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


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#395410 09/24/17 10:20 AM
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Clement Offline OP
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Hello all. I have read all the posts in the past about fuel gauges. Have not read about this. 1946 Stylemaster business coupe. The fuel gauge works, needle registers, but the needle " jumps " . It is not the normal , smooth , slow progression towards empty. When tank is full, needle displays all the way on full. As you drive, the needle goes towards empty , but again, it is a " jumpy " progression. When needle is on empty, the most I have ever been able to put in tank is 8 gallons ( we all know it is a 16 gallon tank ) I looked under dash...connections are tight and clean. Looks like original gauge and tank is original. Thoughts and suggestions ? Thanks experts !

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Sounds to me like you are going to take it out and look. If you can get all the way to the "E" the wiring pretty much has to be fine.

Empty too soon might mean a sunk float.

Smoothness? I dont know if I would be expecting smoothness. Unlike thermal gauges in some other cars, old Chevrolet gas gauges react instantly to signals from the tank. Some of them wave around a lot. In the 30s, I believe there was a little cork brake to minimize waving around due to fuel sloshing in the tank. I'm not sure if that is still part of the system in 1946. I sort of doubt it.

Crud, corrosion, or varnish on the variable resistor in the sending unit could sure cause some wild waving around.

Since the last thing I would want to do is drop the tank. there are a couple of things I might try first. If you short the wire that goes to the tank directly to ground, the gauge should go to zero. If you can find some resistors that add up to 30 ohms, and you put 30 ohms between the tank wire and ground (with the tank disconnected), the gauge should go full. If it does both of these things fine, the trouble is in the tank for sure.

Make sure the tank is grounded well. Adding a ground wire to a good ground on the frame couldn't hurt. This might help with waving if (and only if) the waving is caused by an intermittent ground.

I cant help but wonder if it has the wrong sending unit.

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ChatMaster - 750
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My 41 the gauge always bounced around and my 47 is the same way, it has a new sending unit in it. If you remove the wire from the sending unit the gauge will go past full, if you don't' have a resistor.


Dens Chevys 1927 Speedster 1928 coupe 1941street rod 1947Fleetline 4 door 1949 1/2 ton Pickup (sold) 1954 210 4 door 1972 Monte Carlo 2003 Corvette convt..
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It could be that you have multiple issues. Just recently we had a great series of posts that helped me diagnose my fuel gauge and sender problems.
Gas Gage Issue

In my case I had 2 problems. The main issue was that the cork floats were saturated and were sinking to the bottom of the tank. The other point I corrected was to make sure that the moving arm was properly positioned on the variable resistor in the sending unit.

I put in new cork floats and things work pretty good for now. I do see a lot of needle movement both up and down as a I drive. I expect that within a year of so I will be back in there figuring out on to adapt a brass float to the sending unit.

I agree that you are probably going to have to pull the sending unit out of the tank to get things operating correctly. Thank goodness that is easy to do on my coupe.

Like Bloo suggested do the simple tests first before you drop the tank and pull the unit. Ground the terminal where the wire attaches to the sending unit and make sure the gauge reads "E". Then disconnect that wire and make sure it reads "F".

After you remove the unit reconnect the wire to the gauge (you might need a short jumper wire) and also ground the sending unit. Then manually move the float arm through the full range while someone watches the fuel gauge. That will tell you how resistance is changing as the arm moves.

That was the test that helped me figure out that the floats were not "floating". I could get a full and correct range of gauge readings by moving the arm. When I put the sending unit back in the tank it would read the correct level for about a day. Then all it did was read empty. The floats would dry out while I had them out but it only took a day for them to become saturated again.

Sorry for the long post but I know you want to be sure it is the sending unit you go to all the work to pull the tank. And that you can fix it after you get it out.


Rusty

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Clement Offline OP
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Thanks all. I will do some more investigating !! I just know in my 1950 the needle reads very accurately and has a very smooth motion. Never saw a needle " jump " like this one. Thanks for the great advice. To me it seems odd and obviously does not read correctly.

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Clement Offline OP
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Ohhh BTW I have a business coupe.....tank mounted under the package shelf ( where the actual rear seat would be in a " normal " coupe) ...can't drop tank. lol So if I investigate further I will have to undo the rubber mat carefully and go from there .

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You might have an easier job getting to the sending unit than you originally thought.

Hopefully an expert on '46 Business Coupes can provide more accurate guidance.

On my '37 coupe the tank is in a similar location. It is behind the seat and between the plywood trunk floor and stamped steel car floor pan. There is a 4" diameter steel access plate in a the plywood floor. It is held down by 4 wood screws.

The sending unit is mounted to the top of the tank and can easily be removed through that access hole.

If your car has that access plate I suggest you mark the orientation before you remove it. On my car the 4 holes for the wood screws are not quite in a regular pattern. There is only one position where all the screws fit properly.



Rusty

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Clement Offline OP
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Why thank you !!


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