I have searched the archives, but have not seen the answer to this question. I am working on a 1949 F1 that has been converted to 12 volt by someone else. The truck was purchased with an aftermarket instrument cluster (Dolphin?) and an adjustable fuel level sender. The owner bought an restored original cluster with a voltage reducer installed. I have been trying to get the present sending unit to work correctly with this cluster and have had no luck. Putting a resistor in the wire between the tank and the gauge has stabilised the reading, but the ohm values of this sender only change about 15 ohm between empty and 1/4 tank. This results in no perceptable change at the gauge. I have ordered a stock sending unit and I will remove the resistor when I install the stock sending unit. I see that the behind the seat gas tank is fat at the bottom and then thins out some nearer the top of the tank. This seems to mean to me that half full is not halfway up the tank. My biggest concern is that the gauge reading in the last 1/4 tank will be accurate. Does the stock sending unit give an accurate reading all the way from top to bottom, or is the last 1/4 tank sort of a best guess? I realize that I will have to adjust the arm on the sending unit. Please help this harried mechanic,
Thanks,
Tony

 

The gage in my 56 was doing something similar and, like you, I thought that the problem was different resistance between the gage and the sender. That still might be it, but I solved the problem using a more purely mechanical approach. I realized that a stock sender and gage must have been in the ball park with each other originally and that I could either put a resistor in the sender line to dial it in or I could recalibrate the sender to match the gage. I did the latter

I took the sender out of the tank and reconnected the sender-to-gage wire using a jumper. I also used a jumper to ground the sender to the chassis. I laid the sender on a piece of cardboard marked for the tank height (I stuck a tape measure down thru the sender hole to find an accurate dim for the height). I put the sender on the line for the tank top and started working the float arm up and down. The gage responded. I adjusted the float arm length and the radius position of the float pivot clamp until full up and full down on the float arm read full and empty on the gage. Reinstalled the sender. All has been perfect ever since. The whole operation took about an hour.

The tank in my 56 is like yours, behind the seat and shaped funny. I can't say whether half full is really that, but like you said, it's only important to be able to read 1/4-to-empty within reason.

You could try this method. It might fix the problem, doesn't cost any money and it's pretty easy. Just a thought.

 

Thank you Randy for your reply. I had tried pretty much the same method as you in my attempts to calibrate the universal sending unit. I have the advantage of having the old tank from this truck. I also used my 8.8 liter version of a Mity Vac that one can use to empty and then refill the tank. I tried this in 1 gallon increments, pausing at each gallon to check the reading on the gauge. The problem has been solved by installing a stock, bimettallic spring type sending unit and float. There was a little glitch as the brand new brass float had a hole in it. It took me a few minutes to realize that this was the problem rather than an error on my part. I was in a hurry so temporarily I used the plastic float from the universal sending unit. I will solder the hole in the brass float and install it when I next work on this truck. The method I use for soldering these floats is to solder as best I can, but I usually find difficulty in doing the last bit as the heat from inside the float "blows out" and creates a hole. I have a hot iron at the ready and put the float in hot water. Immediately after removing the float, do the last bit of soldering and the float will be drawing in air as the float is hotter the its surroundings. I don't know if this is a common technique, however it works for me. I have gotten a tremendous amount of advice and knowlege from this site and I'm sure I will continue to do so. Thanks again Randy.

 

I had the basically the same problem with my '48. I was using the stock gage with a newer tank and sending unit, still using the 6 volt system. I played around with the gage and sending unit exactly like Randy. I place a resistor between the sending unit and gage, I also placed a resistor between the two posts on the gage to adjust the resistance of the gage. The gage still does not read 100% but very close and at least I know if I have gas in the tank. The gage in my '49 F-2, which is all stock, doesn't seem to be much more accurate than the new set up in the panel. When I'm driving down the road the F-2's gage moves around a little during the jostling but settles down once I come to a stop, just like my panel. It just takes a few more seconds for the panel's gage to settle, but it seems quite accurate.

 

I had searched the archives and I too found the advice regarding putting a resistor between the tank and the gauge. I tried a 10 Ohm, then a 20 and a 30. I finally settled on a 2.2 Ohm resistor and this seemed to give me the best results. This is merely what I found with the universal sending unit that was in the truck before. This truck was already converted to 12 volt and the owner got an OEM instrument cluster with a 12 to 6 volt reducer installed by the rebuilder. With an OEM sending unit installed, things seem to be better. It is good to hear that the accuracy in the last few gallons isn't spot on as this is exactly what I was attempting to get. I now have, I believe, gotten that gauage to indicate true empty and give the owner fair warning that he wasn't getting before. I do have some questions about the vacuum advance, but that will be another thread. You guys are great!