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Gas Gauge Problems (New)

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For those that followed my other post, this is a continuation of my 1940 Buick super. I grounded the gauge wire at the trunk connection and the gauge dropped to empty. Indicating the gauge and wiring was good. So, I started tackling my gas Gauge problem today. I dropped the tank and took out the sending unit. Much to my delight the tank and sending unit was changed some where along my cars life. The tank is super clean inside! Just like new. When I took out the sending unit, it had plastic washers under the 5 screws. Is that normal? Or should I replace them with metal washers?


anyway, I tested the old sending unit while I had it out, sure enough it’s bad. I got my new sending unit today, hooked it up outside the tank and the gauge works perfectly! Thanks again to everyone who helped me through this frustrating problem. I do plan on connecting a ground wire to one of the sending units screws, that’s why I’m confused about the plastic washers. Any thoughts?

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6 minutes ago, Chuck Griffin said:

I thought the same thing but isn’t it insulated through the rubber sealing gasket?


  The screws are screwing into the tank.   The sending unit flange is grounded to the tank.  A wire eyelet will be grounded to the tank by virtue of the screw.


  My 1950 has metal washers.



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Gas can seep up through the threads and you will smell it. I have not seen plastic washers on one, but in my opinion the sender should have a dedicated ground wire no matter what, so it shouldn't matter. Copper washers are a more likely period fix for potential leakage. I suspect (but can't prove) GM's method was to make the holes in the cork gasket really tight around the threads and hope for the best. I do that when I make a gasket, and I also use purple Loctite on the screws (blue is probably ok). The loctite seals up the threads and also allows you to choose how tight to make them, and how hard to squish the gasket, with no worries about screws backing out. Just make sure the terminal on the ground wire is contacting the sender housing directly.

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The separate ground wire is a good idea. Originally the metal fuel line was used as the ground. Now most cars have a rubber hose connecting the first 10 inches of fuel line from the tank to the rest of the line. That removes the ground that used to go through all the frame clamps that hold the fuel line in place.

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I'm a firm believer in good grounds and I ran several dedicated ground wires to lights, to the accessory electric priming fuel pump, and the fuel sending unit.  

From my battery, I ran two ground cables:  one to the frame to create a "ground hub", and the second ground cable up to the engine.  



This is my schematic tracing my battery cables, the master "kill switch" and the frame "ground hub".




In the car, this is what it looks like.  I have two ground cables (2/0) running off the negative pole.




The one goes up front to the engine, the second grounds to the frame and this is where I attach my dedicated grounds if needed.

This picture shows the "hub" with my electric fuel pump and my fuel sender wire attached.




The fuel sender ground wire attaches directly to the flange mounting screw.  

I routed it down the frame rail and I left plenty of slack so I can drop the tank without any problems.




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