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dmorales-bello

Perplexing "FUEL" light malfunction

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The thermistor has to have enough current flowing thru it to heat up. To low a wattage (high resistance bulb)and it will not heat up, how low wattage is to low is something the engineers can answer if they had the specs.

Here is a good example, if you read the voltage at the light socket with a reg VOM or DVM you will likely see 12v, as there is no load on the circuit. Put a load on it like say a test light and the voltage will drop, not zero but some, bigger load bigger drop, as the voltage drops at the socket, more current is flowing thru the thermistor, submerged I doubt it would ever heat enough to change it resistance (lower). its a very analog setup so you may or may not see the bulb of a test light come on.

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1 hour ago, dmorales-bello said:

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?

I want to hear Steve's explanation about the whole balancing act and competing factors here...  LOL

But in any event, it sounds like you DO have the correct bulb in there, so I think we're back to the thermistor.

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I just completed the experiment with the 4.3 watt and the 6 watt bulbs and the malfunction was triggered after 25 minutes or so in both cases. The gas tank is still over half full.  So (as the Capn' pointed out) it looks like we're back to the thermistor.?

Just thinking "out loud" here: would some kind of resistance placed inline just prior to the lead reaching the bulb be a possible fix? Granted, it would have to be calibrated precisely to allow enough current to reach the bulb once the fuel level dropped and the thermistor was above it.

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5 minutes ago, dmorales-bello said:

Just thinking "out loud" here: would some kind of resistance placed inline just prior to the lead reaching the bulb be a possible fix? Granted, it would have to be calibrated precisely to allow enough current to reach the bulb once the fuel level dropped and the thermistor was above it.

That thought popped in to my head too.  Maybe use the fuel tweaker trick on your fuel light.  A potientiometer (isn't it actually a rheostat, as it's used?) that you can tune until you get what you want.

http://www.atlanticz.ca/zclub/techtips/tempsensorpot/index.html

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@Captain Obvious, do you mean like this?

image.png

So it seems like the thermistor isn't spec'd correctly from the experiments run by @dmorales-bello. As @Dave WM said, a lower wattage bulb would be higher resistance in the line. In theory you could use a potentiometer to adjust the resistance, but it would be cheaper just to try a lower wattage bulb to see if that's a viable workaround.

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2 minutes ago, Zed Head said:

That thought popped in to my head too.  Maybe use the fuel tweaker trick on your fuel light.  A potientiometer (isn't it actually a rheostat, as it's used?) that you can tune until you get what you want.

http://www.atlanticz.ca/zclub/techtips/tempsensorpot/index.html

A pot runs a few dollars (plus shipping since Radio Shack is no more). Low wattage BA9 bulbs were pretty cheap at the link I posted. You would need no more than a 100 Ohm rating on the pot, and then you have to modify the wiring to incorporate it.

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you would prob want to get a WW pot, those cheap carbon track pots would not like the current. I think I have some 100 or 500 ohms ones laying around if you cant get the fix with the different bulbs.

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Fry's is like a super duper Radio Shack.  Georgia is close.  https://www.frys.com/ac/storeinfo/storelocator

I'll bet that zcardepot used a battery to determine which thermistor would work, and didn't consider alternator voltage.

Seems like at this point it needs to be determined if you're trying to find a fix that you can live with or trying to show that ZCarDepot used the wrong thermistor so that you can get a refund.  Or go even farther and determine the right thermistor to use for the application.

A cheap "pot" would be the easy experiment if you just want to know the answer.

 

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The Fry's closest to me (about 3 miles from my office) was almost an empty warehouse the last time I visited. Are they back to stocking products?

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its not a stock thermistor? oh well that could explain things.

 

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11 minutes ago, Dave WM said:

its not a stock thermistor? oh well that could explain things.

 

It wouldn't surprise me if it's an "off-the-shelf" piece that was used in creating the reproduction sending unit. And by "off-the-shelf" I mean that it is designed for a more recent car/electrical system, maybe even for LED lights. That would explain why the light comes on so soon for the incandescent bulbs. Unless ZCarDepot has someone as thorough as the good Captain when specifying the behavior for the reproduction, a thermistor with the wrong curve could have been used.

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Posted (edited)
23 minutes ago, Dave WM said:

its not a stock thermistor? oh well that could explain things.

It's "new".  I assume that the offending part is shown here.  Maybe it has numbers on it.  Just noticed that they put "copy of" in the url.  

https://zcardepot.com/products/copy-of-fuel-tank-gauge-sending-unit-sender-240z-260z-280z-70-78?_pos=3&_sid=b7ce84965&_ss=r

image.png

Edited by Zed Head

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1 hour ago, SteveJ said:

The Fry's closest to me (about 3 miles from my office) was almost an empty warehouse the last time I visited. Are they back to stocking products?

I used to go the one in Wilsonville, by Portland.  It was well-stocked but it's been years.  Maybe they're dying too.

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3 hours ago, Dave WM said:

you would prob want to get a WW pot, those cheap carbon track pots would not like the current. I think I have some 100 or 500 ohms ones laying around if you cant get the fix with the different bulbs.

