Gas gauge help

[RRR18]

I just replaced my underbed tank with a replacement due to leak. My problem I think is my gauge. So I'm leaving it to the experts here to point me in right direction. New tank included new pickup and sending unit assembly already installed in tank. Just swapped the tank basically. I know that my ground for the tank is good and the wiring is good. With the old tank - even when it was full the gauge never read past 3/4 full and would still be running under E. Now with new tank - I put exactly 5 gallons in to get everything running this past weekend and the gauge moved slightly but not past E. So I did same thing today and went and got exactly 5 more gallons and it moved the needle to 1/8 of a tank. In my mind I put 10 gallons in a 21 gallon underbed conversion tank so am I stupid for thinkin it should be at 1/2 tank on the gauge right now??? Or is my gauge kinda bad? I thought either the gauges worked or they didn't. It's a '67 but I don't think it's a '67 cluster. Could be anywhere between 67-72 cluster. Any suggestions???

[HSRACER]

All gauges need to be paired with the right resistance sending unit. Some are 30 to 90 ohms, 40 to 120 ohms etc. you will need to verify what gauge you have then match it to the right resistance sending unit.
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[RRR18]

Ok I understand that. I just thought that since it was "original" gauge and "original" sending unit it would be the same ohms. I believe it is 90 ohms.

[VetteVet]

Since you're getting a reading from the sending unit, You're wiring and gauge power must be OK. The only things in question are the resistance readings on the sending unit and the float movement. The problem in my opinion is the float adjustment and possibly the movement of the arm.

Here''s what I would do. First off I would pull the sending unit wire off at the sending unit and with key on look for a gauge reading at approximately 3:00. This would indicate that the gauge resistor is Ok. Then I would pull the sender from the tank and reconnect the sending unit wire and ground the flange, then move the float arm through its full travel and watch the gauge for corresponding movement from empty to full. Also check the resistance on the sender from 0 to 90 ohms with an ohmmeter. If you get all these readings then it figures that the float arm is not traveling correctly in the tank.

If it's the original sending unit then the float might have a pinhole in it and have some gas in it which would make it heavy and tend to hang down on empty and read less than full.

I think lf the sending unit would have to be compatible with the under bed tank anyhow because it will travel a lot less in that than in the stock tank which is behind the seat and much taller than the under bed one

[RRR18]

Thanks VetteVet. I'll test everything like you suggest. It's a brand new tank and send unit assembly.
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[RRR18]

Pulling the wire off the sending unit with the key on DID make the gauge go to 3:00 position. Put wire back on and ya back to just above 1/8 tank with 10 gallons in it. I guess my next step is to pull the sending unit and test the float travel manually. I can't imagine that there is anything binding it up.

[RRR18]

"COULD" it be possible that when the previous owner had this gauge cluster apart to do the white face gauge look....he put the needle on in the wrong position? Maybe just clocking it in the wrong spot. A long shot I know but figured I'd ask.
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[Sammy]

You guys are on the right track. Keep in mind it is a known common fact that the aftermarket senders run about 50-50 to work properly. Wanted to add you may just need to tweak the float arm to get your 1/2 tank then empty or fill your tank full to see if that corrected your problem.