Fuel sending unit ohms help - JeepForum.com
 
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post #1 of 5 Old 04-11-2019, 09:34 PM Thread Starter
Jeepdvm
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Fuel sending unit ohms help

I recently installed new tank, sending unit and fuel gauge. I have almost a full tank and the gauge still reads empty. I shorted the gauge and it goes all the way to full, so I think the gauge is working.

I measured the resistance from the sender unit at the gauge and got 130 ohms! I then disconnected the sending unit wire at the connection in the rear driver’s side wheel well. I tested the resistance from the side coming from the sending unit and got 70 ohms.

I ran a ground all the way from the dash back to the sending unit to make sure it wasnt a grounding issue and it doesn’t seem to affect it.

Am I looking at two different problems? Bad wire (why I got 130 ohms at gauge) and bad sending unit?

Any other explanations for the high resistance and ways to fix it?

Thanks

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post #2 of 5 Old 04-11-2019, 09:56 PM
BagusJeep
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From recollection it should read 70 ohms empty, 8-12 when full.

HOWEVER, this is pre-war technology and you have to set it up by bending tabs and arms when you install it if you ever want to see accurate readings. If you look closely it is just a wire winding and an arm that runs over it, the arm being pushed by a plastic float.

If you have a reading the coil is intact, the problem is that the arm is not in the right place, and that is only adjusted by taking it out and bending the tabs and arm.

I take it you did not check the resistance on installation in the tank? That could be one of your problems. You could hook it all up off the vehicle and bend the arm so empty means empty. That means setting it so the resistance at the sender PLUS loss in the connection to the gauge = 70 ohms. Easy way is to hook it up and look at the gauge, you can take the sender out and move it up and down as well.

The other problem is that the tank ground is to a bolt on the frame. You then rely on that connection and the frame to engine ground strap and the engine to tub strap and the screws that hold on the instrument panel and then the gauge to panel clamp etc.

To get a loss as small as 8 ohms is expecting a lot of this myriad of junk rusted connections. Hence, even though you can get empty to be empty, the full will be erratic. Hence a seperate ground from tank to instrument cluster is advisable to eliminate the losses.

My recent replumb of Puthijeep was just as difficult, Empty = half full and Full=Full on a 20 gallon tank. It only takes 10 gallons to refill when showing empty, that is a lot of reserve. At least I know it has at least half a tank. My guess is I need to bend the arm further.

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post #3 of 5 Old 04-11-2019, 10:37 PM Thread Starter
Jeepdvm
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I know my tank is at least 3/4ths full. Could bending the arm make that big of difference from 70 ohms to around 20 ohms(what I think it should be with 3/4 tank.
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post #4 of 5 Old 04-12-2019, 04:57 AM
John Strenk
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Sounds like you might have more than one problem.

Your float may be broken or full of fuel. That might be why it is showing empty when it is nearly full.

Or you have a lot of wire/ground resistance going to the gauge or to ground. If you checked the resistance at the sending unit using the same ground you tested it at the gauge, it MAY be a problem with the ground from the sending unit to the tank. I would check resistance from the sending unit ground at the tank to the same ground point you used before and then any point on the frame..

You probably will have to drop the tank to figure out what is going on with the sending unit.


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post #5 of 5 Old 04-12-2019, 06:50 AM
walkerhoundvm
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Quote:
Originally Posted by John Strenk View Post
Sounds like you might have more than one problem.

Your float may be broken or full of fuel. That might be why it is showing empty when it is nearly full.

Or you have a lot of wire/ground resistance going to the gauge or to ground. If you checked the resistance at the sending unit using the same ground you tested it at the gauge, it MAY be a problem with the ground from the sending unit to the tank. I would check resistance from the sending unit ground at the tank to the same ground point you used before and then any point on the frame..

You probably will have to drop the tank to figure out what is going on with the sending unit.
Or, when you installed the sending unit, because of what a PITA it is to sit, you have the float hitting the side wall, unable to float to the top. Been there before.
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