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Username Post: 38 fuel gauge question        (Topic#199484)
navyson 
Member
Posts: 154
navyson
Loc: California,USA
Reg: 01-12-06
09-23-08 03:01 PM - Post#1527939    

I finished the wiring of my 38 and switched on the key and the original 38 fuel gauge needle popped into position. The gas tank is currently totally void of fuel. The problem is that the needle pops up to halfway between empty and a half tank. The gas tank is new from TANKS, INC. and the sender is new from CHEV'S OF THE 40's. I followed the installation directions for the sender to the letter. It seems that the gauge is getting more current than it's supposed to. Does anyoe know what's going on? An adjustment somewhere?
Thanks

1938 Master Deluxe 4 door Chevrolet; 1964 Corvette, Sport Coupe


 
Royer 
Needs to Get Out More Member
Posts: 11411

Loc: Bloomfield Hills, Michiga...
Reg: 09-25-01
09-24-08 04:20 AM - Post#1528291    
    In response to navyson

If you disconnect the wire at the gauge unit that goes to the tank sending unit, you should see the needle move to and beyond the "Full" mark. If it doesn't, Id say your gauge (instument panel) unit is likely defective. Also, grounding the same wire momentarily should cause the needle to move to "Empty".

If both these checks are satisfactory, I'd suggest that you try pulling the sending unit from the tank and bending the arm a bit so that the potentiometer sits lower with the float on the bottom of the tank.

Royer



 
bowtiefan 
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bowtiefan
Loc: Vacaville, CA
Reg: 02-19-04
09-24-08 07:49 AM - Post#1528408    
    In response to Royer

  • Quote:
I'd suggest that you try pulling the sending unit from the tank and bending the arm a bit so that the potentiometer sits lower with the float on the bottom of the tank.




I just went through this last week on our 57 Corvette. Assuming the float is the same basic design as the Corvette, mounts to the tank from the top, you have to bend the arm down if it reads low and up if the gauge reads high. At first this seems backwards, but once you realize how it works, you'll understand.

Ken

38 & 39 2dr sedan, 41 coupe, 54 convertible, 56 Nomad, 57 210 2dr sdn, 70 Camaro RS, Corvettes; 57, 61, 65 396 coupe, 67 427 convert, 69 & 72 T-top, 88 coupe,91 ZR-1, 07 Z06, 16 Z06, 07 Silverado, 09 BMW 135i vert.
http://www.picturetrail.com/ken58


 
navyson 
Member
Posts: 154
navyson
Loc: California,USA
Reg: 01-12-06
09-24-08 10:51 AM - Post#1528526    
    In response to bowtiefan

Wow! I'm confused! Can you explain the arm bending principle to me. I thought the arm when down would have a low current flow , showing "empty" on the gauge and when the arm is all the way up the current flow is high showing "full" at the gauge. Adjustment of the float would just be for bottom of tank and top of tank.
I took this sending unit out of the original tank and put it in the new "Tanks, Inc." tank. maybe the float is sitting different in the new tank and the potentiometer is registering a quarter tank of gas instead of empty?

1938 Master Deluxe 4 door Chevrolet; 1964 Corvette, Sport Coupe


Edited by navyson on 09-24-08 11:19 AM. Reason for edit: No reason given.

 
navyson 
Member
Posts: 154
navyson
Loc: California,USA
Reg: 01-12-06
09-24-08 11:24 AM - Post#1528544    
    In response to navyson

Royer,
This must be what you are telling me to do, for the empty position, bend the arm so the potentiometer is at the lowest point and the float is at the tank bottom?


1938 Master Deluxe 4 door Chevrolet; 1964 Corvette, Sport Coupe


 
Royer 
Needs to Get Out More Member
Posts: 11411

Loc: Bloomfield Hills, Michiga...
Reg: 09-25-01
09-24-08 11:26 AM - Post#1528546    
    In response to navyson

And the arm could very well have been bent slightly in the process of removing it from the original tank, while it was out laying on the bench, or during installation into the new tank. Trust me, you need to assure that the POTENTIOMETER is in the "Empty" position when the FLOAT is in the "Empty" position.

