurate gauge readings ( on vintage Ford vehicles)by bypassing the fuel sender, thereby eliminating it as a source of the problem. Does it sound like I might have a fuel gauge that is not properly calibrated? If so, how do I calibrate it?

[jwc66k]

Some "hints". In a similar low fuel reading, we emptied the tank, removed the sender and found the float had some fuel inside. A new sender installed.
For just the opposite type reading, overfilled, we bent the arm. It was "better".
Jim

[ruppstang]

You also can check the omens resistance in the sender. I cannot remember for sure, but I think it should be 0-7.5 omens over the range of the arm. Often you need to open the case and clean the coils. 

[midlife]

You also can check the omens resistance in the sender. I cannot remember for sure, but I think it should be 0-7.5 omens over the range of the arm. Often you need to open the case and clean the coils.

Correction: full fuel should be about 13 ohms; low fuel about 73 ohms.

[ruppstang]

Correction: full fuel should be about 13 ohms; low fuel about 73 ohms.

Thanks Randy

I thought I may have not got that right.  My point was you can correct some sender inaccuracies by cleaning the coils.

A simple check is to ground the sender frame while holding the sender and moving the arm up and down while someone watches the gage. Them make adjustments. If you have a known good sender, you can do this to check for a sender problem before removing the one from the tank.

[DaveF]

As I may have previously indicated, I am not using a sender. I am using an electronic tool that simulates the resistance of a correctly operating sender. It is designed to be used on vintage Ford oil pressure, fuel and temp gauges. The problem that I am having is improperly calibrated gauges, not the sender. Any tips on gauge calibration? Is it something that I might be able to do, or is it a particularly sticky job?

[jwc66k]

As I may have previously indicated, I am not using a sender. I am using an electronic tool that simulates the resistance of a correctly operating sender. It is designed to be used on vintage Ford oil pressure, fuel and temp gauges. The problem that I am having is improperly calibrated gauges, not the sender. Any tips on gauge calibration? Is it something that I might be able to do, or is it a particularly sticky job?

Not to be sarcastic, but try using the electrical tools of the 1960s.
Jim

[ruppstang]

As I may have previously indicated, I am not using a sender. I am using an electronic tool that simulates the resistance of a correctly operating sender. It is designed to be used on vintage Ford oil pressure, fuel and temp gauges. The problem that I am having is improperly calibrated gauges, not the sender. Any tips on gauge calibration? Is it something that I might be able to do, or is it a particularly sticky job?

As I recall there is a small amount of mechanical adjustment on the gages.

[jwc66k]

Those are the adjustments to get pointer at zero with no signal and at the max reading with the full low voltage. I've only done the low adjustment.
Jim

[midlife]

There are also adjustments on the constant voltage regulator that gives about a quarter of a scale change.  On the back of the CVR is often a potentiometer embedded in plastic or a screw adjustment.  This adjusts the amount of time the flange sits on the BATTERY voltage and/or ground.  What I do is carefully determine when I have a quarter tank of gas, and then adjust the pot on the CVR to give me a quarter tank full reading.  This is the most important reading on a daily basis of any of the gauges.  Otherwise, these gauges are designed to be relative indicators of temperature, oil pressure, etc.  Once you determine what their normal readings are, then anything well away from the readings indicates a problem.