Reproduction Parts for 1916-1964 Chevrolet Passenger Cars & 1918-1987 Chevrolet & GMC Trucks


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Howdy,

I just pulled the sending unit out from the below the seat cushion gas tank and I was curious how to test it. Do I simply place an ohm meter to it and see the varients between float up or down? I get a reading around 6 with float down(empty) and 38 with float up(full). Trying to avoid replacing the original unit and looking to simply replace the two floats on the unit with new ones. As a side note, the inside of the gas tank looks good enough to simple be left alone. I thought I would have to get'er cleaned up, but it looks good....barring any holes of course, but unlikely.

Regards

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The readings on the sending unit sound about right. On our '36 since the tank was mounted in tar paper lined straps I added a ground wire to a frame screw.

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That's exactly how you test a sending unit. It should read ~0 empty and ~30 full. Reading 6 in the empty position could leave you with the impression you have more fuel than you actually have. As long as you realize a reading of about 1/8 tank means you're running on fumes you'll be OK.


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If you have a little corrosion on the terminals your meter will not get to zero and 30 but a little higher. Your values are not abnormal for a properly working sending unit tested with an ohmmeter.
.


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I agree with Chipper that your unit is operating as expected. The nominal reading is 0 to 30 ohms.

That fact that the range difference on yours is 32 ohms means it will make the gauge read as expected with normal variations and accuracy.

Here are 3 tricks you could try to maybe improve the readings.
- Clean/polish the terminal where the wire attaches to the sending unit.
- Spray an electrical contact cleaner into the area of the sending unit where the moving contact moves over the variable resistor. Then operate the arm through the full cycle about a dozen times. This should remove oxidation on those parts.
- Check your ohmmeter by connecting the ends of the test leads together. You might be surprised at the reading. If I do not use my meter for a while I will get a 1 to 2 ohm reading when I connect the test leads. I can get that down to 0.1 to 0.2 ohms by polishing the test lead ends and rotating the ends where they plug into the meter.


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Thanks for the input and recommendations. I think you are all right in that I need to clean/polish the terminal which would most likely do the trick.

On a side note, it caught me off guard how you fill these old gas tanks. I looked and looked for the inlet hose from outside of the vehicle before I realized that the tank has a cap on top sitting under the seat. Seems unsafe and burdensome, but I am sticking with the original tank.

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While fueling, you should remain in the drivers seat at all times. This means your passenger (wife, kid) will be responsible for filling and paying. Let us know how this works out.

Mike


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Funny Mike. I can already tell you how that would work out.

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Yep!! I can imagine there'd be quite an explosion take place!!!

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Do I run the only wire from the sending unit to the fuel gauge and a wire from the gauge to a ground to get a reading on my gauge? I thought I needed a hot/ignition wire to go to the fuel gauge as well, no?

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There is only a single wire between the fuel guage in the dash and sending unit in the tank. Sometimes if the tank earth doesnt connect properly a wire is added from the tank sender unit to the chassis to make a earth connection.
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Check the wiring diagram for your vehicle. There should be one in the shop manual.

1934 Wiring Diagram


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So one to the sending unit and the other to an aux/power on the ignition switch? Makes sense, need power somewhere along the way. Is there no ground needed at all?

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Check Tony’s post and the wiring diagram.

The sending unit does need to be grounded. Normally that ground circuit is completed through the way the sending unit is mounted to the tank and then the tank mounted to the chassis.

Some of us have added a separate ground wire that connects from one of the mounting screws for the seeing unit directly to the body. Sometimes dirt, paint, and light surface rust can interfere with the normal mounting based ground path.

You will also note in the wiring diagram that the body of the fuel gauge needs to be grounded. That normally is not a problem but every now and then it will catch you.


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Thanks Rusty.

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The tank would be better with a ground wire. The gauge also requires a ground as Rusty 37 Master says, but be careful about exactly what grounds if you have things apart wwhile testing.

If the gauge is removed, and you are paying attention, you can see exactly what part of it normally grounded to the cluster or housing. Also if the cluster or housing is out of the dash it wont work because the housing or cluster grounds by mounting to the dash.

Power >> power wire >> gauge terminal

Gauge ground tab >> cluster >> ground (dash metal)

Other gauge terminal >> sending unit wire >> sending unit terminal

Sending unit flange >> gas tank >> ground (body or frame)

So yeah, only 2 wires were used as built, but it still also has 2 grounds. Also it matters which gauge terminal is which, If they are backwards it wont work.

You should add a ground wire for the sending unit (tank).

Last edited by bloo; 02/24/22 09:59 AM.
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Great info from Bloo as always. He explained the gauge ground better than I did.

I learned that one the hard way! We were trying to test a '49 3100 fuel gauge out of the truck. We finally figured out that we needed to attach a ground wire to gauge housing after I read the shop manual description CAREFULLY!

The ground wire for the sending unit is cheap insurance that you will not have problems.


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You have all of the info, what You need, but here is an article form Filling Station:
https://www.fillingstation.com/fuelgaugetesting/
Mihaly

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I am by no means an electrical guru, but with my other older trucks, I have had a booger of a time with fuel gauges and sending units on just about all of them. While working on my 47 Dodge, many told me to "check the ground"...it was by far the most important part of a functional unit I guess. I've worked on the Dodge unit for 3 years and still can't get it to work all the time.

Sounds as though I need to run a wire from one of the 5 or 6 screws on the sending unit to the truck frame to get a good ground, no?

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That is exactly the solution! You can ground to any nearby sheet metal. Use a star washer to make sure the connection digs into the paint.

A gauge reading that is intermittent (needle bounces from 0 to a reading) often indicates a problem with the moving contact running across the wound wire rheostat in the head of the sending unit. Sometimes that can be improved by spraying electrical contact cleaner into the head of the sending unit and working the float arm through its travel.

Last edited by Rusty 37 Master; 02/25/22 12:08 PM. Reason: More details

Rusty

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Thanks Rusty. I'll have to to pull the unit back out and hit it with some cleaner.


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