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Returning Bolter that just introduced myself on the Welcome forum. Getting the truck back on the road after sitting unlicensed for years.
1950 3100 with overhauled 57 235. 12 volt new harness (20 yrs ago)
Original gauge with Runtz resistor. New tank and sending unit (15 yrs ago)
Gauge on empty when started. Tank is approximately half full. Removed wire from sending unit and gauge goes to full. Tested sending unit and reads 0 ohms. Removed sending unit and tested with multimeter and unit registers between 30 ohms and 0 ohms when moving float.
What should I do next?

Did you check the float to see it it's full of gas and no longer floating?

Good question. Seemed to be a solid piece of plastic that didn’t make a sound when shaking it. I could always just pull it out again and test it in a container and take another reading

Did the gas gauge work 15 years ago when you installed the new sending unit? Or never did work right?

Depending on the answer to that, you might want to explore the idea that the float arm is hitting the side of tank somehow. Sounds like your float is floating. Sounds like your sending unit produces the right variation in Ohms

The sending unit screws allow very limited “clocking” options of the float. Adjustment to the sending unit might be needed to ensure no interference.

There is a gas gauge troubleshooting "sticky" thread at the top of this forum.

Just removed the sending unit again and the float is a solid black plastic cylinder. Tested it in a container of water and it floats and ohms change from 0 to 30 depending on water level. Reinstalled it and I now get about 10 ohms reading with gas in the tank. Gauge moves from below E to just above E when key is on.

JW51, yes the gauge worked before the truck sat unlicensed for a number of years. I don’t believe the float arm is hitting anything as I double checked when reinstalling.

Yes I read through the sticky before my initial posting but some of the steps I think assume i should know more about electrical things. Complete noob with my first multimeter but determined to fix it with a little guidance

I'll bet that you will get lots of help on here. The multimeter can become one of your favorite tools, once you get familiar with it.

Just now noticed where you’re from. I’m not far away᠁.in Cape. Drive past Johnston City pretty often for various work and family things.

Hi Blue. At apx 1/2 full you should get around 15 ohms of resistance, so it sounds to me you may be pretty close with 10 (since we don't know how close to 1/2 full you are). Furthermore, it is really good news if your sending unit is registering between 0 and 30 ohms and not rubbing on the side of the tank or hanging up anywhere. If the float rod is adjusted so that it comes very close to hitting the top of the fuel tank when the tank is full, all will be pretty much Jake. One thing that occasionally is overlooked: These fuel tanks have no baffling to keep fuel from sloshing from side to side, so that means when you open the door, stand on the running board and sit down in the driver's seat, the truck leans a bit to that side (which it will do even if you're built like Frank Sinatra). So when that happens, the sloshing fuel will create sort of a mini tidal wave effect in the tank...and it will slosh side to side for a while, too. Which naturally will move the float up/down as the fuel moves side to side. Could just be that causing the needle to move from just below E to just above E.

But here's another thing. Inside your fuel gauge is an hourglass shaped permanent magnet which is moved up/down by electromagnetic force. That little permanent magnet will have its magnetism changed if your fuel gauge was run on 12v in its life at some point. Then if that happens (and some think it could take a few minutes to a few hours) your fuel gauge will work fine with 12 volts but might register lower than it should on a reduced voltage. True your fuel gauge cares more about amperage than voltage, but still in all the two are interconnected. For many years I've told folks to think of voltage as the size of your arm and amperage as the strength of your arm. Anyway, if your gauge seems to be registering lower than it ought to, you might try removing the Runtz device and see what happens.

Hey Jon,

The gauge stays in the same place all the time, sitting still, moving doesn’t matter. The needle moves barely when turning the key off and on only. I will wiggle my way under the dash and take a look and/or take off the resistor and see what that does.

Have you verified that you've clocked your sending unit correctly? It should move parallel to the length of the tank. Any other position will cause it to hang up on the sides of the tank. If I recall, the float arm goes toward the driver side.

OTTO, yep float is towards driver side

Finally got some time to work on the truck today. Gauge still showed empty when ignition is on. I ran a jumper cable from the sender to the resistor and it showed empty with ignition on. I noticed the resistor was bent at a 90* angle so I straightened it and the gauge went to full. Actually it went as far past full as far as it could go. I decided to take the resistor off and it pegged past full again. Could it be a bad resistor?

Ohms at the sending unit are around 20. Ordered a new runtz resistor that has a ground cable just in case.

Any ideas out there?

Try removing the runtz resistor and leaving it alone for a while. Then, please try connecting your power wire to your fuel gauge without using that resistor. Please trust me, nothing bad will happen if the 12v wire connects to the right side of your fuel gauge (as you look at the back side of it). If the ohms at the sending unit are around 20, things are sounding good there. Assuming the resistance is 20 ohms where the signal wire connects to your gauge (the left hand post as you look at the back side of it), your gauge should show about 2/3 full...since you have 2/3 of 30 ohms of resistance being sent to the gauge (30 ohms would be full). Please let us know.