And make certain that the tank has a good solid ground to the frame (I recommend running a dedicated wire or ground strap) as too often the nicely painted surfaces on a restored vehicle present more problems in this regard than did the original as it came out of the factory seventy years ago.

Royer



 
navyson 
Member
Posts: 154
navyson
Loc: California,USA
Reg: 01-12-06
09-24-08 04:02 PM - Post#1528725    
    In response to Royer

Royer
Looks like it's going to be a troubleshooting weekend!
Thanks!

1938 Master Deluxe 4 door Chevrolet; 1964 Corvette, Sport Coupe


 
bowtiefan 
"10th Year" Gold Supporting Member
Posts: 2271
bowtiefan
Loc: Vacaville, CA
Reg: 02-19-04
09-24-08 08:51 PM - Post#1528947    
    In response to navyson

Yes, you bend the arm as you said, but you need to check the 'full' location also. On my Corvette, when I raised the arm to the 'full' location, the float was actually higher than the mounting plate where it goes into the tank. What I did was fill the tank and then bend the arm until it read full on the gauge. (Since the tank is right behind the seat, I could see the gauge as I made the adjustments. You don't have to totally mount the sending unit to the tank to check it, at least I didn't on the Corvette, but be sure that the bracket is parallel with the mounting surface. A little bit off there translates to a bigger difference on the sending unit and thus an inaccurate gauge reading) Don't know where it will read when empty, haven't used up enough gas yet. If you have an empty tank, it might be easier to adjust to empty, and not worry about the 'full' location. You'll find that out when you eventually fill it up.

At least, this is how mine went, so hopefully it will give you some idea of what you might be up against. Good luck.

Ken

38 & 39 2dr sedan, 41 coupe, 54 convertible, 56 Nomad, 57 210 2dr sdn, 70 Camaro RS, Corvettes; 57, 61, 65 396 coupe, 67 427 convert, 69 & 72 T-top, 88 coupe,91 ZR-1, 07 Z06, 16 Z06, 07 Silverado, 09 BMW 135i vert.
http://www.picturetrail.com/ken58


 
navyson 
Member
Posts: 154
navyson
Loc: California,USA
Reg: 01-12-06
09-25-08 04:20 PM - Post#1529439    
    In response to bowtiefan

Thanks Ken,
I'll be taking the tank back out this weekend for the sending unit adjustments. Hopefully a bend here and there will tweek it to the right reading.
Jim

1938 Master Deluxe 4 door Chevrolet; 1964 Corvette, Sport Coupe


 
David Hayward 
Deceased RIP David
Posts: 7051
David Hayward
Age: 62
Loc: New Forest, UK
Reg: 04-10-99
09-26-08 12:39 AM - Post#1529689    
    In response to navyson

I am making this thread sticky as it is of such interest.

Automotive Historian, Writer & Author

Avatar: sole surviving 1939 Chevrolet truck assembled in Southampton, England


 
Royer 
Needs to Get Out More Member
Posts: 11411

Loc: Bloomfield Hills, Michiga...
Reg: 09-25-01
09-26-08 04:27 AM - Post#1529719    
    In response to navyson

I haven't worked on a '38 model year Chevy gas tank myself, but are you sure that pulling the tank is necessary? Usually there is a small acces panel in the trunk floor that can be removed to access the sending unit. And if there isn't, you might want to consider cutting such an opening (which can be closed with a piece of sheet metal after working on the sending unit) rather than going to the trouble of pulling the tank.

Royer



 
bowtiefan 
"10th Year" Gold Supporting Member
Posts: 2271
bowtiefan
Loc: Vacaville, CA
Reg: 02-19-04
09-26-08 07:00 AM - Post#1529792    
    In response to navyson

If you are going to remove the tank, my thought would be to make sure that you set it so that it is in the same position as when it is in the car. Then use some jumper wires, including ground, so that you can see the results of your adjustments without having to replace the tank. In theory, this ought to work!