That's a great offer, Dave. I will try to find as low a wattage bulb as possible and order it to finish off experimenting with the bulbs. If that fails I will gladly take your offer and purchase one of your potentiometers (whichever you think best suits the application) and attempt that remedy.

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2 hours ago, Zed Head said:

It's "new".  I assume that the offending part is shown here.  Maybe it has numbers on it.  Just noticed that they put "copy of" in the url.  

https://zcardepot.com/products/copy-of-fuel-tank-gauge-sending-unit-sender-240z-260z-280z-70-78?_pos=3&_sid=b7ce84965&_ss=r

image.png

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.

20200724_164856.jpg20200724_164922.jpg

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Posted (edited)

If they're the same you have one to experiment on.  Results should translate.  Today's electronics manufacturing processes are all about process control to produce identical parts.  For that type of part you really shouldn't see a defect.  I wonder how many they have sold, and how many have actually been installed.  You might be the first.  p.s. I say they but I think it's just one guy.

I wonder if some of the delay is due to all of the metal surrounding it.  Might be fun to hook it up to a battery and see how warm it gets.

Edited by Zed Head

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5 hours ago, SteveJ said:

If you want to experiment further, here is a site with different wattage bulbs: https://www.bulbtown.com/9mm_Diameter_Bayonet_Base_s/1421.htm

In looking at the available bulbs in your link I'm a bit lost because they all have different voltage ratings and none are 12 volt (like the one I'm replacing). I understand that I need to go below 3.4 watts but how about the voltage rating? Which bulbs would you suggest for the test?

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6 minutes ago, dmorales-bello said:

In looking at the available bulbs in your link I'm a bit lost because they all have different voltage ratings and none are 12 volt (like the one I'm replacing). I understand that I need to go below 3.4 watts but how about the voltage rating? Which bulbs would you suggest for the test?

Automotive bulbs are rated for 14 VDC. (sometimes 14.4)

https://www.bulbtown.com/9428049_GM_General_Motors_Replacement_Bulb_p/9428049.htm

https://www.bulbtown.com/274020_GM_General_Motors_Replacement_Bulb_p/274020.htm

https://www.bulbtown.com/1813_Miniature_Bulb_Ba9S_Base_p/1813.htm

https://www.bulbtown.com/274004_GM_General_Motors_Replacement_Bulb_p/274004.htm

https://www.bulbtown.com/363_Miniature_Bulb_Ba9s_Base_p/b363.htm

There are probably others within that list. I didn't go down each page.

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33 minutes ago, SteveJ said:

Thanks for the help. Just placed the order and should have six different bulbs to experiment with by middle of next week.

For each bulb I will first check to see if it illuminates with the connected sender OUT of the gas tank and the car key in the "ON" position. If any of the bulbs fail to illuminate, that one will be eliminated from further trial.

Next, with the sender installed in the tank with fuel, I will run the engine for 15, 20, and 30 minutes and see which bulb (if any) light up. 

Hopefully at least one of them will not and the problem should be solved. If not then it's pot time. (By that I mean "potentiometer", not weed. LOL)

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I have the same sender from zcardepot and my fuel light doesn’t light up but it never has... The fuel gauge does seem very accurate. Sorry I have added nothing helpful to this topic but I am following with interest. I replaced my bulbs with LEDs and still doesn’t work. I only got it because the last sender didn’t move my gauge needle properly and this fixed it.

 

 

Sent from my iPhone using Tapatalk

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1 minute ago, AZDatsun said:

I have the same sender from zcardepot and my fuel light doesn’t light up but it never has... The fuel gauge does seem very accurate. Sorry I have added nothing helpful to this topic but I am following with interest. I replaced my bulbs with LEDs and still doesn’t work


Sent from my iPhone using Tapatalk

Did you try plugging in the unit before putting it in the tank with your key to the "ON" position? That should trigger the light in about 10 seconds or so.

BTW, what alternator are you running?

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As an aside, does anyone know how current production vehicles trigger a low fuel warning?

Perhaps this is an area where a leap forward would be of value.

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Posted (edited)

Turns out mine is not working, was working awhile ago. Spent the day trouble shooting. Back prob the connector at the center console. Key on fuel in tank disconnect the lamp and you should get 12v at the back prob (with out the load of the lamp you get the full battery voltage due to the high impedance of the DVM). Plug the light back in and the 12 drops down to about 20-30 millivolts (again back probing the connector from the lamp the harness that is under the dash). I next pull the sending unit. Tip remove the tank mount by the fuel pump but leave the tank in place. This will give you way more room to work around the sending unit ring (this is with the side mount sending units). Visual of the thermistor can shows a fine layer of corrosion where it snaps in. I remove the thermistor can clean both the can and the retaining clip with some fine sand paper, Try it again this time with the sending unit outside of tank. It works but not as well as I like plus I see some intermittent drops of the voltage while I monitor the back probes. down to zero... hmm must be a break in the circuit. I noticed the can has a small wire that comes out the bottom and is not firmly attached. I presume this is the electrical ground for the thermistor. I solder that to the can bottom. Much better action now. I am posting up a video will link here when  done.

 

Edited by Dave WM
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On ‎7‎/‎24‎/‎2020 at 1:02 PM, SteveJ said:

@Captain Obvious, do you mean like this?

image.png

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!!

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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.

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