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dmorales-bello

Perplexing "FUEL" light malfunction

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    As some of you might remember, I swapped my original fuel sending unit on my "78 280Z a few weeks ago. Although the original sender measured fuel level adequately, the "FUEL" warning light had stopped working a couple of years ago so I decided to spend some quality quarantine time swapping it for a new reproduction unit from Z Car Depot. I had already tested the yellow/blue lead and assured I had continuity from the "FUEL" light in the dash all the way back to the sender plug.

    I plugged in the Z Car Depot sender to the harness before inserting it into the tank and upon turning the car key to the "ON" position the "FUEL" illuminated and I could change the position of the needle on the fuel gauge by moving the floater arm on the sender. Everything seemed to work as it should. Once I put the sender into the tank and locked it, the needle in the dash positioned itself to the right (I had an almost full tank of gas) and the "FUEL" warning light turned off (as it should). Needless to say I was very happy with the way everything went.

    I drove the car for the first time since the sender swap about a week later and went on a nice 25 mile run. After driving for about 15 minutes the "FUEL" warning light came on! The tank was still pretty much full and the needle on the gauge was close to the extreme right. I drove another 15 minutes to get home and the "FUEL" light did not go off until I turned off the ignition. I turned the key back to the ON position to power the fuel gauge and the "FUEL" light did not illuminate BUT upon turning the key to START and running the engine the darn light came back on.

    I performed the same routine a few days later and the malfunction remained exactly the same. "FUEL" light comes on after the car has been running for about 15 or 20 minutes. The tank being over half full. The "FUEL" light will turn off when I shut the engine it and will remain off even with the car key turned to "ON" with the fuel gauge working but will illuminate as soon as I start the car again. After the car sits overnight the light will be off until the engine runs for about 15 minutes and the whole "Groundhog Day" scenario repeats itself. In other words, the engine needs to be running for the malfunction to occur. Leaving the key in the "ON" position (which powers the fuel gauge) will not trigger the "FUEL" light. I'm totally stumped!!

    I contacted Scott at Z Car Depot and he said he hadn't had that malfunction reported before and he immediately sent out another sender. Amazing customer service!!

    I installed the "new" sender the day it arrived and the malfunction remained exactly the same. I made sure all the contacts on the plug were clean, applied dielectric grease, looked for anything that might look strange, successfully tested the sender by plugging it into the harness before putting it in the tank and nothing changed. I'm even more perplexed by the malfunction. As always, any help from the vast knowledge base will be greatly appreciated.

    Keep safe everyone. 

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    15 minutes ago, dmorales-bello said:

     The "FUEL" light will turn off when I shut the engine it and will remain off even with the car key turned to "ON" with the fuel gauge working but will illuminate as soon as I start the car again. After the car sits overnight the light will be off until the engine runs for about 15 minutes and the whole "Groundhog Day" scenario repeats itself. In other words, the engine needs to be running for the malfunction to occur. Leaving the key in the "ON" position (which powers the fuel gauge) will not trigger the "FUEL" light. I'm totally stumped!!

    As I understand those sensors they are resistive elements that heat up to actuate the light.  I don't really know how they work, but when they're covered by fuel they stay cool and when they are exposed they heat up.  Something like that.  I might be completely wrong.

    So, one thing that would cause more heat would be more current.  And when the engine is running you'll have alternator voltage instead of battery,  so, more current.

    You might check your system voltage with the engine running.  Maybe your alternator is going bad and overcharging.

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    Posted (edited)

    Hi Zed Head, you are correct in the basic function of the sensor. From research on this site I've learned it is a "thermistor" which will close a circuit when it stops being cooled, in this case, by the "cold" fuel in the tank. The previous owner of the car changed the factory alternator to a 105 amp unit form Z Car Specialties in 2014 which (as per the voltmeter in the dash cluster) is charging a tad over 14 volts at idle. Isn't that within normal range? I have no other electric snafus at this time. 