Good luck
Ken

38 & 39 2dr sedan, 41 coupe, 54 convertible, 56 Nomad, 57 210 2dr sdn, 70 Camaro RS, Corvettes; 57, 61, 65 396 coupe, 67 427 convert, 69 & 72 T-top, 88 coupe,91 ZR-1, 07 Z06, 16 Z06, 07 Silverado, 09 BMW 135i vert.
http://www.picturetrail.com/ken58


 
navyson 
Member
Posts: 154
navyson
Loc: California,USA
Reg: 01-12-06
10-01-08 02:44 PM - Post#1533189    
    In response to Royer

Yes, in order to get to the sending unit the gas tank needs to be dropped down.

1938 Master Deluxe 4 door Chevrolet; 1964 Corvette, Sport Coupe


 
navyson 
Member
Posts: 154
navyson
Loc: California,USA
Reg: 01-12-06
10-01-08 02:54 PM - Post#1533195    
    In response to bowtiefan

Here's the results of last weekend: I dropped the gas tank and removed the sending unit. It was positioned wrong and was hitting the curved side of the tank. I read the installation directions and adjusted the float so it hit bottom when the tank is empty. I carefully measured and remeasured. I then reinstalled and repositioned the sending unit as it described. Then I did what you have described exactly. The gauge needle popped up to a quarter of a tank! It is till a mystery. Seems like it is getting too many amps or something. I even tried my really old sending unit with the gauge, same thing. Could the old gauge be to weak to resist the amperage flow or something? I don't think the needle is bent, it is otherwise in good shape.

1938 Master Deluxe 4 door Chevrolet; 1964 Corvette, Sport Coupe


 
bowtiefan 
"10th Year" Gold Supporting Member
Posts: 2271
bowtiefan
Loc: Vacaville, CA
Reg: 02-19-04
10-01-08 08:14 PM - Post#1533426    
    In response to navyson

I know it's a PITA, but try bending the arm so that it is at the top when full and let the empty be where ever it ends up and see what happens. Beyond that, you are talking electrical and I only do mechanical.

Ken

38 & 39 2dr sedan, 41 coupe, 54 convertible, 56 Nomad, 57 210 2dr sdn, 70 Camaro RS, Corvettes; 57, 61, 65 396 coupe, 67 427 convert, 69 & 72 T-top, 88 coupe,91 ZR-1, 07 Z06, 16 Z06, 07 Silverado, 09 BMW 135i vert.
http://www.picturetrail.com/ken58


 
Royer 
Needs to Get Out More Member
Posts: 11411

Loc: Bloomfield Hills, Michiga...
Reg: 09-25-01
10-02-08 04:04 AM - Post#1533530    
    In response to bowtiefan

Forget about putting the sending unit in the tank for a minute. Simply connect the sending unit to the wire which runs up to the instrument panel display unit and connect a temporary grond wire from the body of the sending unit to the negative battery terminal. Turn the ignition key "on" and see if you can get the display unit to read "Empty" at some point. And see if you can get it to read "Full" at some other position of the float. If you can't, then try running a new wire from to connect the sending unit to the instrument panel fuel gauge display. If you still can't achieve both "Empty" and "Full" readings at some position of the float, you have either a defective sending unit or disply unit.

I'd suspect the sending unit if the gauge can be made to read "Empty" with the ignition off.

Royer



 
David Hayward 
Deceased RIP David
Posts: 7051
David Hayward
Age: 62
Loc: New Forest, UK
Reg: 04-10-99
10-02-08 12:39 PM - Post#1533799    
    In response to Royer

I am discounting a holed sender unit float..they did and do rust and you can get a pinprick hole not visible to the naked eye! However, I suggest that you do check the 'business end' anyway.

Automotive Historian, Writer & Author

Avatar: sole surviving 1939 Chevrolet truck assembled in Southampton, England


 
David Hayward 
Deceased RIP David
Posts: 7051
David Hayward
Age: 62
Loc: New Forest, UK
Reg: 04-10-99
10-04-08 03:14 PM - Post#1535270    
    In response to David Hayward

This may be of some interest: a link to the gas gauge page in the '38 Shop Manual:
http://chevy.oldcarmanualproject.com/shop/1938/38c...

