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. 

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.

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.

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

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.

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 July 24, 20204 yr by Zed Head

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?

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.

33 minutes ago, SteveJ said:

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.

 

 

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)

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

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?

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.

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 July 25, 20204 yr by Dave WM

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!!