Fuel Gauge 1971

[JDININO57]

The fuel gauge on my 1971 is acting odd.

With the key off, the gauge points just above empty.
When I turn the key to the on position, the needle moves to the left and points to empty.

When I disconnect the brown wire from the sending unit, it moves rapidly to the right and stops at the 3 o'clock position.

When re connecting the brown wire to the sending unit, the needle moved rapidly to the empty position.

It is a replacement sending unit.
The float is intact.
There are 10 gallons of gas so it should read roughly 1/2 full.
I measured the resistance of the sending unit and set the reading to OER with no response.

The resistance of the sending unit is near zero when in the full position.
There is measurable resistance when in the empty position.

Voltage at the sending unit is 10.4V

Any experience is appreciated

[dmjlambert]

Originally Posted by JDININO57

The float is intact.

It is possible for the float to be intact and not float. It can fill with gas. It is possible you have a bad float. It may also be possible the float arm is stuck on something keeping it in the full down bottom of the tank position. If you have not had the tank completely out and empty and looked around in there, perhaps you have something in there interfering with the motion of the arm.

Originally Posted by JDININO57

I measured the resistance of the sending unit and set the reading to OER with no response.

I don't understand this sentence at all. I think OER is a brand name, but I don't know what you are trying to say.

Originally Posted by JDININO57

The resistance of the sending unit is near zero when in the full position.
There is measurable resistance when in the empty position.

Ohms measured with an ohm meter between the sending unit terminal (with wiring disconnected) and the sending unit metal mounting plate should be near 0 ohms when the float arm is down (empty tank) and near 90 ohms when arm is up (full tank). This makes me wonder how you measured.

I wonder if you can use alligator clip test wires to connect the sending unit to ground and to the sending wire while the sending unit is outside of the tank, and then watch the gauge while moving the float arm, and see if it seems to read correctly. I would probably tend to try something like that. With the sending unit removed that is your chance to look around in the tank with a flashlight. Siphon the gas out of the tank if necessary, and look around in there, to determine if the tank is nice and clean and free of debris. And it is possible you need to make minor adjustment to the bend of the float arm to keep the float from rubbing on or contacting the sides or bottom of the gas tank.

[davischevy]

Probably the float.

[JDININO57]

Hey thanks for your note. OER is a parts manufacturer of resto parts. www.oerparts.com

I took the tank out of the truck added a gallon of rust cleaner and marbles rotated it for several hours over several days, drained it an repeated the process. Then thoroughly treated it with POR Metal prep, followed by POR tank sealant. There are no obstructions.

I measured the resistance of the sending unit with an ohm meter and called the support line at OER, who never responded. I don’t believe the float is damaged or think it is filling with fuel, however that is worth checking out. Thx

While I have the sending unit out of the tank, I will retest the resistance of the sending unit. I’ll look at the potential of binding.

Thanks again for your thoughts

[FirstOwner69]

The brass floats on the new sending units have a horrible reputation. My new one failed within a few weeks. I'd spend less than $10 on the float in the attached thread. You'd probably need it soon regardless.

http://67-72chevytrucks.com/vboard/s...d.php?t=666161

[pjmoreland]

If you have the sender out of the tank, connect the tan wire to it and touch the mounting flange to ground. With the ignition switch in the run position (the engine doesn't need to be on) you can move the sender float arm back and forth and see if the gauge responds appropriately.

[JDININO57]

Originally Posted by FirstOwner69

The brass floats on the new sending units have a horrible reputation. My new one failed within a few weeks. I'd spend less than $10 on the float in the attached thread. You'd probably need it soon regardless.

http://67-72chevytrucks.com/vboard/s...d.php?t=666161

Very good. Ill look in to that.

[JDININO57]

Originally Posted by pjmoreland

If you have the sender out of the tank, connect the tan wire to it and touch the mounting flange to ground. With the ignition switch in the run position (the engine doesn't need to be on) you can move the sender float arm back and forth and see if the gauge responds appropriately.

Yeah. I thought of that as well. thx

[PbFut]

Did you treat the tank with Por15 with the sending unit installed? I would think the process would destroy the potentiometer on the sending unit.

[JDININO57]

Hell no. The tank treatment is like plastic when cured. The sending unit, pickup and float would be encased in the POR forever.
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[54blackhornet]

Zero to ninety is the proper ohms reading for a functional sending unit.

[JDININO57]

So I was able to address the faulty fuel gauge/sending unit and it turned out to be several factors.

The “new” 4 year old OER sending unit has a few problems.
The insulation on the 12V “strip” powering the rheostat was split. Not the root cause but it was failing.

The brass float on the “new” 4 year old replacement OER unit was in fact split.

Since the ‘new” OER sending unit had problems I ordered a “new” replacement from OER including the JEEP plastic float as FirstOwner69 suggested.

I installed all of it and unfortunately, there was no change. I checked the “ground” and “power” wire and everything checked out OK.

I continued trouble shooting, checking continuity to ground and power, re inserted the sending unit and Voilà, it read ¾ of a tank, as it should have.

While the mystery is not definitively solved, I suspect it was a ground/power issue. Anyway it works and I appreciate every ones input.