Perplexing "FUEL" light malfunction

[Dave WM]

11 minutes ago, Captain Obvious said:

You ought to be able to de-solder the lead from the can and then just pull the plastic cap out with the thermistor along with it. (If the whole thing isn't too fragile by now...)

yes I will do that, I have a solder sucker and to some solder braid standing by to use.

Edited August 3, 2020 by Dave WM

[Captain Obvious]

Yeah, I'm thinking that inside that black cap there is a ferrule crimped onto the end of the yellow wire. That ferrule would be crimped onto the wire at one end and have a flat disk with a small hole in the other. Stick the thermistor lead through the hole and solder it to the disk. Transition from the semi-rigid thermistor lead to the flexible stranded wire.

Of course, I've never even held one of these things in my grubby hands so it's all speculation and analysis from afar. 

[Captain Obvious]

I updated the pic above to show more details about how I think the crimp ferrule is used to transition from device lead to wire.

[Dave WM]

some items I noted, besides the wide open reading on the thermistor, the slider on the fuel gauge is actually 2 sliding contact points. the one on the WW resistance element AND the one that grounds the slider arm to the metal of the sender. I was having some shaky readings with my analog VOM so I cleaned with some contact cleaner, a little scraping, and then some deoxit and it worked much more smoothy going from around 7 ohms to about 80 ohms full spread.

back to the thermistor

 I tried various contact of the hot lead thinking maybe a corrosion point on the crimp from the wire to the terminal on the sending unit no joy, I cant get any resistance reading even with my HP analog VTVM on the 1 M scale. This makes me think it has to be disconnected as I cant imagine it being that open. another test will be to send a voltage thru it and see if the VTVM can pick that up. I have found this useful when trying to read extremely high resistances. Clearly this high a resistance is not usable but I like to test things before disassembly in an attempt to collect data.

doing all this before disassembly just in case its gets destroyed in the process of removal.

Edited August 3, 2020 by Dave WM

[Dave WM]

here you go. It seems like there is a meshed liner that acts as a filter for the gasoline that enters the chamber, then you can see the tiny disk shaped thermistor

[Dave WM]

it def was NPT I was able to hold the disk in between the ohm meter probs, got about 1.5k and it would drop when heated. I was not able to resolder the broken piece, I suspect it was a elec weld but not sure about that. Maybe a conductive epoxy. I cleaned it but was unable to get solder to stick. I suspect the material I was "cleaning" was some semi conductor as it sanded very easy leaving what looked like a metallic shiny surface. I am not sure what pure germanium looks like but maybe something like that.

[Dave WM]

https://www.amazon.com/Cher9-Values-Thermistor-Temperature-4-7K/dp/B083XSLGXC/ref=sr_1_5?dchild=1&keywords=thermistor+value+pack&qid=1596476471&sr=8-5

think I may pop for this and experiment with the 1, 2, and 3k to see how they behave.

[Zed Head]

On 7/28/2020 at 8:21 AM, dmorales-bello said:

 

Maybe the plastic is insulating the thermistor itself, allowing it to heat up over time.  If I had that spare one I'd be tempted to tug on that plastic piece.  Looks like it is slipped in after soldering, as a stabilizing piece.  I bet that it will slide right up that wire.

[zclocks]

Dave, 

Very nice. I've been following this thread as I have the same problem with my 75 280. I don't suppose there are any markings on the disk( had to ask)? The resistance is a good piece of data. 

I suspect the lead material is Tin or lead that may have been solder coated at one time. This is 45 year old technology and tin/lead is what they were using then. From  the looks of the broken /deformed lead on the thermistor wire it appears to be severely  oxidize -reduced. This is why you couldn't solder  it.

One last question. I think you have a side mounted fuel gauge in your gas tank. Were you able to remove the gauge  while the tank was in the car or did you have to drop the tank?

Thanks for all your work.

[Dave WM]

yes easy to do, get the car up, jack stands or ramps, what ever makes you comfortable (or both for security). I found removing the tank mount strap on the passenger side (sender side) gave me enough room to have good access to the retaining ring, and was able to snake it out without removing any hoses or dropping the tank. removing the wheel would be a good idea as well but I did not. You want the car up a pretty fair amount so you have room to maneuver the sendor (and your body while doing that). It may help to undo the fuel pump mount leaving everything hooked up, but I did not have to do that. Really depends on how flexible you are.

I suggest you use a grease pin or some other indelible marking to note the orientation of the sender before removing. Its hard to know for sure if you have the sender in at 12 oclock

Edited August 3, 2020 by Dave WM

[Dave WM]

I am guessing anything in the 3000 ohm (tested and works) to 1000 ohm (prob just come on faster) NTC small thermistor would work. The one shown in the pic is prob about 4mm in diamenter and about 1mm wide. You can see the relative size to the canister it fits in. I suspect you want something of the same approximate physical size, not only to insure fitment but maybe it is a way to get somewhere in the correct power/heat/ resistance curve properties as well.

[Zed Head]

On 8/2/2020 at 6:19 AM, Captain Obvious said:

 

The zcardepot spare part should still have its color code.  Might be good to know.