Gas Gauge Problems

[Chuck Griffin]

When I turn my key on in my 40 Buick super,my fuel gauge goes from empty to past full. Where do I start diagnosing the problem? Can I access the sending unit wire in the trunk? I just bought the car and the gauge was not working.

[Matt Harwood]

https://www.oldcarsweekly.com/news/fueled-frustration-fixing-faulty-fuel-gauges

[neil morse]

Hi Chuck:

 

I had the exact same problem on my '41 Super.  As you will read in the link supplied by Matt, your problem is most likely either a poor ground or a bad sending unit.  The ground is through the fuel line connection, so often people use a small length of rubber fuel tubing where the line connects to the sending unit and don't realize they are blocking the ground.  You can first test the dash unit by grounding the wire to the sending unit.  Look in your trunk on the driver's side rear corner, and you will see where the wire goes through a hole in the trunk floor.  There should be a "bayonet-style" connector in the line just before it goes through the hole.  With the ignition on, remove this connector and ground the wire from the gauge by touching the body or trunk floor.  If your gauge goes to empty, you know your dash unit is good.

 

Next, you have to remove the tank.  I know this sounds crazy -- and I agree.  I don't know why they couldn't have just put an inspection opening in the trunk floor like on the '48 Chrysler that I used to have, but that's the way they decided to do it.  Here's a link from my thread showing what I did.  I ended up having the tank cleaned out and coated and put in a new sending unit (my old sending unit was not working on top of not being properly grounded).  Good luck, and feel free to send me a Private Message if you have any questions.  (Click on the arrow in the upper right hand corner to get to the right post, and just follow from there.)

 

 

 

 

 

 

 

 

Edited May 18, 2021 by neil morse (see edit history)

[Morgan Wright]

The fuel gauge is just a volt meter. The sender is just a rheostat. A cork float attaches to the variable arm on the rheostat sender in the tank.

 

To test the gauge, get a rheostat from somewhere, either a volume control from a radio, a speed control from a fan, they were $4 at Radio Shack not sure who sells them anymore. One wire from the hot side of the gauge to the rheostat, another wire from the rheostat to ground, and you should be able to make the fuel gauge go from full to half or 3/4 or anywhere, depending on how you turn the rheostat.

 

If not, the sending unit is bad. Needs a new cork, you can get one from a wine bottle. Or maybe it's busted.

 

Two metal straps hold the tank up, easy to remove.

[Bloo]

GM in that time frame used 0 ohms for empty and 30 ohms for full. A volume control from a radio is typically something like 10,000 or 50,000 ohms, so I think you might have a lot of trouble getting that to work.

 

Get hold of the wire Neil Morse mentioned and ground it to a GOOD ground. The gauge should go to empty. If it does, everything in the dash and wire harness is likely OK. Hook the wire back up and try temporarily grounding the tank to a good clean ground on the frame or body. If that doesn't make the gauge start working, you need to drop the tank. From the post where the wire connects (one meter lead) to the sender case (the other meter lead) should measure very close to zero ohms with the tank sitting on the ground right side up and empty.

 

Don't forget to zero your meter, If analog, there should be a knob to adjust for full scale (zero) with the leads shorted. if the meter is digital the zeroing function may be marked "delta". Short the meter leads together and press the "delta" button. If you do not have a delta or zeroing function, short your leads together and note the ohms reading. Subtract this number from whatever you get when you measure the tank.

 

Flip the empty tank upside down, shake it a little, and measure again. It should be at least 30 ohms higher. If not, the sender is screwed up and needs to come out and be repaired or replaced.

 

No matter what, if you drop the tank add a ground wire. Connect one end to a sending unit mounting screw, and the other end to a good clean ground, probably on the frame but the body should be ok too. This prevents the slow degradation of the grounding over time that causes the gauge to read above "E" when actually empty.

 

 

Edited May 15, 2021 by Bloo (see edit history)

[Gary W]

I just went through this very same thing on my '37.  If you follow along from "I ran out of gas" to the next few posts detailing how the problem was diagnosed and fixed I think it'll help you out.  I received great advice from many of the same people (Bloo) giving you input.  They know their stuff.

