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‘78 CJ5 fuel gauge calibration

7491 Views 5 Replies 4 Participants Last post by  John Strenk
I replaced my fuel sending unit a number of years ago. It was never calibrated correctly, when it’s empty it showed about 1/4 tank. Just recently it is empty at about 5/8 tank on the gauge. How do I fix this? New gauge? If so, how do I make sure I get the correct one and don’t end up with the same issue as the last one. The sending unit doesn’t appear to be bad as it is sending current to the gauge.
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· Super Moderator
24,616 Posts
What gauges did you buy? Some are not worth the effort to calibrate.

Anyway, you will have to make sure the the sending unit float has enough movement between a full tank and an empty tank otherwise calibrating the gauge would be pointless.

Take the sending unit out and using an ohmmeter make sure when you move the float up and down, you can go from 78 ohms when the gauge is sitting on the bottom and can read 10 ohms when the float it at the top.

Measure the depth of your tank and then make sure the float travels the same distance. Make sure when the float is all the way up, as if the tank is full, it doesn't drop below 10 ohms. It can go to 0 ohms which could burn out your gauges.

This should be good enough for for your gauges to operate properly but over time your gauges calibration can vary a little.

To take things a little further. One final adjustment....

MAKE sure you have a good ground in the remainder of this adjustment procedure.
If the ground falls off, things will get hot fast.

This is how I hook up the gauges for testing:
Font Circle Parallel Rectangle Auto part

First lets have a look at the back of a typical OEM style Fuel gauge.
Wood Circle Auto part Metal Font

You probably never noticed the extra slots in the back but those are used for doing a calibration. They are actually quite clever in coming up with such a simple adjustment.

If you look inside an OEM gauge you will see two windings around two sperate bi-metalic strips. One is the often miss-labeled "Voltage Regulator" side and the other is for the needle or indicator movement.
Automotive tire Rim Automotive wheel system Gas Spoke

As the windings heat up around the bi-metalis strip, one end of the strip will move.

This free end is attached to the needle. The way the calibration works is there is a slot in the end of the needle were the pin on the end of the bi-metalicstrip fits.

Depending upon the position of this pin in the slot determines how much the needle moves in relationship to the movement of BM strip.
The closer the pin is to the pivot of the needle the greater the travel of the needle.
This is the Full Scale Adjustment. So if your gauge is not reading "Full" when the resistance is 10 ohms then you need to adjust the position of the pin.

Product Slope Font Parallel Circle

THe other adjustment just sets the starting position of the needle or "Zero Adjustment" So if your gauge isn't reading empty when it's at 78 Ohms then you need to change this adjustment.

Note these adjustment are interrelated meaning that if you adjust the "Zero Point", then the needle might not reach "Full" so you will have to go back and re-adjust the "Full Scale" adjustment.

Start with the Zero Point first.

I attached the proper resistance (73 ohms) to the "S" terminal and ground and attached the power and ground leads.

Audio equipment Gas Electrical wiring Bicycle part Machine

I used 10-32 nuts and wire lugs to make the connection easier.

Then I adjusted the Zero point to read "E" as best I could using a flat bladed screwdriver. Then you replace the 73 ohm resistor with a 10 ohm resistor and adjust the FUll Scale adjustment.

Note it might take a minuet for the needle to stop moving so you might want to read a chapter in your FSM while your waiting. :). It also makes several jumps when going to "F". This is normal.

Now the Crown replacement gauges I have work a bit differently. Mine was quite bent inside and needed an adjustment. Maybe yours was knocked out of cal also.
Motor vehicle Auto part Font Circle Gas

The Zero and Full Scale adjustment are in one place. Put the screw driver in one slot and you can adjust the Zero Position.
Turn the screwdriver 90* and you can adjust the full scale movement.
The pin and needle mechanism is the same as the OEM gauges. It's just they are made out of plastic instead of metal.

You may have noticed the screw on the side of the newer gauges. It is for adjusting the regulator. It controls the 'bounce' or 'steps' of the needle movement.
One way the needle moves in big jumps instead of moving slowly. Might not even reach 'full'. To far the other way and the regulator does not have time stop heating and the gauge moves too far or burns up.
This controls the Pulse Width Modulation 9PWM) of the regulator. Think of it as the 'voltage' adjustment even though it doesn't really change the voltage, just how long of a power pulse is applied to the heating coils on the regulator.

It has a funky screw so you don't accidentally mess up this adjustment.
Household hardware Gas Auto part Nickel Circle

the OEM gauges have a slot on the back and you have to bend the position of one of the points to adjust the regulator.
Font Gas Circle Metal Auto part

Font Parallel Rectangle Slope Diagram

They have a nifty tool to make this adjustment that I only saw once on a picture on the internet and never again. It looks like a bent rod with a slot on the end so you can slip it in and bend the adjustment.

Irregardless of how the adjustments work internally, the procedure is still the same as above.
If you can't get the gauge to work properly after all this then I would recommend you send them back and get another set.
There have been some "Quality issues" with the vendor.

There are a couple of other types of after market gauges but I haven't needed to play with them yet. One is terrible and one didn't need any adjustment at all.

For more information on Gauges and stuff, Go here:
Quick Gauge test

· Registered
3,432 Posts
I would grab a "New Folder" (From my pics on your computer)

Rename it Jeep Forum pics (or whatever)

Go to each desired pic and "Right Click" it

This will put it in a file (find the JF pics) file

When satisfied it is going in the correct folder,

Click on "save" to actually put it into folder.

Then go fetch the next one!

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