1939 Special Fuel Gauge Sender Voltage

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Well, I put a new tank sender unit in (Cars Inc). It was a 1958 model, which they said was the same except it had the sock on it.

I had gas in the tank and my gauge went from showing "E" to half-full, which was encouraging. That is until I got gas.


Filled her up, and all I get is half a tank reading.


When I disconnect the sender pig-tail in the trunk the gauge jumps to Full, as I believe it should based on other posts.


The sender is grounded by new 12 g. wire to a frame bolt, which I just sanded to shiny metal.  Voltage at the pig-tail is not quite 3 volts, though.  Being as ignorant as I am...3 volts is half of 6 volts, which might be equal to about half a tank. I know the gauge measures by ohms of resistance, but that's where the ignorance kicks in.


So - what say you...1) run a new wire from the gauge to the sender, 2) scrap the dash gauge for an after market hidden in the glove box or 3)some other thoughts?


Many thanks - 

Edited by JRHaelig (see edit history)

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What do you mean by pigtail?


There are all sorts of things that could have gone wrong. The good news is that 1958 and 1939 are electrically compatible. IMHO socks are a great idea. If it were me I would add one even if using the original sending unit.


This is a 0-30 Ohm system, with 0 being empty. What you need to accomplish now is 0 ohms empty and 30 ohms full.


The float could be bad or leaky, or it could be hitting on something in the tank, or the arm could be bent wrong, or it could be too long.....


When you get the tank out measure the ohms. Short the leads of the ohmmeter first to see how many Ohms the leads are. If it reads 1.2 Ohms for instance, that is really zero. Now measure the sending unit in the tank with no gas. It should be very close to "zero". If the float hits the bottom it won't get there.


Turn the (empty) tank upside down. It needs to go to 30 ohms or higher (given your symptoms, it probably wont). If it wont, take the sending unit out and try to see if it goes to 30 ohms out of the tank. Check to be sure the float isn't leaky or full of gas. If it is a brass float you can dunk it in hot water and look for bubbles. If all that is OK, see if you can tell what the arm was hitting on. If it is not hitting anything, maybe the arm is too long and the float is hitting the top of the tank before the sending unit gets to 30 ohms.


Incidentally, the float should really never QUITE hit the tank. The sending unit should have stops of some sort. If the float hits, over time it might crack.


Once you have figured that part out, I would also try it right side up with some gas. Suck out the gas through the pickup with a transfer pump (do NOT use a fuel pump or anything electric). Do not buy the orange/red plunger type transfer pump at Harbor Freight. It is crap and wont even last long enough to fix one gas tank. Get something with a squeeze bulb or a crank. The idea here is to make sure that the gauge gets to zero (0 Ohms) before you run out of gas.


Ideally, you should be able to keep pumping fuel after the sender hits 0 ohms. Pay attention to how much you can pump before it sucks air and stops pumping. The difference between these two points is your reserve. The service manual sometimes even tells you how much the reserve was originally. It probably shouldn't be 5 gallons for instance, maybe 2. If it is wrong, you probably need to give the arm a little bend.


You can't suck all the gas out, and any left in the bottom after sucking out the reserve is simply lost capacity. For this reason, the hole in the end of the pipe with the sock on it needs to be REALLY low in the tank.


If you have to bend the float arm to fix empty/reserve (0 ohms), then turn the tank upside down again after emptying it. Make sure it still gets to 30 ohms upside down.


Work outside. Have a fire extinguisher handy.



Edited by Bloo (see edit history)
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May be that the (dash) gauge is faulty.

Have you tried putting different ohm's to it, from a different source

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I ran a dedicated ground directly from the battery to the sending unit.  It's real easy to check the quality of your ground by running a temporary wire from your battery to that wire you have bolted to the frame.  

At least you can remove "bad ground" from your list of possible reasons.


Good Luck!

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You may want to check to see if the wire that supplies the positive voltage from the meter is good. Run a temporary wire from the positive supply of the meter to the gauge. That 3 volts may jump to 6 and solve the problem. If it does and the gauge works you will need to replace the wire currently in the car.

Also, without installing the sending unit at all you should be able to work it through its range by lifting and lowering the float to see if the gauge responds properly. Just have ground and supply properly connected.

To test your ground point (where you cleaned it) attach one end of the ohm meter to that ground and the other via a long jumper wire (maybe no. 16 size) to the battery ground strap. It should read close to 0 ohms.

Edited by Roadmaster71 (see edit history)

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