72 Chevy C-10 Fuel Sending Unit Problem
 

I am having the most difficult time understanding the way my fuel gauge / sending unit circuit works. I have original the "behind the seat" gas tank set up.
If I may ask a few questions about the circuit...

OK, I just installed my second sending unit and the results havent changed, so I doubt both sending units were problematic.

When full of gas, my sending units both send a reading of 3/8ths.
When empty of gas, my sending units both send a reading of 1/8th.

When I remove the tan wire, the gauge swings to about 2:30 position (past-full). Check!
When I ground out the tan wire, the gauge swings to below empty. That's good. Check!

The gauge demonstrated full range of motion between 100% open circuit and 100% grounded out. Check!

So, I doubt the new fuel sending unit or the fuel gauge is to blame.

The brass floats are soldered air-tight on the sending units. I checked, no leaks.

Where do I go from here?

Thanks, auminer49r.

Re: 72 Chevy C-10 Fuel Sending Unit Problem
 

Have you tried testing the sender outside of the tank? You will need to ground the mounting flange then you can move the arm by hand to see if it moves the needle from E to F.

If you can get a full range on the gauge that way it could be that there is some sort of obstruction preventing full motion in the tank or perhaps the replacement senders aren't the correct part for the tank.

If moving the arm through its full motion doesn't give you full range on the gauge then the sender doesn't match the gauge. Somewhere around here someone should have the impedance range expected from the sender.

Re: 72 Chevy C-10 Fuel Sending Unit Problem
 

I have not bench-tested the sending units. I will test the old one tonight. That's good advice. I'll take it!
I will speculate that fuel tank obstructions are probably unlikely since it would have to have an obstruction from sinking and an obstruction from floating in order to have the gauge reading within the 1/8 to 3/8 range always. I have even ran the truck out of gas with 1/8 showing on the gauge.


I know for sure that the sending units are both 90 Ohm. (it is printed on the unit). So, ... maybe the fuel guage isn't 90 Ohm? Could that be? Does Chevy have factory gauges that have a different Ohm range?

The new sending unit, I just installed, is called-out specifically for a 72 C-10 with Automatic Transmission, just like my truck.

I'm still looking for things to test when I get home tonight, assuming the tank doesn't have obstructions in it blocking the float from floating past 3/8 and/or sinking below 1/8.

Thanks, auminer49r

Re: 72 Chevy C-10 Fuel Sending Unit Problem
 

The gauge and sender should both be 0-90 ohm. You can also test the sender with a multimeter to verify that it is 0 ohm empty and 90 ohm full.

Re: 72 Chevy C-10 Fuel Sending Unit Problem
 

I will test the fuel sending unit tonight, out of the tank, with a multimeter just as you described. Judging by past needle locations on the fuel gage, I might expect a "full" float swing to read about 34 Ohms and an "empty" float swing to read about 11 Ohms (1/8th of 90 Ohms empty, and 3/8ths of 90 Ohms full). That'll be useful info. If that's the case, would I have a mis-matched gauge/sender combination? or, could something else change the 90 Ohm range that is required for the sending unit to function properly?

thanks, auminer49r

Re: 72 Chevy C-10 Fuel Sending Unit Problem
 

If the sending unit checks out, I would check the wiring for bad connections, high resistance and possibly a faulty gauge. There are some resistance specs here for testing the gauge.

Re: 72 Chevy C-10 Fuel Sending Unit Problem
 

OK Guys, I tested my sending units.

When the float is up it reads 96 Ohms, when down it reads about 1 or 2 Ohms. that Checks!

I fueled my truck up yesterday so my float should be riding high in the tank.
The sending unit in my tank is currently reading 69 Ohms. My fuel gauge reads 1/4 of a tank (bogus). At 69 Ohms shouldn't my gauge be reading up around 3/4 of a tank? The gauge is not properly interpreting the resistance that the sending unit creates.

I tested the tan wire, grounded to the gas tank, with the key on, I get a reading of 69 ohms (coincidence)?? Shouldnt I get a low number, close to zero?

thanks,
auminer49r

I think my sending unit is good. Would you agree?

Re: 72 Chevy C-10 Fuel Sending Unit Problem
 

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Re: 72 Chevy C-10 Fuel Sending Unit Problem
 

Re: 72 Chevy C-10 Fuel Sending Unit Problem
 

with the tan wire disconnected at the fuse panel and the key on you should read past full to 3:00 and with the tan wire connected at the fuse panel and the sending unit tan wire grounded you should read dead empty. Or you could run a ground jumper to the fuel ga plug on the fuse panel. This will verify the gauge readings are correct but you may have a short to ground on the sending unit wire between the fuse panel and the sending unit.

Re: 72 Chevy C-10 Fuel Sending Unit Problem
 

I have a '68 C20, and need to remove my sneding unit. I am just checking to see that once I disconnect the fuel line, all I need to do is tap those 4 tangs counter clockwise until that metal ring/holder is free, correct?

thanks much!