I have a '72 240Z with a Rebello engine, TWM EFI/etc, and a '76 fuel tank and pump. But the fuel gauge (not unexpectedly) doesn't show accurate fuel levels. That is the gauge indicates less fuel than actual, and never points higher than about 3/4 with an actual full tank of gas. Short of replacing the gauge with a '76, might there be an electrical component I could install to correct the signal sent from the sender unit to the gauge?

Have you tried grounding the signal wire and see if the gauge goes to full?

Have you cleaned all of the connecters between the fuel gauge and fuel sending unit? Corrosion will show up as extra resistance. Extra resistance means lower readings.

Have you tried grounding the signal wire and see if the gauge goes to full?

Good tip. The car is in storage for the winter but will try that next Spring. Thanks.

Have you cleaned all of the connecters between the fuel gauge and fuel sending unit? Corrosion will show up as extra resistance. Extra resistance means lower readings.

Makes sense. Almost certain there is no corrosion as I remember the connectors being very clean and this is a CA/UT car, but worth checking. Thanks.

Trust me. Corrosion can happen even with a "California" car. The horns on my 240Z wouldn't work until I took a dremel and wire brush to the horns and the wiring. That's a little more extreme because the horns are out front and get spray, but I have found corrosion at many connectors.

I have a '72 240Z with a Rebello engine, TWM EFI/etc, and a '76 fuel tank and pump. But the fuel gauge (not unexpectedly) doesn't show accurate fuel levels. That is the gauge indicates less fuel than actual, and never points higher than about 3/4 with an actual full tank of gas. Short of replacing the gauge with a '76, might there be an electrical component I could install to correct the signal sent from the sender unit to the gauge?

Once you corroborate that all the electrical sources of failure check out, you can also pull the sender from the tank, bend the floater arm downward about an inch (toward the bottom of the fuel tank) and reinsert in the tank. That should allow your fuel gauge needle to move to the right with the same amount of fuel in the tank. It will take some experimentation but you should be able eventually to get the gauge needle to go fully to the right with a full tank of gas.

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There are two adjustment pots on the back of the gauge for syncing the needle with the sender. Ya, that's why no one does it...

Now I feel like, duh! Should’ve known that. 

On 1/13/2022 at 2:38 PM, cgsheen1 said:

There are two adjustment pots on the back of the gauge for syncing the needle with the sender. Ya, that's why no one does it...

Can anyone give more detils on these adjustment pots?  I removed the center controls face and removed the gauge and tried turning the pots with a flat blade screwdriver but they don’t budge and I don’t want to force them. Appears a substance was possibly applied to lock them in place. 

Edited August 5, 20222 yr by JayZ12

Could not adjust the gauge. Either the pot screw was seized or locked and I didn’t want to force it. 
 

So let me ask this. Again, my Z is a 72 model but has a custom setup including fuel injection and an ECU, so I have a 76 fuel tank (and sender unit) and upgraded alternator with solid state voltage regulator from Motorsport.  I believe these factors render both the AMP and FUEL gauges inaccurate — the amp gauge regularly shows around +15A while running and the fuel gauge shows only 3/4 with a full tank of gas.  The question is would a 280Z AMP/FUEL gauge show more accurate measurements, and would it fit in my dash as a replacement?

Edited May 16, 20231 yr by JayZ12

1 hour ago, JayZ12 said:

Could not adjust the gauge. Either the pot screw was seized or locked and I didn’t want to force it. 
 

So let me ask this. Again, my Z is a 72 model but has a custom setup including fuel injection and an ECU, so I have a 76 fuel tank (and sender unit) and upgraded alternator with solid state voltage regulator from Motorsport.  I believe these factors render both the AMP and FUEL gauges inaccurate — the amp gauge regularly shows around +15A while running and the fuel gauge shows only 3/4 with a full tank of gas.  The question is would a 260Z AMP/FUEL gauge show more accurate measurements, and would it fit in my dash as a replacement?

The ammeter is probably not inaccurate. You probably put the new loads on the battery side of the ammeter. That means in order for the alternator to provide power to the EFI, the current has to flow through the ammeter.

Did you check the resistance of both sending units? It may be a difference in resistance between the two that is affecting the fuel gauge.

As for putting a 260Z AMP/FUEL gauge in the car. NO! DON'T! FORGET YOU EVER HAD THE IDEA! The 260Z is wired completely different from the 240Z for how this gauge works. For better or WORSE, all of the current flowing from the alternator to the battery in a 240Z goes through the dash, and the terminals on the ammeter don't have boots. (At least that I have seen.) The 260Z is measuring the current through a shunt. (Good luck finding that shunt, to install in your 240Z, too.) To wire the car to handle the 260Z ammeter is likely more hassle than it's worth. As for the fuel gauge, the sender for a 260Z is the same as a 240Z, and the gauge is the same.

The correct answer to your question is to move the power source for your EFI away from the battery side of the ammeter and try again to adjust the fuel gauge.

 

Oops my severe typo — I have a 76 tank, sender unit and the engine converted to EFI so I meant to ask about a 280Z gauge (not 260). But nevertheless thank you for the very thoughtful analysis most of which probably still applies. Will need to check where loads are relative to gauge. And when you say “try again to adjust the [fuel] gauge” do you mean one of the two pots on the back of the gauge, which pot and how hard should I attempt to crank it for I can’t afford to break the gauge!

And after some browsing, I found this image of the back of the gauge. The two holes circled have slots visible, but when I attempted to turn them there was severe resistance so I backed off. I also found this image which I assume depicts the slots visible in the holes, so there doesn’t seem much play here…