67-72 Chevy Truck fuel gauge diagnostics 101

[jjzepplin]

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!

[VetteVet]

Originally Posted by jjzepplin

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!

Have you pulled the tan wire from the sender off the fuel gauge in the fuse panel. It is located on the end of the panel in the center. Pull it off and turn on the key. The gauge needle should go past full to about the three o'clock position.

Next you want to run a ground wire to this terminal from any good ground. Go to the battery negative if you want to be sure. With the key on, the gauge should read dead empty.

If you have a multimeter like the one shown in this thread above, you can check the sending unit. Just connect the red lead to the wire and the black lead to the tank. You won't need the key on because you're reading resistance. The reading should be somewhere between 0 and 90 ohms. zero being empty and 90 being full. If you can do this as you're adding fuel you should see the meter resistance climb from where it's at to 90 ohms.

A failure to get the readings means either the sending unit is defective or the sending unit to tank ground is poor or the tank to frame ground is poor.

Read through Toms excellent posts above and you'll get the visual of all this, and you'll understand how the indicator system works.

[TBONE1964]

Originally Posted by jjzepplin

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!

JJ,

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,
Tom