"sealing" gas tank sending unit

[cdsmith]

It's me again...

Had to remove the gas tank to have it cleaned out (big uglies in there!). So decided to have a go at fixing the sending unit (it was corroded and frozen up).

After some judicious spraying with various chemicals I have managed to get the float arm to move freely with full range of motion.

Now the problems are; the seal at the pivot point of the float arm (inside the tank) seems to leak, and I the float itself has a pinhole leak. So the question is are these fixable?

Thanks
cd

 

[Dave Russell]

If you feel like it is worth the trouble, It can maybe repaired. The float moves a sliding arm over a resistor wire winding. One end of the winding is the output, the slider goes to ground. If this winding is intact you can possibly make things work. Connect an ohmmeter from the terminal to the case. Move the float arm slowly through its travel range. The meter should go smoothly from near zero at on end of the travel to something in the neighborhood of 70 ohms at the other end of the travel.

To my knowledge, there is no inside "seal" at the pivot point. The sliding contact & the resistance winding are often operating in gasoline. The gasket between sending unit & tank must be leakproof, & the gage top cover gasket, if it has one, must be good.

If the float is metal, submerge it in hot water to see where bubbles come out. The hole, or holes, can be soldered. Warm the float enough to boil off any fuel or water inside first. If the float is plastic it is probably best to replace it.

I don't think replacement sending units are too expensive if the "resurrection" fails.
D

 

[dklawson]

I agree with what Dave said but I'll add that I have friends who have repaired the plastic floats by finding the damaged area and remelting the plastic with a soldering iron (AFTER they've drained all the trapped fuel out). Also, when sealing fuel system gaskets DO NOT be tempted to use silicone (RTV). Use a non-hardening compound like Aviation-Form-A-Gasket, or Permatex-2B.

 

[George Zeck]

I bought a new one from NOS Locators (ebay) for < $25 shipped. You might be interested in that option.

Tx-

Geo Zeck
/ubbthreads/images/graemlins/hammer.gif

 

[cdsmith]

Thanks for the replies...

I didn't realize the sliding contact & the resistance winding is operating in gasoline - why does that scare me?

I've measured the resistance and it does go between 0 and 70, so I guess that part is okay. I have also set the float (it's metal) outside in direct sun to "boil out" any residual fuel (90+ degrees in the shade, so in direct sunlight it’s a lot hotter)

The reasons for "fixing" vs "replacing" was cost (local parts quoted $80 for replacement), but I will do the "ebay" thing - the "< $25" sounds like a deal. The other reason is I just like fixing things that people habitually just replace (fixed the dipper switch instead of replacing) -- I like the simple challenges.
cd

 

[dklawson]

For what it's worth, I had to fix a brass (carb) fuel float for a VW recently. I drilled a small hole in the float and pressurized the thing with about 2-5 psi air so I could find the real leak when I held the float under water. I quickly found the pin hole, drained the fuel out through my drilled hole, and soldered all the holes shut. The hole I drilled served quite well for both troubleshooting and draining the trapped fuel.

 

[Dave Russell]

[ QUOTE ]
Thanks for the replies...
I didn't realize the sliding contact & the resistance winding is operating in gasoline - why does that scare me?
cd

[/ QUOTE ]
The theory is that there is too little current (typically 6 milliamps) flowing in the circuit to create a spark. It must be true. I've never heard of a gas gage blowing up a car.
D

 

[dklawson]

I was told that another reason the sender never causes problems is that the fuel/air mix in the tank is way too rich to ignite. I like that low current explanation better!

 

[sparkydave]

To add to Doug's comment, nearly every car built today also has the electric fuel pump immersed in the gas tank, brushes and all, since it needs gasoline to help lubricate it and cool it. The gas gauge is a low current device, and not likely to spark, but since the mixture is much too rich to ignite it's not a problem.