Damned gas sending unit!!!

[Da Flash]

I recently cleaned out the tank in my '71 and installed the tank, sending unit and O-ring. Now the gauge is pegged at full even though I know I only have ~1 gallon in there!! While removing and checking the sending unit, I discovered that it would read fine as long as the retaining ring wasn't on but, as soon as I the ring comes in contact with the unit and tank, the gauge goes off. The ring looked a little rusty and it wouldn't ohm when I used my multimeter so I cleaned it up with some emery. Still the same result!!

From my very limited electrical background, I've figured out that this has something to do with grounding.

Has anyone else run into this problem, and if so what did you do to remedy it??

Any and all suggestions are appreciated!!

[Duffman]

Try switching your connections on the sending unit and see if that makes any difference.

[240260280z]

The resistance on my 77z's sending unit is from 4 ohms to 80 ohms.

You should be able to read this using an ohm meter first on the wires at the top of the sensor on the tank then on the wires to the fuel gauge in the dash. If this is ok then your sender and wiring is good.

The next culprit is the fuel gauge. It works off standard bi-metal heating and cooling. Just clean the "carboned-up" contacts within it and you should be good to go.

Edited March 17, 2011 by Blue
whatever

[Da Flash]

Well I can't switch the connectors because I did a connector mode a few years ago, I basically eliminated the bayonet ends and soldered on a wire with a male spade end on one wire and a female on the other. Tried as I may, I couldn't find new bayonet fittings!

I've ohmed the sending unit and a new one ( I hate buying new stuff that doesn't fix the problem!!) they both measure the same resistance give or take. On my 71 it's about 11-80 ohms. I'll checkout the connections at the gauge to see if that helps.

Do you guys think that it could be a grounding problem?? The reason that I ask is that all seems fine until I put on the retaining ring.

[240260280z]

Here are schematics for the fuel sender and the gauge.

If you have the wires reversed then you may get the problem as the sender wire (yellow) may ground when you connect the sender to the tank <<<<watch out for sparks when you do this!!!>>>>

Edited March 18, 2011 by Blue

[IdahoKidd]

Yes I think it is a grounding problem. Try a jump wire from the sending unit (metal housing part) to the frame and see if that doesn't make the gauge work. If so, then figure out if there is a problem with the ground wire (bad connection, broken, etc.)

[Da Flash]

Yes I think it is a grounding problem. Try a jump wire from the sending unit (metal housing part) to the frame and see if that doesn't make the gauge work. If so, then figure out if there is a problem with the ground wire (bad connection, broken, etc.)

When I check continuity between the sending unit body and the frame, it's good ( 0.3 ohms). When I check continuity between the tank and the frame, same thing but, when I check the ground wire going to the sending unit and the ground wire going to the gauge, I get a resistance of ~45 ohms!! I'll double check the gauge connectors and let you guys know!!