    Edited by dmorales-bello

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    Posted (edited)

    I thought it was worth a check.  Most of today's regulators regulate about 13.8 to 14.3 so yours looks right.

    Edit - The following is wrong...

    Since "open circuit" turns the light on, maybe check your wiring connections.  There might be one that is by something that gets hot when the engine is running.  Maybe by the exhaust system or on the floor or even back by the tank.  I know somebody that had a BMW with a fuse holder in the trunk that would get hot and go open then close when it cooled.  It killed all electrical, he'd just have to wait to get going again.  Took him a long time to figure out.  

    Edited by Zed Head

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    its a resistor that changes resistance with temp. So you have a constant current flowing thru it (its in series with the bulb to the battery). Current is always flowing generating some heat in the thermistor, but since its immersed in fuel the heat it soaked away keeping the temp cool and the resistance too high to allow enough current to flow and thereby causing the light to be seen (there is always a small current flowing thru the filament, just not enough to cause it to glow). With the gas gone, the thermistor no longer has its heat sink, so it starts to heat up, resulting in more current to flow (and more heat by the way) until the is a large enough flow in the series with the lamp for it to begin to glow. You need the correct incandescent style bulb for it to work, including the correct wattage bulb. Its a VERY simply circuit as long as the correct bulb is used.

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    another useful purpose of these handy devices in in vintage electronics to soften the start up of power supplies. there is often a surge current when the power switch is closed, and some of those power switches are very hard to find. So to extend the life of them its common to put a thermistor like a CL-90 in series with the power line. IIRC they present with about 100 ohms at typical room temps, enough resistance to significantly reduce the surge current on power up. As current flows thru them they heat up (quite hot actually) and the resistance drops to just a few ohms. This all happens in about 10-15 seconds and is not noticeable on most tube amps. they are good for about 2 amps total which works well with most of the typical tube amps of the day.

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    Dave WM, the bulb that's in the "fuel" light housing is the stock incandescent bulb. It's round in shape. At least that was the bulb in that housing when I purchased the car and one of the very few incandescent bulbs I didn't change to LEDs.

    Sent from my SM-N950U using Tapatalk

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    Posted (edited)

    Told you I didn't really know how they work.  Mine never went on, I've never seen it.

    Here's the bulb specs.  

    Maybe.

    image.png

     

     

    Edited by Zed Head

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    Maybe ambient air temperature is contributing. Warm fuel and all...

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    Ok I have to ask, you are sure you have the mount wire correct and the thermistor is in the tank such that its near the bottom?

    Next check the function of the plug by shorting out the wire harness, there is a common neg lead, connecting this to one lead will turn on the fuel light the other will peg the fuel.

    The last thing you can test would be to actually take out the sending unit, hook it up and dip the thermistor in a glass of water, make sure everything works as it should while you can observe the effect of the thermistor in and out of the water.

     

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    One mine the onset of the light is gradual very dim and slowly brighter as the current increases. Driving the car around corners/stopping/starting, all has an effect a the gas sloshes around, so it will go out dim come back on etc.... My setup has it so it will start to light at about 14 gallons empty (that is what it will take if I fill up so I guess I have about 2 usable gallons left).

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    1 hour ago, Patcon said:

    Maybe ambient air temperature is contributing. Warm fuel and all...

    I thought about that as well but it's been 94 degrees at most. The same thing has happened during a morning drive when the temperature was 84 degrees. Not what I would think of as extreme temps.

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    Posted (edited)
    22 hours ago, Dave WM said:

    Ok I have to ask, you are sure you have the mount wire correct and the thermistor is in the tank such that its near the bottom?

    Next check the function of the plug by shorting out the wire harness, there is a common neg lead, connecting this to one lead will turn on the fuel light the other will peg the fuel.

    The last thing you can test would be to actually take out the sending unit, hook it up and dip the thermistor in a glass of water, make sure everything works as it should while you can observe the effect of the thermistor in and out of the water.

