The Stovebolt Site Forums The Shop Area The Electrical Bay Using an Ohmmeter to check fuel level?

None of my gauges work including the fuel gage, probably from the 12v conversion somebody did, I was wondering if I could simply check the fuel level using an ohmmeter? I know the float is supposed to be 0-30 ohms, assuming 30 ohms is full, and 0 for empty. Where would be the easiest place to connect a meter? I just got my 4100 on the road and have about 50 trouble free miles on it so far, but have no idea as to my fuel level. I have aftermarket gauges for rpm, oil pressure, temp., and volts, and eventually I might try and fix the gas gauge, but in the meantime looking for a easy way to check it from trip to trip.

If you do that, be sure that nothing but the ohmmeter is connected to the tank sensor. An ohmmeter has its own internal voltage supply so if any voltage from the vehicle system is present the meter will give an incorrect reading, or the meter could be damaged. I believe the meter works the other way- - - -full at 0 ohms, empty at 30.
Jerry

I am certain Jerry knows more about the 4100, but in the 3100 full is 30 ohm and 0 is empty. Or at least it is that way on the 1952 model. You can check the fuel level with an ohmmeter if the prior owner didn't burn out the sender with his 12v conversion. If he connected 12v to it, it could have been fried and that means you'll get no ohm reading (open circuit). If so, you might want to remove the sender and see what is happening there. I will tell you rebuilding the original sender isn't easy. I've tried it and the problem is the shaft is steel and the housing is some sort of potmetal which over time wears out to the point the wiper flops around and only makes occasional contact. If you need a replacement sender, there's a guy on eBay selling a decent 30 ohm sender. But as I posted a month or so ago, some of the senders sold by some of the suppliers are junk. The makers (whoever they are) use 90 ohm senders and drop blobs of solder on the windings to reduce the ohm rating. You can see this here:

http://i1154.photobucket.com/albums/p535/JonGoodman/IMG_8579_1.jpg

This was bought from a well-known supplier of old truck parts and it never worked right. I'm sure you can see why.
Jon

Jon, it's been quite a while since I've been into the circuitry on a balancing-coil gauge, bit if the dash unit goes to "full" when the tank sensor wire is grounded, it's likely that it's seeing zero resistance or close to it. Real hands-on experience trumps what seems to be logical every time, though!
Jerry

Hi Jerry,
Logic sometimes seems to send me off on the wrong track. I had to sit and think about this whole thing a few weeks/month or so ago myself. And even then I wasn't completely positive until I hooked everything up to see how that gauge read. So that we'll have them here, I've taken 4 images and I'll leave them on my photobucket thing. This is the best of all senders I've found because (a) it works by wiping a vertical plane. Most of them as you know work by wiping a curved surface. To me vertical is better. (b) as you'll see it is accurate to 30 ohm. Most of the ones I've seen might read somewhere between 28 and 45 ohm. (c) it wasn't expensive and (d) the moving parts are nylon so they ought to last. It is the sort of sender I think a detail-conscious guy like you would appreciate.

Here is the sender in full position:
http://i1154.photobucket.com/albums/p535/JonGoodman/In%20full%20mode.jpg
Here is the ohm reading in full position:
http://i1154.photobucket.com/albums/p535/JonGoodman/Full%20ohm%20read.jpg
Here is the sender in empty position:
http://i1154.photobucket.com/albums/p535/JonGoodman/In%20empty%20mode.jpg
And here is the ohm reading in empty position:
http://i1154.photobucket.com/albums/p535/JonGoodman/Empty%20ohm%20read.jpg

Now having said all this, the tank in my truck is baffled horizontally what looks to be about 2 inches from the bottom. This is a new tank made in Canada and it seems like they did a good job. I didn't notice if the old tank was baffled this same way, but it had developed a rust hole which meant it wasn't going to be used any longer. Net effect obviously is that when the gauge says empty, there will still be a gallon or two of gas in here. Not a problem, but a fact. The sender in empty mode can't fall to the bottom of the tank.
Jon

Good info, Jon! Thanks! A balancing-coil gauge uses two wire wound magnets with the needle pivot between therm. One has a constant current flow provided by either the ignition switch or a voltage-reducing circuit, and the other one has a variable current produced by running the current through the variable resistor in the gas tank. The combination of constant magnetism from one coil and variable magnetism from the other changes the position of the gauge needle. Depending on how the coils are wound and the polarity of the magnetism they produce, the needle changes position to indicate the level in the tank. It's a really a pretty simple system once the physics and electronics are understood.
Jerry

Happy to post it, Jerry. The eBay seller's ID is renbusan in case anyone needs or wants to get one of those senders. As of this morning he has sold 585 of them. They are $28 and that includes shipping in the USA.

Jon

I finally got around to fixing my fuel level sender. I should have diagnosed it prior to ordering a new sender and saved $30, since all I needed was a float, which I robbed off of the new sender. The original sender works fine now, but not the gauge. Using an ohmmeter works so well I might just install a cheap meter in the truck somewhere I can easily check when needed. With the meter I know exactly how much gas I have right down to the gallon, or less. Ohms X .583 = gallons.
Hopefully I can get partial credit back on the new sender minus float, or I'll have another new part I'll never use added to my collection.

I had the same problem on my truck, float filled and sank
However being on a farm with a PO box no one ships to up in Canada where delivery times would take forever I used copper from an old stove top boiler pot and pounded out a new float from scratch, seems to be working well so far.
Related note, in my 50 truck the gas gauge in the dash is suppose to be a for a 0-30 ohm sender, and my sender is still perfect 0-30, but through the years the gauge drifted and was now expecting 0-4x ohms for full scale, turns out if you put the sender and gauge together on the ground with wire clips to the battery you can crack the tiny adjustment nuts on the back of the gauge and tweak it back to reading full when the sender just reaches the top. There is lots of adjustment range, you could prob match up an original instrument gauge with many aftermarket senders.

By using an ohmmeter I was able to calculate my fuel mileage from a 13 mile round trip to a car show yesterday. In 13 miles the tank dropped 2.7 ohms x.584 = 1.57 gallons of fuel used to equal 8.28 MPG.

There are hundreds of digital programmable fuel gauges out there for not much money. Why would you mess around with using an ohm meter with its associated problems?

Autometer makes a very good 0-30 ohm gauge that can mount under dash.

I have a yardstick behind the seat, 2.5 gal per inch

Ed

I picked one up off of Amazon. Works good with the stock 0-30 ohm sender on 12v. It's very accurate.
Mounted it under the dash on ther left side of the steering wheel.
Pic