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$1 Low fuel level warning light repair


Sam280Z

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I performed this repair a few months ago and it has been working fine, so I thought I'd report here.

 

The low level warning lamp lights when current is allowed to flow to ground through a small thermistor located in the small metal can located at the end of the metal rod protruding from the fuel level sending unit. Over time this component fails - it appears that the electrical leads corrode away at the thermistor. If you can shake the sending unit and hear a rattle in the can, the transistor has failed. When you open the can, you will find a small ceramic disc.

 

A thermistor's electrical resistance changes inversely with temperature - higher temperature results in lower resistance. Also, it heats up when current flows. So, when this thermistor is submerged in gasoline, it stays cool enough so that not enough current can flow to illuminate the warning lamp. When it is out of the gasoline, it starts to warm up and allow current to flow, illuminating the lamp.

 

I used Mouser part #: 527-2004-1k

Manufacturer Part #: RL2004-582-97-D1

 

You will have to pry apart the little can and solder the new thermistor in.

 

While I was in there, I noted that the contact on the fuel level sender wiper was basically worn away. A small drop of silver solder on the end of the wiper and it seems to be working fine again.

 

Sam

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  • 10 months later...

Dude, that is exactly how the "Firebar" style Low Oil Level Switches work!

I never knew this, but was familiar with the technology from work. Doubt I could use a Firebar, though---it heats similarly, and uses a thermocouple to sense the bar's heat...tripping a relay at 430F! Bit hot for use in the fuel system for my tastes! LOL

 

This  makes sense, and sounds like a good way to retrofit to a car someone may be converting to EFI to give a warning similarly through an LED on the dash!

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Just to be a little more exact on your terminology, there are two different types of thermistors: Negative coefficient and positive coefficient. The one you are describing is a negative coefficient thermistor. Positive coefficient thermistors are rare in the automotive industry, and I don't believe Nissan has ever used them, so quite a few of us will never see one in a car. Just need to keep that in mind, because there is a possibility that you could run into a positive coefficient thermistor with an aftermarket sensor. If a person is unaware of this fact, and happens to get one, they could think it is a defective part, when it is actually working properly.

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