break in continuity when test gas sending unit

[64longroof]

Hi, when measuring ohms of the fuel sending unit of my 64 Dart, the ohm readings are a correct 15 and 78 at the far ends of the gauge, but I have to jiggle the float arm to get a steady reading, otherwise the continuity breaks. It's especially hard to get a middle range , or half tank reading. Ohmmeter leads are clamped to clean metal with Jr. Visegrips....any ideas??? Thz

 

[67Dart273]

Well it's "simple" the thing has problems. This is just a simple wirewound rheostat, that is a moving contact arm sliding across a coiled resistance wire.

So any poor contact in the moving part due to loose, rust, wear, corrosion, loss of arm tension, whatever will cause a problem

First tho, LOL make darn sure your ohmeter leads are making GOOD contact. I always use alligator clips.

 

[RedFish]

You can open the little box and see if there is anything repairable. Try bending the sliding contact for better conduction. Theres a brass contact inside that slides against a resistance wire. That contact wears away until eventually becomes like a fish hook of sorts and snags on the wire. At that point its stuck wherever it is.

 

[64longroof]

Thanks for responding. Is that sweeping contact supposed to have that little hole at the tip, or did it wear through after 200,000 miles? I sanded and bent contact so it would contact the
windings more firmly...some winding loops did snag the sweeping contact , so I slid a finely-whittled bit of toothpick under the winding s from the backside of the winding s' support plate to tighten up the windings. I also twisted the sweep so it's tip doesn't snag windings.

 

[cudascott]

The hole at the tip is wear through

 

[67Dart273]

Yes. lol. Even though I'm a very low buck backyard guy, I don't think I'd try and rebuild one, personally.

 

[RedFish]

wont work. Gasoline will eat the tooth pick and contaminate your fuel.
That contact never was a solid spoon shape with single small contact point. More like fillet of a donut. This lets it contact the wire at 2 to 6 points at all times. It would wear thin and through to a 'C' shape and snag the wires. It couldn't stretch the wire and raise it from the phenolic board either. That board shrinks with age. The change is amplified at wider span of board so longer wires appear more loose.

 

[moe7404]

try some marvel mystery oil in the fuel

 

[TrailBeast]

As already stated, the hole in the end of the contact arm is where the contact button used to be connected to the arm.
I have also seen the wound wires worn through, and this makes it where if the gauge reads at all it only reads the (upper range or lower range) can't remember which.

The problem with trying to fix that arm by attaching another button to the end of it is that if you heat that arm (solder the button on) the arm looses it's spring ability, and you will have the same problems but for a different reason.
Then in contrast, somehow otherwise attaching the button messes with the ohms and causes a wrong reading.
(like if you glued it on with something that has continuity, but not enough)
Then there's the whole fuel eating some materials thing to consider also, so by the time you get through trying to solve all of these issue's you are far ahead by replacing the sender anyway. (even with it's own can of worms being involved)

It sucks I know, and every time we replace a sender unit it never seems to work quite right again.
I attribute this to sloppy quality control in manufacturing, because even senders with advertised correct ohm readings seems to have some problem or another in it's range when used with the older gauges. (I'm sure some worked right the first try, but it's rather rare in my own experience.)