fuel gauge wackiness 1949 Chev 1/2 ton PU

[davidyavidy]

I'm a novice. From what I've read, the fuel gauge on this vehicle is a 30 ohm circuit requiring a 30 ohm sending unit. I installed one, but the gauge needle only moves about the width of the needle near the empty hash mark when the float is moved from one end of its travel to the other. I had a universal sending unit on the shelf that is 240 ohm resistance, and when hooking it up to the gauge, the needle works perfectly. At one end of the float's travel the needle is on the empty hash mark, at the other end the needle is just above the full hash mark. Perfect, except the gauge reads opposite of the floats position. When the float is up, the gauge reads empty, when the float is down, the gauge reads full.

 

What I need is a sending unit that's wired opposite of the one I have, OR reverse the wires on the back of the gauge. If I reverse the wires on the back of the gauge, will I damage the gauge? That is the question. Someone please help if you can. Thank you.

 

Dave

 

P.S. I've heard that Ford wires their fuel system opposite of Chevrolet. On one make, grounding the sending unit wire coming from the gauge will peg the needle empty while the other make will peg the needle full. Can't remember which is which, but since I have a Chevrolet, would installing a Ford unit remedy the problem?

[rsb]

Dave,

 

Reversing the wires on the gauge will not change the reading and may damage the gauge. I would investigate the ground path of the sending unit and check if the gauge is original, damaged, or has been replaced. If it were me, I would first disconnect the sending unit wire from the gauge. Then ground that terminal on the gauge and see what it reads. Next I'd pick up a 30 or 33 ohm (more common) resistor from the local electronics shop and wire it between the terminal and ground to see what it reads. You should see full deflection between E/F with these two tests. Once you have eliminated the gauge as the cause, you can work your way back toward the tank.

 

-R 

[Beltfed]

You can not reverse the wires from the sending unit. Might be better said...you can't reverse the wire. Only one wire to the sending unit. Other connection is through chassis ground or common.

As to what rsb said. I use a couple of variable resistors instead of just a fixed value one. 0-100 ohms and 0-300 ohms. Couple bucks apiece. Using these and an ohm meter you can calibrate your gas, oil pressure, and temp. gauges. Ain't electricy neat? Either way will work.

[Beltfed]

I should have noted that you can calibrate the gauges if they are the resistance type and not the bulb with capillary tube. Sorry.

[ply33]

If the universal sending unit worked but read backwards, then there is a chance you can reverse it. At least the universal ones with a resistance range that can be modified to work on my Plymouth are designed so that you can assemble them "wrong" and have the float arm on the opposite side of the resistor card.

[Guest MORRISGAUGE]

Your gauge is a 30 ohm. Your basic continuity test would be to ground the sending unit post and turn the ignition on to have the gauge read empty, then disconnect the sending unit post (open) to have the gauge read full. No sending unit is "universal", the 240-30 ohm range is Stewart Warner/VDO/Datcon. I would question if anyone has serviced your gauges previously and whether they are original?

 

www.morrisgauge.com

[davidyavidy]

To rsb, Beltfed, ply33, and MORRISGAUGE:

Thank you for your responses. According to the shop manual, the gauge is the dual coil type. I think I may have extra parts around including a gauge. I'll try the wire reversal on that one to see what happens. If I sacrifice it, so be it. Guess I'll need to go to an electronics store and purchase one or more variable resistors for testing. Not sure when I'll do this as I'm involved with several other large projects at the moment, but will try to keep all informed as to what I end up with. Thanks again.

To Beltfed:

I wondered about reversing the wires on the gauge, not the sending unit.

Dave

[Beltfed]

Dave, Sorry I missed that.

[Guest MORRISGAUGE]

Technically, switching the wires can damage the gauge, if it has a short. The posts are marked IGN and TANK or SENDER on the insulator. Your gauge also has a ground connection to the mount plate. With your ohm meter, check the reading from IGN post to ground (mount plate), Post to Post and Sender Post to Ground (mount plate). You could still have an issue, but that gives you a basic idea of continuity and transverse coil condition. Very often, the gauge loses continuity to the ground connection. Other than that, the sender is usually at fault and needs to be the correct ohm range (30 ohm) for the application.

[davidyavidy]

To rsb, Beltfed, ply33, and MORRISGAUGE:

Thank you all for your helpful information, but I finally figured out a solution. The "universal" 240 ohm sending unit was the trick with some modifications. The rheostat mechanism bolted to the stem, so I just flip-flopped it 180 degrees, and now the gauge reads correctly: float up gauge full, float down gauge empty. The other modification was to turn the stem 90 degrees from its original position. The stem is two piece for vertical adjustment to match the tank depth. I removed the lower portion (where the rheostat is fastened to) from the upper portion and installed a small corner brace between the two stem halves. I then matched the stem length and float rod length to the original sending unit. It's now installed and works great.

What puzzles me to no end is why the 240 ohm unit works while the 30 ohm unit does not. My friend has a second truck in the wings but wants to go to 12v on that one. I'll have to pay closer attention to everything next time around.

Thanks again everyone,

Dave