Fuel Gauge Frustration

I have a 65 CJ5 my wife inherited. We did a nut and bolt restoration. And everything turned out great except the dang fuel gauge. And its driving me crazy. I have attached the meter readings on the gauge sending unit. Its leading me to think I may have a bad gauge and bad sending unit, but I am not sure. New wiring (which tests out), new gauge, new sending unit and most everything else is new as well. But this is my Achilles heel.
Anyone have thoughts or ideas? Both Sending unit and gauge readings look suspect to me.

Tank Sending unit 84.3 ohms - Gas tank Full

Gauge (no wires attached):
S to ground - 62.5 ohms
S to I - 154 ohms
S to A - Open
I to A - Open
I to ground - Open
A to Ground - Open

 

Have you tried grounding the sender to see if that helps? The tank may not have a good ground to the body. It's also worth making sure the body, chassis, and engine all have good grounds back to the battery while you're at it.

 

The sending unit is easy to test. Looks like you have a multimeter and know how to use it? Connect the sending unit to the multimeter leads and measure the resistance. The expected readings for full-half-empty are given in the TSM. As Fireball points out, the sender must be grounded to function. You can likewise measure the resistance of the sender terminal and base to the battery ground cable. Your meter should show little (an ohm or less) or no difference in the resistance readings with the sender on the bench or in the tank.

You can damage the fuel gauge by powering the gauges without a ground. The fuel gauge contains a pulser (CVR for "constant" voltage regulator) that pulses power to the gauge needle movements about once a second. The case of the fuel gauge must be grounded, or the pulser will not pulse. I expect a digital multimeter will not show you anything useful if you try to measure the pulser output. Try a test light.

Go here https://oljeep.com/gw/74_tsm/3-Electrical(3-41toEnd).pdf starting at page 3-45 and read the diagnostic instructions for the CJ gauges. There is an excellent diagram (3-79) of the gauges circuit. Assuming your '65 is a 12 volt system, the cluster works the same in the '74 CJ. The discussion/directions in the TSM should be much better than what you have in the '65-era TSM. If you are using a Haynes or Chiltons book, don't.

 

Yep. When I ground the S terminal the guage reads full and I checked ground continuity on both guage and sending unit ground. I'm leaning towards a bad sending unit but pulling the tank (that's full of gas) isn't going to be fun. Might just create an access door in the bed to get to it.

 

A '65 should have the tank under the driver's seat. The rear-mounted tank did not appear until mid-1970. The rear tank is an improvement, but access to it is indeed harder.

 

The fuel gauge kicked my butt for quite a while. I found help with the following info.....

https://www.morris4x4center.com/knowledge-center/CJ-Gauge-Troubleshooting/

 

In the end it was a bad sending unit (brand new). Ended up cutting an access door under the rear seat so if it happens again tank removal wont be an issue.

 

Fuel Sending Unit | ECJ5 (earlycj5.com)