Automotive Historian, Writer & Author

Avatar: sole surviving 1939 Chevrolet truck assembled in Southampton, England


 
navyson 
Member
Posts: 154
navyson
Loc: California,USA
Reg: 01-12-06
10-11-08 07:26 AM - Post#1539580    
    In response to David Hayward

David,
Thank you for the 38 Chevy on-line manual. So far it appears that there is too much current flowing through the fuel gauge when the tank is empty. This is probably due to not enough grounding. I am still doing tests to see if an additional ground will decrease the current flow to the gauge when the tank is empty.
Thanks!

1938 Master Deluxe 4 door Chevrolet; 1964 Corvette, Sport Coupe


 
Royer 
Needs to Get Out More Member
Posts: 11411

Loc: Bloomfield Hills, Michiga...
Reg: 09-25-01
10-11-08 08:55 AM - Post#1539631    
    In response to navyson

The addition of a dedicated ground wire from one of the retaining screws on the tank sending unit to a good, solid ground on the frame will NEVER do any harm.

Royer



 
David Hayward 
Deceased RIP David
Posts: 7051
David Hayward
Age: 62
Loc: New Forest, UK
Reg: 04-10-99
10-11-08 10:23 AM - Post#1539665    
    In response to Royer

I agree with the very 'well-grounded' suggestion!

Automotive Historian, Writer & Author

Avatar: sole surviving 1939 Chevrolet truck assembled in Southampton, England


 
Don D 
Member
Posts: 50

Loc: SC, USA
Reg: 04-13-06
03-07-09 05:32 PM - Post#1646093    
    In response to David Hayward

I also have an issue with my 1939 Master Deluxe fuel gauge. I replaced the fuel tank and float assy. With the float assy removed and connected to the gauge (tank side)with key on, I move the float from empty position toward full the needle drops below the empty mark. Move the float position back to empty the needle moves up to the empty mark on the gauge. Wires are connected per my 39 wiring diagram. Any suggestions. Ground is good.

Don

Don
39 Chevy Master Deluxe, 216 engine


 
Royer 
Needs to Get Out More Member
Posts: 11411

Loc: Bloomfield Hills, Michiga...
Reg: 09-25-01
05-09-09 11:58 AM - Post#1694316    
    In response to Don D

Are you certain that the replacement sending unit is really correct for your car? It sounds to me as if the resistance range (from the "Full" to the "Empty" positioned isn't correct for this application.

Royer



 
navyson 
Member
Posts: 154
navyson
Loc: California,USA
Reg: 01-12-06
05-21-09 07:05 AM - Post#1701879    
    In response to Royer

Hi Royer
Good question! I bought the sending unit from Chev's of the 40's so I don't know anything about the sending unit's resistance vs the gauges. I have put in about 2 gallons of gas in the tank and it still doesn't register anything but empty. When I turn on the ignition the gas gauge arm jumps into position to about 1/8 of a tank. I did all the test that have been mentioned previously except grounding the tank. I felt the tank is grounded to the engine via the stainless steel fuel line to the fuel pump. It is still a mystery and I have been reluctant to fill up the tank and have the gas gauge register empty. I would have to either use up all the fuel, 14 gallons, or drain the tank to fix the problem. The gas gauge passed all of the test procedures so I know it's the sender that might have the problem. Seems like it's getting too much amperage or something. It's still an unsolved problem.
Jim

1938 Master Deluxe 4 door Chevrolet; 1964 Corvette, Sport Coupe


 
gary635g 
Contributor
Posts: 455
gary635g
Age: 79
Loc: Oldtown Idaho Yuma Az
Reg: 10-28-09
09-25-10 06:24 AM - Post#1980663    
    In response to navyson

Just replaced sending unit on 41 Chev from chev of the 40's 2 and 1/2 gallons barely move gage off mt another 5 moves to 3/8 ohm Readings 0 mt 30 full.Gary



 
gary635g 
Contributor
Posts: 455
gary635g
Age: 79
Loc: Oldtown Idaho Yuma Az
Reg: 10-28-09
02-03-11 04:16 AM - Post#2041351    
    In response to gary635g

Still working this problem. The phonelic(sic) float would only lift the sending unit i/2 I told Chev of the 40's they provided me another unit because they do not have only the floats. Re-installed yesterday with two floats and added 7 gallons reads 1/2 will see how it does when filled. Poly tank being only 6 inches deep where unit goes in aggravates the problem.



 
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