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. 

 I noticed in the OE one there's a hole in the top and side but in the new one the holes are in the ends. Is it possible there's an air bubble in the top of the unit that could be eliminated with a hole in the top?

in order for that light to come on you have to get much lower resistance, at 12/943 ohms is .013 amps or 130mA seems low. CO needs to chime in...

just guessing really as to the normal operating parameters. maybe it self heat up to a much lower resistance. Must be for the light to come which means the thermistor is sitting in a hot hot pool of gasoline to turn on. maybe 124f is still not that hot, try boiling hot. that has to turn it on with a much lower resistance.

feel free to ignore that last bit about boiling, I am just guessing now, but would like to see that resistance get down to about 100ohms (.1k)

Edited July 28, 20204 yr by Dave WM

in order for that light to come on you have to get much lower resistance, at 12/943 ohms is .013 amps or 130mA seems low. CO needs to chime in...

just guessing really as to the normal operating parameters. maybe it self heat up to a much lower resistance. Must be for the light to come which means the thermistor is sitting in a hot hot pool of gasoline to turn on. maybe 124f is still not that hot, try boiling hot. that has to turn it on with a much lower resistance.

feel free to ignore that last bit about boiling, I am just guessing now, but would like to see that resistance get down to about 100ohms (.1k)

Wouldn't you have to have the thermistor receiving power in order for it to heat up enough to drop the resistance even lower?

 

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heating from within (current passing thru a resistor)or outside (hot water heating up the resistor) should have the same effect, drop in resistance. I don't know the specs of the thermistor so really just guessing as to what temps things happen. The only hope is a resistor in series with the circuit will keep the already  high resistance (3k) a bit higher to keep the thermal runaway from happening (more heat generated than is dissipated by the fluid surrounding it). adding resistance will how ever limit the overall max current allowed (a fixed resistance in series) so as to limit the brightness the lamp could ever achieve. Sounds like a balancing act that Nissan got right but is hard to reproduce from you experience. Has the vendor commented on any of this? would be nice to hear that they have fielded working units.  

I've not contacted the vendor waiting for us to come up with a viable solution. I did ask him for a couple of weeks time to try out the second unit he sent but that was before I realized it was having the same issue as the first unit I got from him. He seems very amicable so I don't expect anything negative but... You never know.

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Well I'm no help on the sensor resistance front. I pulled my HVAC bezel off and measured the resistance. Open circuit. Thinking that maybe I had a bad wire or connector somewhere, I pulled my little access hatch off in the rear and measured it right at the tank. Open circuit.

I know the rest of the system works because if I ground the bulb wire back at the tank, the lamp comes on, but without a working sender, it'll never come on. I don't usually run the tank down so far as to really care, but my warning sender is fubar.

8 hours ago, Dave WM said:

in order for that light to come on you have to get much lower resistance, at 12/943 ohms is .013 amps or 130mA seems low. CO needs to chime in...

I can help with that tomorrow. I will push current through one of those little bulbs until it starts to "noticeably" glow. The current measured at that point will enable me to extrapolate how low the sender resistance needs to go in order to light the bulb.

I had previously measured the bulb current (directly connected to a power supply) at about 230 mA. So that's "full bright". In order to reach that level, the sender unit would have to drop to 0.0 Ohms. Clearly the current at which the bulb is "noticeable" will be less than that.

I think I did my math wrong, current = 12/943*1000 (milliamp) 13mA no where near enough.

I think I did my math wrong, current = 12/943*1000 (milliamp) 13mA no where near enough.

Could you explain this a little further?

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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

 

Edited July 29, 20204 yr by Dave WM

be glad we are not talking about capacitors and Farads...

Capt, I remember using uuf vs pf...

Edited July 29, 20204 yr by Dave WM

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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