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Posted By: Eureka Jim How dangerous?... - Tue Apr 02 2019 11:21 PM
...is it to install a gas gauge sending unit into a tank still on the truck? My '48 GMC has a between-the-frame-rails tank. I have easy access to the top of the tank by removing a few boards from the flatbed. The truck is outside under a canopy tent with no side or end panels. Because there is some gas in the tank there will be fumes when I remove the old sending unit.

The only anticipated trouble spot is getting the old gasket off, it's stuck pretty tight. I have a battery disconnect switch, I won't have a cell phone with me and I can run an external ground from the tank to the metal stakes that hold the tent to the ground. I think I can scrape off the old gasket with a thin plastic tool. No power tools will be used.

What I don't want to do is be tomorrow's headline: "Old Fool Kills Self & Nice Old Truck"

Suggestions? Thanks, Jim
Posted By: 78buckshot Re: How dangerous?... - Tue Apr 02 2019 11:41 PM
I wouldn't think twice about swapping the sending unit, you have covered all the bases with safety, if you want to go one step further you can fill the tank as full as possible with fuel, the less room for vapor the more safe it will be. Many folks here have done welding on gas tanks including myself, cleaned the tank as best I could with engine degreaser, then HOT water, then fill the tank with water with the repair site as high as possible, wave the torch over the hole and you get a little gyser, then your ready to weld.
Posted By: Hotrod Lincoln Re: How dangerous?... - Tue Apr 02 2019 11:44 PM
Go for it- - - - -unless you happen to be puffing on a foot-long stogie at the time, it's highly unlikely you'll have any problems. Contrary to popular legends, gasoline isn't really that dangerous. There's only a fairly narrow gas/air mixture ratio that will even light off. Get the mixture too lean or too rich, and it just won't burn.
Jerry
Posted By: Eureka Jim Re: How dangerous?... - Wed Apr 03 2019 03:18 AM
OK good, thanks for the replies . I was unable to find an original part so I'm forced to use an aftermarket one. Although it's from a reputable supplier and advertised as a 'universal' fit, it's clocked 90* off with no adjustment built in. I'll have to re-drill the mounting flange to make it work. Sounds like a tomorrow job!
Posted By: cmayna Re: How dangerous?... - Fri Apr 05 2019 03:49 PM
Fly me over, isle seat, first class.......and I'll give you a hand. The only thing I have with new sending units is knowing whether they were properly tweaked at the factory to read 1/4 full when you for sure have 1/4 worth of gas in the tank. Do you have the ability to fill the tank to 1/4 and then install the sending unit and find out if infact it reads 1/4?
Posted By: Hotrod Lincoln Re: How dangerous?... - Fri Apr 05 2019 04:14 PM
Be careful- - - -some of the aftermarket sending units have the wrong resistor installed, and have globs of solder dripped onto the variable resistor to sort of approximate the correct full-sweep resistance value. You definitely don't need to install one modified like that!
Jerry
Posted By: Eureka Jim Re: How dangerous?... - Sat Apr 06 2019 12:41 AM
Sorry Craig, I got the job done without your generous offer of help!

I used the "universal" sending unit from Jim Carter Truck Parts to replace the two year old and dead "universal" sending unit from Jim Carter Truck Parts. I didn't mean to order the same part that failed, I just forgot where I got the first one from and didn't realize it was the same part until I recognized it and checked my records.

But I know more now about electrical stuff than I did 2 years ago thanks to Jerry and others here on Stovebolt. My truck was converted to 12V before I got it and I elected to run a resistor in front of the gauge to lower the voltage. I tested the sender before I installed it and it looked good. I followed the instructions for adjusting the depth and arm length for my tank and just today I put in what I figured to be 1/2 a tank. Success! The gauge is pretty much dead centered.

The biggest challenge was that the hardware was clocked 90* off from what I needed, making it impossible to install. The float would be hitting the side of the tank and there would be no way to bolt it down. The only adjustment was 180* which gave me the same problem just on the other side of the tank. I ended up having to change the orientation of the holes in the flange that bolts to the top of the tank. I also had to cut off some length of the framework to allow it to wiggle its way into the small tank opening. Thanks to my paternal grandfather machinist's genes, I was able to figure out how to do both those little jobs, though I had to think really hard.

Is there any such thing as visual dyslexia? If there is, I got it! Thanks everyone, Jim
Posted By: Jon G Re: How dangerous?... - Sat Apr 06 2019 03:48 PM
Jerry is 100% correct about the modified senders. I'll post the junk I've found again in case anyone is interested. The sender won't work worth anything when this is done. I bought this unit from a popular vendor of AD truck parts. Since the seller didn't make the unit I won't tell you who it was.

Here is in my opinion the best gas sender I've yet seen:

https://tinyurl.com/y4htzvew

It is $28 with free shipping, it operates with a flat wiper and imbedded resistance wire and it reads right on the dot. Plus the thing is made of nylon. 30 ohms is correct and 0 ohms at the other end of travel is correct.

Attached picture IMG_8462.JPG
Posted By: 49 3100 Re: How dangerous?... - Sat Apr 06 2019 04:10 PM
Jon g what year range will this work on? I have a 49 AND 51 3100--niether fuel gauge works--Thanks Bill
Posted By: Jon G Re: How dangerous?... - Sat Apr 06 2019 07:18 PM
Both of those trucks require a 0 to 30 ohm sending unit, Bill. If you are using a 12 volt system, you need a voltage reducer for the gauge itself. This sender only affects ohm readings, however and it should work fine for either 6 or 12 volt. When you get the gauge you will need to adjust it and on the one I bought I cut the height back to fit in the tank. Seems to me it was about 13 inches down to the horizontal baffle plate. Then you have to adjust the length of the swing rod. Typically you divide the fuel tank height by 2 and multiply that figure x 1.4 but there will be some data with the sending unit telling you how the maker designed it. On tanks like these (vertical tanks) you can hold the sender outside the tank at the height it will mount to see how the arm will swing...that is to say when it will bottom out and when it will top out.
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