Jon,
The resistor is off, 12v is directly to the gauge and it reads past full as far as the needle will go. As stated the tank is actually about 1/2 to 2/3 full

Ok, let's wait until you get the new runtz device and see what that does. All the runtz device is going to do is to reduce your voltage to apx 8 volts. Let's see what it does.

Latest update. Went into town with the truck and the gauge was still showing full but while coming back the gauge would move erratically from full to empty. That was new had never seen that. When I got home the new resistor was in the mail and I proceeded to install it. Once installed and key turned on it went to full. Same thing as without a resistor with direct 12v. Reading the installation/diagnostic sheet that came with it, the last item said “if the gauge only shows full and you know less fuel is in the tank the problem is the wiring to the sender or the sender itself. Run a jumper from the sending unit to a good ground, if the gauge goes to empty, the sending unit is bad” Well the gauge went to empty, so I guess it’s time for a new sending unit?

Possibly so. If you want to get the one I believe is the best option for the AD truck, this is it: https://tinyurl.com/8tyapykt

There are many reasons I endorse this sender above others and I've discussed them here before. If you want me to repeat them, please let me know. You'll need to shorten the mast and the rod for the float.

If your gauge is moving erratically a couple of things could be happening. If you remove the wire going to the center of the sending unit (we can call that your signal wire) your gauge should go to full. And if you touch that wire to a known good ground, your fuel gauge should go to empty. That's pretty simple but there are two things that can goof that simple test up...one is a broken wire inside the insulation. Very rare but I've seen it happen before. Two is a bad connection somewhere at the sender or at the gauge...a connection that is making contact and then not making contact.

The other thing is your sender. Which in this case is probably the answer. It operates as a wiper (like a windshield wiper, sort of). It "wipes" a contact point across a variable resistance affair and if that wiper gets weak it will start skipping on the resistance winding. And when it does that you'll lose contact/gain contact and your gauge needle will start jumping from what it should be telling you to Full...back and forth. Or depending on how intermittent the connection between the wiper and the resistance winding is the jumping may only go from say 1/4 tank (or whatever level of fuel is in your tank) up to 1/2 tank and back again. In my experience the senders which operate on a flat plane last longer and are more accurate than what I call the inverted "C" senders. My 2 cents worth...please spend them wisely.

Update on the sending unit.

I decided to try the model you showed and the bottom line is the old unit was defective and it is now working. I don’t think I will be using that source again due to a few things. The item took almost three weeks to get here. They never communicated with me as to the delay. Shipping is free since they send USPS Media Mail (slowest way). It’s a generic model so you have to cut the shaft and the rod, rebend the angle on the rod, I had to re-solder one of the lines and the gasket did not line up and had to punch new holes. Next time, for a couple bucks more I’ll just buy the USA made one from a specialty vendor in Kansas City. Thanks to all for the help!

I'm very sorry to hear that. I've never met that fellow but we have traded email messages over the years and I wonder if somebody else is working with him now or whether he might be on a last of the summer vacation. I have never gotten the feeling he would cheat anybody at all. In fact, just the opposite. All those sending units are generic. Sorry if that wasn't clear. They were designed by a man in Taiwan and he made them so he could vary the resistance as needed for the application.

Somebody in Kansas City is making a made-to-fit sender for the AD tank? I didn't know that. I spent 2 years developing several different types before deciding (a) I could make the best sending unit around (b) I couldn't do it for $30. What is the name of the company?

Jim Carter is selling USA made units as of a couple weeks ago

Once again sorry but I wasn't able to find any sender at Jim Carter's site other than for the 54 and up Chevy pickup.

I know that I bought a sending unit for my '51 from Jim Carter a few years back. No idea of it's origin country though.

Jon G,

Here is what I found available from Jim Carter 1949-1953 Gas Tank Sending Unit wit...t Chevrolet and GMC Pickup And Big Truck.

His info doesn't specify country of origin, but says "Manufactured for Jim Carter Truck Parts"

Hope this is of some help.

Dan

I tried a search earlier and didn't find the 47-53 sending unit, but his web site isn't know for being that good.

Could have sworn the brass float one had a USA made stamp. My bad if not. I had already bought the other but couldn’t get a response from the vendor so I was looking around. It still doesn’t change my mind with the work I had to do to make the new one operational. The good news is the gas gauge is reading as correct as a 73 year old truck can!

That is very good news, Blue. I've sent the vendor of the one you bought a message but he hasn't replied (which is not common as he has heretofore been quick to answer). So I assume he is currently out of the picture...vacation, getting kids back to school, etc. You never know what might be happening in somebody's life, you know. That sender you got should work for a very long time.

Ooops, forgot to mention that I also installed a new Runtz resistor to the original gauge.