     

    I'm not sure what you mean by having "the mount wire correct". The factory connector only fits one way.

    The whole unit fits only one way and straight into the tank which was full of fuel so yes, the thermistor must be in the fuel. As a matter of fact, with the unit plugged into the harness but not installed in the tank, the thermistor triggered the "FUEL" light on. It went out as soon as I introduced the sender into the tank as the thermistor hit the fuel.

    I will run your suggested check by plugging in the connector with the sending unit out of the tank and putting the thermistor in water (alcohol might be better perhaps) and also shorting the leads to the common negative wire to check function.

    Is there a chance that perhaps the rating on the thermistor is wrong and therefore triggers the light while fuel warms up a bit from the heat outside plus whatever other heat sources could be involved (engine?, exhaust? etc...)??

    Edited by dmorales-bello

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    I got no silver bullet. The only explanation I have is what you already mentioned... Sounds like the thermistor is not quiiiiite right for the application. CLOSE, but not enough headroom for tolerances and environmental stressors like ambient temp and system voltage.

    And as Zed mentioned, why it only happens when the engine is running is probably because the system voltage is higher then. That extra volt-n-a-half matters.

    Thermistors come in all sorts of base resistances and with all shapes different curves. For this application, you want one with a "knee" in the curve as opposed to linear, and you want the knee to be at the correct temperature. I'm thinking that it's too close to the limit most of the time, and the combination of ambient temperature and system voltage (when the engine is running) puts it over the edge.

    @Dave WM, Did you find any specs at all about the thermistor? Anything at all? I didn't look at any of the FSM's, but I bet you did.

    I wonder what thermistor ZCD picked for their unit. And why.

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    Thanks for your input,@captain obvious. It's good to see we're all going in the same direction. It's also clear that if we're correct, there seems to be no easy fix unless a properly calibrated thermistor replacement can be sourced.

     

    Sent from my SM-N950U using Tapatalk

     

     

     

     

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    8 hours ago, Captain Obvious said:

    I got no silver bullet. The only explanation I have is what you already mentioned... Sounds like the thermistor is not quiiiiite right for the application. CLOSE, but not enough headroom for tolerances and environmental stressors like ambient temp and system voltage.

    And as Zed mentioned, why it only happens when the engine is running is probably because the system voltage is higher then. That extra volt-n-a-half matters.

    Thermistors come in all sorts of base resistances and with all shapes different curves. For this application, you want one with a "knee" in the curve as opposed to linear, and you want the knee to be at the correct temperature. I'm thinking that it's too close to the limit most of the time, and the combination of ambient temperature and system voltage (when the engine is running) puts it over the edge.

    @Dave WM, Did you find any specs at all about the thermistor? Anything at all? I didn't look at any of the FSM's, but I bet you did.

    I wonder what thermistor ZCD picked for their unit. And why.

    See, now you made me go look. The FSM doesn't have much in the FE section, and the BE section refers to the FE section.

    image.png

    I asked my frenemy Google about this and found a couple of interesting threads in other forums.

    https://www.element14.com/community/thread/56806/l/which-thermister?displayFullThread=true

    https://advrider.com/f/threads/honda-thermistor-fuel-light-to-led.954021/

    As was mentioned earlier, the wattage of the bulb and characteristics of the thermistor come in to play. (Thanks for the re-hash, Steve. Do you have anything to add to the conversation?)

