Perplexing "FUEL" light malfunction

[dmorales-bello]

milli often used in electronic terms as 1/1000th
so 1000mA = 1 amp
the math I used was ohm law current in amps=volts / ohms
so you had .943k ohms or 943 ohms (in resistance K is 1000, confusing eh?)
12 volts/943 ohms=
.0127 AMPs
.0127 AMPS * 1000 to convert to milliamps
12.73 mA rounded to 13mA
Lots of times values are stated as milliamps or mA due to the smallish current and its easier to say 130mA that .013Amps
 

Ok, so what's the practical application in this case?

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[Dave WM]

since your reading of .93k at even 125f there is no way that light should come on CO says more like 230ma to glow full so I would guess you need prob a min of 75ma to notice it.

that being the case the thermistor must be getting much hotter that 125f. more on the order of .15k or 150 ohms to flow about 80mA just to get it to light dimly.

So the issue now become if you insert a pot to lower the resistance in an effort to keep the light from coming on, anything more that 150 ohm in series with the thermistor will prob keep it from lighting no matter what the thermistor is doing.

therefore any added resistance should be no more that 150ohms and clearly more than 0 (otherwise it has not benefit). I will dig around my pot stash and look for a 100 ohm WW rheostat. I have some that were used in old color convergence boards for TV's in the vacuum tube era.

[dmorales-bello]

since your reading of .93k at even 125f there is no way that light should come on CO says more like 230ma to glow full so I would guess you need prob a min of 75ma to notice it.
that being the case the thermistor must be getting much hotter that 125f. more on the order of .15k or 150 ohms to flow about 80mA just to get it to light dimly.
So the issue now become if you insert a pot to lower the resistance in an effort to keep the light from coming on, anything more that 150 ohm in series with the thermistor will prob keep it from lighting no matter what the thermistor is doing.
therefore any added resistance should be no more that 150ohms and clearly more than 0 (otherwise it has not benefit). I will dig around my pot stash and look for a 100 ohm WW rheostat. I have some that were used in old color convergence boards for TV's in the vacuum tube era.

Excellent explanation! Got it, thanks!
BTW, the bulbs will arrive this afternoon and I'm ready to test them also.

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[Dave WM]

roger, I have a 150 ohm WW pot think its 2 watt rated (it will get warm, so really just use to size the correct fixed resistor once the value is known) ready to send. Would like Capt O to confirm my value as a good starting point (any higher resistance I think would render the light dark not matter what the sender does). You would want to use some jumper wires (3) to insert in series with the connector plug that you back probed. 1 jumper is a direct pass thru, the other two connect the pot between the remaining plug lead. Start off with max resistance and see if the light stays off, if it does, then you can sneak up on the lower resistance until it just starts to light, back off and you should be good. Note the resistance value and substitute the pot for a 5 watt WW resistor. This all assume it actually works and you are not able to get it to work with the other bulbs.

incase it was not already covered regarding bulb experiments:

On the other bulbs, remove the current installed one, check the resistance , then check the replacements. pick the one that has the highest resistance. We are trying to limit the current to keep the thermistor from self heating beyond what the fuel cooling can dissipate. install the highest resistance bulb and try again. This is the same as adding the pot in series and adding resistance with it.

Edited July 29, 2020 by Dave WM

[dmorales-bello]

roger, I have a 150 ohm WW pot think its 2 watt rated (it will get warm, so really just use to size the correct fixed resistor once the value is known) ready to send. Would like Capt O to confirm my value as a good starting point (any higher resistance I think would render the light dark not matter what the sender does). You would want to use some jumper wires (3) to insert in series with the connector plug that you back probed. 1 jumper is a direct pass thru, the other two connect the pot between the remaining plug lead. Start off with max resistance and see if the light stays off, if it does, then you can sneak up on the lower resistance until it just starts to light, back off and you should be good. Note the resistance value and substitute the pot for a 5 watt WW resistor. This all assume it actually works and you are not able to get it to work with the other bulbs.
incase it was not already covered regarding bulb experiments:
On the other bulbs, remove the current installed one, check the resistance , then check the replacements. pick the one that has the highest resistance. We are trying to limit the current to keep the thermistor from self heating beyond what the fuel cooling can dissipate. install the highest resistance bulb and try again. This is the same as adding the pot in series and adding resistance with it.

