Perplexing "FUEL" light malfunction

[Captain Obvious]

On ‎7‎/‎24‎/‎2020 at 1:02 PM, SteveJ said:

@Captain Obvious, do you mean like this?

Steve, LOL. No. I'm talking about the balancing act between the changing resistances of the thermistor and the bulb filament.

I measured the filament resistance of a couple of the 3.4W bulbs from the Z. When cold, the resistance is about 5.4 Ohms. If you calculate the power that bulb will consume when connected to a 12V source, you get about 31 Watts. 31 Watts!!! And the current that would flow through that 5.4 Ohm "resistor" would be about 2.4A!!

However, it doesn't work that way. The resistance of the bulb filament increases significantly when it heats up and glows. That's what I meant a while ago about the bulb being a PTC.

To show this effect, I connected the bulbs to a 12V source and measure the current draw while the bulb was lit. It was 220 to 240 mA. And if you back calculate the filament resistance, you find that the resistance has increased about tenfold from 5.4 Ohms to about 50 Ohms.

So as the resistance of the thermistor decreases (allowing more current flow), the resistance of the bulb goes up (disallowing more current flow). That's the balancing act I was talking about.

It's just a light bulb and a thermistor and it's giving me a headache. HAHA!!

[Zed Head]

I've always wondered why the low fuel light on my other Nissan, a Pathfinder, would start very dimly then slowly get brighter and brighter as the fuel level got lower when I got down past "E" (sometimes I like to live on the edge).  I could never figure out how it did that.  Seems to make more sense now.   That was a 1995 model.  I'd guess that modern cars still use the technology, it looks simple, effective, and cheap.

But, I still don't really understand why it would suddenly turn off, then begin the dim-brighter-brighter cycle again.  Maybe it has something to do with that knee.  Or maybe it was just a slosh of gasoline.

[Captain Obvious]

I bet that's just a slosh quickly dousing and cooling the sender. My DD does the same thing.

7 minutes ago, Zed Head said:

(sometimes I like to live on the edge). 

I always pictured you as that kind of guy.       Zed "Danger" Head

Edited July 26, 2020 by Captain Obvious

[Zed Head]

11 minutes ago, Captain Obvious said:

I always pictured you as that kind of guy.       Zed "Danger" Head

Thanks.  But I have to confess that there was some calculating behind it.  I knew my numbers and what should be left.  Still, it was thrilling...

[Captain Obvious]

No!   Own It!    

But I know exactly what you mean... Off quick, and slowly comes back. I'm sure you nailed it and it's just fuel sloshing onto the sensor.

[Captain Obvious]

Oh, and BTW, the discussion of bulb filaments changing their resistance dramatically as they heat up reminds me... Sorry for the diversion, but since we're so close...

The higher inrush current from a big-ish incandescent load (like headlights) isn't the most gentle thing that can occur to switch contacts. Between that, and the lack of arc extinguishing when you turn stuff off, you can eat up the contacts in the switch even though you aren't switching an inductive load.

@ETI4K  We were having a discussion some time ago and the million dollar question you didn't ask me is "OK, smart guy... Then if the headlights aren't an inductive load than why do the switches burn up like that? Huh? Why's that?" 

[Zed Head]

So much of the early electrical systems on cars were(was?) developed around a deep understanding of the fundamental properties of the materials.  Today, it seems like everything is handled with computer code.  So, the ballast resistor works on the same principle as the filament in the bulb of the fuel light, I think, right?  Using that increase in resistance with current for a control purpose.

Here's a stretch, another cross-reference.  Would a condenser/capacitor on the headlight circuit damp the spark across the headlight switch contacts, like the condenser on a set of points in a Kettering type ignition system? 

Just testing my brain, the switch will still get hot and break the solder joints just from the current, over time, I'd guess, so a relay is the best solution.  But the pitting might be reduced, which contributes to the heating effect.

