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Old 08-24-2014, 09:14 AM   #23
Cluster King
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Join Date: Nov 2012
Location: Junction City, OR
Posts: 4,812
Re: 67-72 Chevy Truck fuel gauge diagnostics 101

Originally Posted by jjzepplin View Post
I have a 35 gallon tank and it's a mother to drop. Couldn't I fill it up and check the reading on the sender?
I would of course check the gauge first and do this if it checks good.(30-45 ohm) LOL!


To add to what Vette explained, here is how I would address your issue.

Get yourself an OHM meter and hook it up as I explained. The OHM meter should be set on the 200 scale and hook the positive (red) lead to the tan wire and the black lead to the sending unit ground. This will show you ohm resistance on the sending unit.

I would try and do this test starting from a empty tank or as near empty as you can get. Get your OHM meter hooked up and take a freind to the gas station.

For a 35 gallon tank, lets break it down into 1/8 increments. IF you are empty, you will need 4.375 gallons of fuel to get 1/8 tank. The ohm reading should be 11.25 ohms for an 1/8 tank.

35 gallons divided by 8 = 4.375
90 ohms divided by 8 = 11.25 ohm.

I would pump 4 .37 gallons at a time and watch the ohm meter.

1/4 tank would be 8.75 gallons and 22.5 ohms and so on and so on.

If you pump another 4.375 gallons and your ohm meter does not move, then I would suspect a bad sending unit. If it goes up to 33.75 ohms, then continue and keep watching the ohm meter compared to the amount of fuel added.

This may take some time but you are trying to isolate a problem.

Keep us posted on how it goes.

Take care,
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