Perplexing "FUEL" light malfunction

[dmorales-bello]

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. 

[Zed Head]

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.

[dmorales-bello]

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 July 23, 2020 by dmorales-bello

[Zed Head]

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 July 23, 2020 by Zed Head

[Dave WM]

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.

[Dave WM]

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.

[dmorales-bello]

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.

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[dmorales-bello]

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

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[Zed Head]

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.

 

 

Edited July 23, 2020 by Zed Head

[Patcon]

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

[Dave WM]

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.

 

[Dave WM]

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).