[Dave WM]

did some more test after the video, the engine on (14v vs about 12.6 engine off battery voltage) resulted in about 8v at the back probe when the same test as the video was done. I filled up the tank this morn just to make sure I did not goof up my gas gauge. The light was coming on and off while driving to the station, gas slosh I presume, this was after adding about 2 gallons to the tank. I am pretty sure I was nearly empty as it took over 14 gallons to fill (so 2 I put in at home plus 14.5, 16.5 total, I must have been on fumes). So with lets guess 2.5 gallons in the tank it would come on and off, Gauge indicated "E" even with the 2.5 est gallons. Filled up it goes right to Full. I may have cocked the sensor a bit on the install. I think it should read a bit higher on empty., I will drive it around to the 1/2 mark, to make sure my gas is below the sensor hole on the side of the tank, then release the hold fast ring, and see if I can tweek the position of the sensor in the hole. Try to get a max reading on the gauge. fyi, the cold resistance of the thermistor reading it at the plug was about 3k which seems very high to me I=E/R 12/3000 or .004 amps, seems like it should be a lower resistance to allow a faster ramp up of the heat once its clear of liquid. I think there maybe other causes for the high resistance other than the thermistor. I should have soldered on lead from the thermistor can to the mounting clip, as noted before there was a fine white residue build up that I had to sand off. So my only fix was to clean that residue and then solder the loose wire that I presume was from the thermistor to the bottom of the can (it was just bent over making a poor contact).

[Dave WM]

Dr Dave, what the cold reading resistance of the thermistor in your new sending unit?

[Captain Obvious]

Dave, that's a good question about the cold resistance. That would be good to know. I guess I could check mine too without too much trouble. It's a little bit of a pain to take the stuff out of the hatch area to get to the connector, but if I get that curious, I'll do that. I bet there's a connector up under the dash I could use too if I dug into the wiring diagram.

I'm assuming your 3K is not correct. At 13V supply, that will dissipate just over 50mW. I don't think there could possibly enough internal heating at that low power level to change it's resistance. It would have to be uber sensitive, and have an almost vertical knee in the curve. I'm with you... I think you've got connection issues elsewhere.

[Captain Obvious]

22 hours ago, Zed Head said:

Today, it seems like everything is handled with computer code. 

So, the ballast resistor works on the same principle as the filament in the bulb of the fuel light, I think, right?  Using that increase in resistance with current for a control purpose.

Yeah, that's very true. If I were solving this problem now, I would throw a couple hundred thousand transistors and a one time programmable chicklet at it and call it a day. In fact, that's what I did with my temperature/oil pressure warning lamp. Microcontroller reading a D/A and using digital filtering to weed out spurious false positives.

As for the ballast resistor, it has a completely different purpose. They don't want the ballast resistor to change value, they just want it to limit the current to protect the coil and whatever is pulling it to ground. The reason it's necessary is because of the way inductors work.

When you first apply voltage to an inductor, it acts like an open circuit. Infinite resistance. Won't allow any current to pass.

Then as time goes by, the resistance of the inductor goes down, and it allows current to pass. More and more and more current as time goes on. And in fact, with a theoretical perfect inductor, after enough time, it appears as a dead short. Zero resistance.

So the problem is, if there isn't something somewhere in the circuit that limits that current at the upper end, it can reach the point where it gets so high that it will damage something. The ballast resistor limits that upper end of the coil current.

You see, when you have points, you really have no idea where the points were when you shut off the motor last time. Points may be open, or points may be closed. If you turn the key on and don't crank the engine, you will quickly cook your coil or your points if you don't limit the current.

And the early electronic ignitions systems sit in a stable default state with their output transistors turned on. There is no current limit built into it at the upper end. So the same thing goes there... You could cook the coil if you don't have a ballast resistor. It wasn't until 78 that they started making the ignition modules good enough such that you didn't need the ballast resistor. I haven't played with one, but I assume they did that by building in an upper end current limit into the output stage.

[dmorales-bello]

Dr Dave, what the cold reading resistance of the thermistor in your new sending unit?

I didn't check that because I didn't perform any readings with a voltmeter. I simply checked if the "warm" thermistor triggered the light and that once the thermistor was dunked in fuel the light turned off.

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