Perplexing "FUEL" light malfunction

[dmorales-bello]

hmmm well there you go, very close to what I got. for cold resistance.
So just that I am clear, right now with all hooked up and gas in tank say at least 1/2 way full, the light will come on after about 25 min and stay on?
If that is the case, seems like the next logical step will be to back probe, and check the lamp voltage from off to on over the 25 min period. will talk about that later, but its what I was doing in the video. You just have to find that plug, set the voltmeter to volts, and insert in the back so the probes contract the inside of the plug. I am assuming starting with the "2000m" to see if you get a reading if not switch down to "200m"

The malfunction is exactly as you describe it.
As far as testing voltage at the lamp plug, sounds good and I think I understand it.
Do you still want me to try all the different higher resistance bulbs with simple 25 minute runs to see which ones go on with a full tank?

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[Dave WM]

14 minutes ago, dmorales-bello said:

The malfunction is exactly as you describe it.
As far as testing voltage at the lamp plug, sounds good and I think I understand it.
Do you still want me to try all the different higher resistance bulbs with simple 25 minute runs to see which ones go on with a full tank?

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I think it would be a good idea to try the back probe the connector at the center console. This is the white connector that is on the lamp harness wire, and the connector that is part of the wire harness that runs along the firewall. Its the one you have to undo (along with a bunch of other) that when you are pulling the center console off. You know you have the right connector when if you UNPLUG it then back prob it should be 12v with the key in the "run" position. Its 12v since there is no lamp load plugged in. The 3k ohm resistance will not drop any voltage since the circuit is open (no lamp, and the meter is not enough of a load to effect it). As soon as you plug the lamp in with the probs still in place the lamp load will cause a current to flow and the voltage will drop due to the high (3k) resistance. The point to all this is to try and duplicate a known working system (mine). so far the resistance of the thermistor checks out the same, so I am curious to see if you have the same starting voltage (lamp load and battery will effect that), and then watch it to see how fast it ramps up. I will pm you my phone number if you want to talk thru it when you are ready to test.

[Dave WM]

here is a thought

is it possible the thermistor in the can is not actually coming into contact with the gas? on my thermistor the can has holes in it to allow fuel to enter and exit the can. IF for some reason this was not the case in the new sender, then the cooling effect would be lost. You should see bubbles come from the can when inserted into a fluid, and likewise fluid should drain out when removed. Not a lot mind you, but something showing the gasoline is actually coming into contact with the thermistor.

[dmorales-bello]

I think it would be a good idea to try the back probe the connector at the center console. This is the white connector that is on the lamp harness wire, and the connector that is part of the wire harness that runs along the firewall. Its the one you have to undo (along with a bunch of other) that when you are pulling the center console off. You know you have the right connector when if you UNPLUG it then back prob it should be 12v with the key in the "run" position. Its 12v since there is no lamp load plugged in. The 3k ohm resistance will not drop any voltage since the circuit is open (no lamp, and the meter is not enough of a load to effect it). As soon as you plug the lamp in with the probs still in place the lamp load will cause a current to flow and the voltage will drop due to the high (3k) resistance. The point to all this is to try and duplicate a known working system (mine). so far the resistance of the thermistor checks out the same, so I am curious to see if you have the same starting voltage (lamp load and battery will effect that), and then watch it to see how fast it ramps up. I will pm you my phone number if you want to talk thru it when you are ready to test.

Got it Dave, thanks. I'll try the test and call you if I get stuck.

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[Dave WM]

another test for the one on the bench. test the ohms, then stick the can in some hot water, see if it changes. I am trying to confirm the thermistor is actually seeing the fluid it is immersed in.

in the hot water the resistance should go down A LOT and it should happen fast (seconds). if the change is very gradual or not at all then you have the answer.

I think this test is more important than the prior suggestion, I suspect the back probing will only confirm that you have approx. the same starting voltage, and give a data point on how long it takes to go to max voltage and what that is.

this latest test addresses perhaps the only other possible cause assuming everything else is correct (hookup, voltages, thermistor properties). a lack of physical contact with the coolant (gasoline) fits the problem. the 25min to full on well I get there in about 5 min, could be the thermistor curve, but it should still work as long as the cooling fluid is allowed to come and go to activate the system.

Edited July 28, 2020 by Dave WM

[dmorales-bello]

Back probe test results with key in the "ON" position :

12.29 Volts without the bulb

18.9mV with the bulb

 

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[dmorales-bello]

Same back probe test but with the car engine running:
14.5V without the bulb
34.7mV with the bulb


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[Dave WM]

pretty much exactly the same I get. the only other test you can do on the installed sender would be to keep an eyeball on it and see how long it takes to start ramping up, it has to for the bulb to come on, but at this point I would say the only thing left is the thermistor is not really in the gasoline due to perhaps a blocked hole in the can or some other manufacturing defect that is insulating it from the cooling effect of being immersed in a fluid.

Edited July 28, 2020 by Dave WM

[Dave WM]

On ‎7‎/‎24‎/‎2020 at 4:59 PM, dmorales-bello said:

Here are are couple of pics of the Z Car Depot thermistor. It has no markings on it whatsoever. I must say that the whole sender unit seems to be a high quality reproduction. This thermistor issue is unfortunate.

BTW, the pics are from the first sender I received from them which is out of the car. When I notified them of the malfunction they immediately sent out another one which is currently installed in the tank. They are both identical.

is there holes on the top plastic part?

see the below OE one

[dmorales-bello]

There are holes on the top and bottom of the thermistor. After submerging it in water (another experiment I just performed and will post in a minute) the can fills with fluid which I can expel from the bottom holes by blowing through the top holes. So the unit is not clogged and fluid will flow in and out of it unobstructed.


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[Dave WM]

I am afraid I am at a loss for any other help. Let me know if you want to try the rheostat, but I honestly wonder if that will help. The whole system in predicated on a variable resistance. Try used a different light while monitoring the voltage at the lamp connectors. You want to drop that mV reading below what you are seeing now.

[dmorales-bello]

So I experimented by measuring resistance in the thermistor while exposing it to different temperatures.
Exposed to 78 degree "Florida early morning" ambient air, the reading was 2.2K Ohms.
When introduced into 100 degree water (as I imagine the temperature of fuel could reach during driving in 95 degree weather for a while? ) the reading dropped to 1.44K Ohms.
Finally, raising the water temperature to 124 degrees the reading dropped further to 0.94K Ohms.
That would seem to show that the thermistor is responding to the temperature around it, be it immersed in fluid or dry. My limited knowledge of electronics however, doesn't allow me to infer if those values are consistent with normal function as intended for this particular application.


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