Got it. Thanks again.
I hope CO can confirm values.

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[Captain Obvious]

I got data.

I applied voltage to a FUEL warning lamp indicator (red lens and all) and "analyzed" the brightness. I came up with the highly subjective brightness levels of:

Barely noticeable
Clearly noticeable
ON
Attention getting
Angry

I measured these values in both a "poorly lit room" and a "well lit room", and here's what I got. It's neat (geekwise) that you can clearly see the filament resistance increasing as the current (and temperature) goes up:

It appears that what you really want is for the thermistor to work the way it's supposed to. Failing that, I would try maybe a 50 Ohm pot in series and see what happens? I'll leave it to Dave to calculate the power dissipation in the pot and determine if it's feasible.    

Edited July 30, 2020 by Captain Obvious
typo

[Dave WM]

roger that, so any more than 50ohms added it a waste of time as it will keep the light from functioning. I have my doubts about the feasibility of the pot fixing the issue, seems as you say it really needs to work as it is supposed to (correct specs on thermistor), but short of that I have a 50 ohm WW pot ready to go to Dr. Dave. He is messing with some other bulbs now. I think I will get a cold filament resistance reading of my car and report back.

lamp at leads that plug into wire harness resistance 6.0 ohms

Edited July 30, 2020 by Dave WM

[Captain Obvious]

20 minutes ago, Dave WM said:

lamp at leads that plug into wire harness resistance 6.0 ohms

Yup. I measured mine at approx. 5.4 cold.

Maybe yours is a couple hundred milli-Ohms higher because you're in hot sunny FLA.  

[Dave WM]

I am posting up a video, will link here, showing my hookup and how it effect brightness of the lamp. I can control the horz I can control the vertical, ….

ok CO what show is it...

[Dave WM]

 

[dmorales-bello]

I received 5 bulbs with different ratings yesterday. They do not show the Volt or Amp rating on them as other bulbs do. Therefore I decided to identify them by the measuring each bulb's resistance on the bench (cold resistance?).
Once again I ran the car with the original 12V 3.4 amp bulb in the "fuel" light housing with the tank at about half full. Once again that bulb lit brightly after about 25 minutes. With the car still running I swapped the stock bulb for each one of the new bulbs. All of them illuminated immediately although dimly. The difference lies in how dimly each one lit up. In general, the bulbs with the highest cold resistance were dimmest. I was also measuring voltage at the bulb harness for each bulb. That voltage varied when each new bulb was placed in the socket but stabilized within a minute or less. I identified the bulbs from #1 to #6 in order of decreasing resistance. Bulb 1 being the new bulb with the highest resistance and bulb 6 being the stock bulb which had the lowest resistance.
RESULTS :
BULB 1: 19.8 Ohms, weakest glow, stable at 2.9 V.
BULB 2: 12.7 Ohms, weak glow, stable at 3.1 V.
BULB 3: 11.5 Ohms, very weak glow, stable at 4.0 V.
BULB 4: 11.4 Ohms, slightly brighter glow, stable at 3.3 V.
BULB 5: 6.3 Ohms, brighter glow, stable at 5.8 V.
BULB 6: 3.8 Ohms, very bright, stable at 7.7 V.

The glow from bulbs 1, 2 and 3 can barely be seen through the red lens of the housing even inside the darkened cockpit (I was outside at 9pm. Wife thinks I'm going insane! ). With the roof light on, you can't detect light through the lens.

The glow from bulbs 4 and 5 can be readily seen through the red lens in the dark (5 more than 4) and are somewhat visible with the roof light on.

Bulb 6 (stock) is very blatantly visible in the dark and quite bright with the roof light on.

I ruled out the stock bulb (6) , and the new bulbs at each extreme (1 and 5) for being too dim or too bright.
Bulbs 2, 3 and 4 are dim enough that are very hard to be seen through the lens, day or night. But should be brighter and visible when the thermistor is above fuel and it's resistance drops (I think).
I placed bulb 3 in the housing and closed everything up. I will run the tank dry and see how the light behaves. I will have bulb 2 and 4 as alternatives.

I know my "report" is not very technically correct but it kinda gives me options.


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[dmorales-bello]

 

Thanks for the video once again, Dave. I would think the pot could be used with the lower resistance bulbs. If I were to use it with the higher resistance bulbs they would probably never light up even with the thermistor out of the gas, correct?

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