Perplexing "FUEL" light malfunction

[Captain Obvious]

I don't think it's as simple as picking a 3K Ohm thermistor out of a catalog and tossing it in there. The 3K is the room temp resistance, but that's not the only important reading. You also have to know how it responds to temperature.

@Dave WM If you had an environmental chamber, you could rig up some way to temporarily hold a test lead to the other side of your thermistor remains and plot it's temperature characteristics.    

[Zed Head]

This is why someone like me would just go to the wrecking yard and get some 1995 Pathfinder thermistors.

http://www.sfu.ca/phys/231/archived, 041-061/061/data_sheets/thermistor.pdf

Edited August 4, 2020 by Zed Head

[AZDatsun]

I just use my fuel gauge instead of trusting the light


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[dmorales-bello]

So here's a preliminary report.

I tried out the stock bulb (#6 on my list) once again just to have a baseline value. It has a resistance of 3.8 ohms. As had happened before, the light went brightly on after running the car for 25 minutes. Easily seen with the bulb inside the housing even under bright sunlight.

Next I tried bulb # 5 which has a resistance of 6.3 ohms. After 25 minutes of running the engine that bulb illuminated very faintly, very hard to detect in daylight and subtly seen in the dark car. This one could work the way @Dave WM suggested which is as a monitor that the system is operational when lit dimly and as a low fuel warning when lit brightly. However, I've yet to run the tank down to empty.

Next was bulb # 4 with a resistance of 11.4 ohms. It also illuminated after 25 minutes or so but the glow of the filament is so faint that it can only be seen directly when the bulb is out of the housing. Once inside the housing it's invisible in daylight and almost undetectable inside a dark cabin. If this bulb (as predicted) will glow brighter once the fuel level falls below the thermistor, this would would work well as intended.

With this trend, one could extrapolate that the remaining bulbs could illuminate even more dimly (directly proportional to the increasing resistance values) at the 25 minute mark as well but might be a bit dim even when the fuel level drops below the thermistor, rendering the low fuel warning function ineffective.

Again, I will finish reporting once I run my tank empty but seemingly there are 2 alternatives so far I could live with ?.

Edited August 6, 2020 by dmorales-bello

[SteveJ]

Can you figure out the wattage of those two bulbs? It may help someone else who buys the ZCD sending unit.

[dmorales-bello]

Can you figure out the wattage of those two bulbs? It may help someone else who buys the ZCD sending unit.

These bulbs don't have the wattage in their specs. As a matter of fact they have nothing more than a serial number and the "made in China" stamp. I'll be happy to test them with my dvm of you can explain how it's done.
Additionally, I plan to contact Zcar Depot to explain the problem and give them the solution. They should simply include the "correct" bulb along with their sending unit. I want to finish testing the bulbs with an empty tank before drawing the final conclusions.

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[SteveJ]

Does the serial number correspond to the information on the website description?

[dmorales-bello]

Does the serial number correspond to the information on the website description?

Good idea. I'll check and post.

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[Dave WM]

went to the JY today (yea no cover charge!) and found a 1991 Toyota, 4 door sedan, the tank sending unit was easy to get at so I pulled it and found the same style thermistor but it was inside a plastic holding tank (like 2 oz size). removed it and ran some test. It was pretty grody so I sprayed some electronics cleaner in it  but could not get the light to go out unless I sprayed the cleaner direction into the tiny holes. I used some fine wire to open them up but I think there is still junk inside that is causing issues with getting the fluid in and out.

I ended up opening two holes (top and bottom) to about 1/16" with a drill, now it works reliably. will post up the video in a few. I wanted to see if warm fluid (in my case water since I don't want to smell up the house with gas) would keep it from coming on so I used hot tap water, about 110f.

 

Edited August 8, 2020 by Dave WM

[Dave WM]

I was able to peel down a strip of the can to get a better look, while I was not able to focus the IR gun on the tiny thermistor (looks like the size of a 1/8 watt resistor) I was able to hear it hiss as I dunked it in the water. 

From what I can see it seemed to have a dark burned looking band in the middle. Further test show its resistance to be polarity sensitive at least with the DVM. It would range from 1.6k to 2k just by swapping leads. the VOM did not seem to notice it but being analog and the scale pretty small it may have moved a bit. the VOM showed something like 1.7k as did the VTVM. I suspect the solid state nature of the device may be a factor in this. I am not sure if I can salvage the thermistor with out destroying it, at this point the can is toast so I would have to if I wanted it in a uncompromised can. Not a big deal but would be fun to try.

another interesting observation was at 1st I was unable to get any resistance reading with the DVM, it would just toggle between moHms and koHms, not settling on a scale (auto ranging DVM) I did not have this issue with either of the fix range VOM or VTVM. After a few min (maybe when it had cooled down as I tested right after using it in the light circuit) I was able to get the 1.6-2k readings (depending on polarity) with the auto ranging DVM.

Edited August 8, 2020 by Dave WM

[Dave WM]

some of the harvesting and transplanting operation. I pigtailed the remnants from the hot side of the can inserted one lead of the thermistor, crushed and then soldered. Removed the old filter material (copper weave I think) soaked in vinegar to remove the scale, then reassembled soldered up the other lead to the can. The OE can works much better as far as getting the fluid in and out of the can.

 

I am going to clean and touch up that solder on the bottom. I thought I had the lead cleaned and tinned but it looks like it could be better.

Edited August 8, 2020 by Dave WM

[Zed Head]

Maybe they are designed to drain and fill slowly so that the light does not go on and off as I described earlier, from gas sloshing around. 

 

The little gas pump/robot symbol light came on, were running out of gas!  Wait, it's off...  It's on again!  Nope...wait...