    I tried a search to see if one could buy a fuel light thermistor. Sure enough, there are some listings on ebay for some Chinesium thermistors. I'm not sure I would want to experiment with incorporating one into an existing fuel sending unit. 

    https://www.ebay.com/itm/Automotive-Fuel-Level-Sensor-Fuel-Pump-Alarm-Sensor-NTC-Thermistor-/283606453547?_trksid=p2385738.m4383.l4275.c10

     

     

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    Posted (edited)

    "The whole unit fits only one way and straight into the tank which was full of fuel so yes, the thermistor must be in the fuel. As a matter of fact, with the unit plugged into the harness but not installed in the tank, the thermistor triggered the "FUEL" light on. It went out as soon as I introduced the sender into the tank as the thermistor hit the fuel"

    Since the above was done, it pretty much means everything is working electrically. I mentioned the mount cause on mine its a side entry on the tank, and the mount wire can be bent up or down to adjust when the sensor is in the fuel. I cant see any other explanation other than an intermittent short in the harness and/or sensor wiring  or a misplaced sensor. Maybe perform the test again with the engine running to confirm its not a voltage issue (higher voltage from alt running). Thing about testing is you want as few variables as possible.

    Edited by Dave WM

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    2 hours ago, SteveJ said:

    See, now you made me go look.

    Ha!! Made you look!

    So the only thing they can muster is "It's resistance is different". Not much help!

    The whole thing is a balancing act... The thermistor in the tank is a NTC and the bulb filament is a PTC. You need the current through the thermistor high enough to achieve the desired power dissipation such that there is self heating when not submerged in fuel, but never enough power to damage it.

    And when the thermistor does heat up and it's resistance drops, you need the resistance low enough to get the bulb filament to glow.

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    16 hours ago, dmorales-bello said:

    Here's a pic of the bulb. It reads 12V 4W 3E. Does that seem right?

    I had originally thought there was simply some dyslexia going on here, but now I'm not so sure... The original bulb spec is 3.4W, but the one in your pic (and part number) appears to be 4.3W instead. If that bulb really is 4.3 instead of 3.4, that might be part of the problem.

    Do you have another (known to be correct) bulb you can toss in there just to see what happens?

    My original bulbs were Toshiba A12V3.4.

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    29 minutes ago, Captain Obvious said:

    I had originally thought there was simply some dyslexia going on here, but now I'm not so sure... The original bulb spec is 3.4W, but the one in your pic (and part number) appears to be 4.3W instead. If that bulb really is 4.3 instead of 3.4, that might be part of the problem.

    Do you have another (known to be correct) bulb you can toss in there just to see what happens?

    My original bulbs were Toshiba A12V3.4.

    I was wondering about the bulb wattage, too, based upon one of the links I posted.

    So @dmorales-bello, are you game to try a different bulb?

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    Posted (edited)

    @Captain Obvious, @SteveJ , I'm definitely open to try any experiment that might clear this up. I must first admit I made a mistake with the bulb identification. I assumed the other bulb I had in my "bulb drawer" was the same as what was in the "FUEL" light housing because they looked the same. In fact, when I pulled the actual bulb form the housing it was a Toshiba branded 12V 3.4 amp bulb. In the "bulb drawer" I not only have the 4.3 watt bulb I photographed, I also have a 6 watt bulb of the same size. So I will now experiment with all 3 bulbs and report.

    Question: In theory, is the wattage of the bulb reversely related to the amount of current needed to illuminate it?

     

    Edited by dmorales-bello

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    2 minutes ago, dmorales-bello said:

    @Captain Obvious, @SteveJ , I'm definitely open to try any experiment that might clear this up. I must first admit I made a mistake with the bulb identification. I assumed the other bulb I had in my "bulb drawer" was the same as what was in the "FUEL" light housing because they looked the same. In fact, when I pulled the actual bulb form the housing it is a Toshiba branded 12V 3.4 amp bulb. In the "bulb drawer" I not only have the 4.3 watt bulb I photographed, I also have a 6 watt bulb of the same size. So I will now experiment with all 3 bulbs and report.

    Question: In theory, is the wattage of the bulb reversely related to the amount of current needed to illuminate it?

     

    Wattage is proportional to current. It is also proportional to voltage.

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    6 minutes ago, SteveJ said:

    Wattage is proportional to current. It is also proportional to voltage.

    I'm sorry Steve, I've never been able to really understand this. So, keeping all other variables the same, would the thermistor illuminate a lower watt bulb sooner than a higher watt bulb?

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