 

Here's the link starting when I ran out of gas and it continues on from there:

 

 

 

Here's the posts detailing tank removal and diagnosis and repair of the sender in the tank:

 

 

Edited May 16, 2021 by Gary W (see edit history)

[Morgan Wright]

8 ohm radio volume control for dirt cheap

 

https://www.ebay.com/itm/193844478871

 

https://www.amazon.com/Speco-WAT50W-Stereo-Control-Wallplate/dp/B00FCZQLC4/ref=sr_1_11?dchild=1&keywords=8+ohm+volume+control&qid=1621268311&sr=8-11

[Rock10]

1 hour ago, Morgan Wright said:

8 ohm radio volume control for dirt cheap

 

https://www.ebay.com/itm/193844478871

 

https://www.amazon.com/Speco-WAT50W-Stereo-Control-Wallplate/dp/B00FCZQLC4/ref=sr_1_11?dchild=1&keywords=8+ohm+volume+control&qid=1621268311&sr=8-11

The 8 ohm is the speaker rating, not the rheostat.

[Eddie-O]

Now that everyone has covered the basics of either empty or full readings, my gauge has just started to act up. Although I haven’t jumped into it yet, my gauge goes to empty when ignition is off ( normal ). When I turn on the ignition it goes to around half full and stays there. I’m going to assume it’s a bad tank sensor but if anyone has had a similar problem it would make my search easier. Did not check anything yet since it just started but it has become my new weekend project this Sat.

[Bloo]

Normal gauge will go full with 30 ohms between the sender wire and ground, and will peg with the sender wire disconnected. Shorting the sender wire to ground sends the gauge to "E".

 

When you get hold of the wire back at the tank sender, key on, disconnnected should peg the gauge, shorting the wire to ground should give you empty. If you do this back at the tank and it passes the test, then most likely the wiring and the stuff in the dash are all OK, and either the sender in the tank is bad or the tank is not grounded.

 

Add a ground wire while you are there to prevent future problems. Run it from one of the sender mounting screws over to the frame.

 

You should mention make model and year. I assumed 1940 Buick because that is what the thread is about. The test should be valid for GM cars through about 1964 and maybe quite a bit later with a caveat or two.

 

Good luck and let us know how it turns out.

 

 

 

Edited May 18, 2021 by Bloo (see edit history)

[suchan]

On 5/17/2021 at 5:56 PM, Eddie-O said:

Now that everyone has covered the basics of either empty or full readings, my gauge has just started to act up. Although I haven’t jumped into it yet, my gauge goes to empty when ignition is off ( normal ). When I turn on the ignition it goes to around half full and stays there. I’m going to assume it’s a bad tank sensor but if anyone has had a similar problem it would make my search easier. Did not check anything yet since it just started but it has become my new weekend project this Sat.

I have the same problem with my '37 LaSalle. Goes from zero at startup to 3/8. The Cad/LaSalle forum suggested it was a rusted float mechanism, and recommended adding Marvel Mystery Oil to the gas to try to free it up. Hasn't worked yet, but worth a try.

[Eddie-O]

Going to dive into it tomorrow.

[Gary W]

I don't know if you saw my post a couple posts up the chain here, but  I just went through this with my 37 Buick.  Try to read through the posts starting with "running out of gas".  Bloo basically walked me through the entire diagnostic procedure and then I detailed how I dropped the tank and removed the sender.  By grinding a little off my sender AND bending the arm a bit, it now reads accurately and doesn't bottom out inside the tank. Maybe you'll pick up a hint here or there to make the job a little easier. 

 

 

[Eddie-O]

Well, worked on my half full reading this past weekend and as I thought, it was the sending unit. Open circuit gauge goes to full. Ground it and shoots to empty. Drove the car for awhile since I had a pretty full tank for winter. After burning enough off, around half tank, the gauge started working again registering from around half and then reading fine the more I burned off. Down to a quarter now, so I’ll be dropping tank and see what’s up with the sender. Glad it’s the sender and not gauge. Whew!