Houpty GT

I am planning to replace the thermistor which sensors if there is gas in the fuel tank. I have found several I can chose from to try but I need to know the resistance values of a functional one to determine what will work without making several trial and errors.

The way the sensor works is: Inside the bullet shaped sensor on the fuel pump/level sensor there is a thermistor. The thermistor generates heat when current travels though it. The warmer the thermistor gets the less resistance it has. When the sensor is submerged in gasoline it cools off quickly and the resistance stays high enough that the low level fuel light will not activate. When the gasoline level falls below the sensor the thermistor heats up and the resistance drops. This raises the voltage across the low level fuel light which then illuminates.

I need to know the ambient temperature of the air and a full tank of gas and whatresistance readings you get across the sensor. Take the resistance reading when the car has not been running so that there will not be any influence from the thermistor trying to heat itself. The value I am going to be comparing this to is 25 degs. Celsius (77 degs. Faranheit) so the closer to the temperature the better.

The second number I need is the resistance of the sensor when the car has been running and the light has been illuminated for a descent period of time. Try to take the resistance value while the thermistor is still warm from its self heating.

A new thermistor is only going to cost 66 cents or so. This will be a very inexpensive fix for most RX-7 owners but I would like to get the thermistor right on the first try so I don't have to disassemble the sensor multiple times.

Thank you in advance and we should have lots of these fixed in no time.

Acesanugal

iTrader: (5)

So technically...to test if your fuel light is working or not, you could just pull the pump assembly out and turn on the ignition???

Houpty GT

Well the car would have to be running because all the light are going to be illuminated if the ignition is on but the engine is not running. This is the test mode to check for bulb function. If the fuel pump is out of the car it could be hard to keep the car running though.

Acesanugal

iTrader: (5)

My CPU is kaput. Horn doesn't work and the idiot lights do not self check. My fuel level gauge is inaccurate and never reads above 3/4...I've only ever stretched my full tank 259 miles driving purely interstate and the light did not come on, so I dunno how far I want to push it before gassing up again.

A way to trip the sensor to see if the system works would be great..

HAILERS

IF you pull the small, two wire connector off the back of the alternator, then when you turn the key to ON, the whole bank of warning lights won't come on.

The warning light cluster lights all come on when there is no Ground from that small plug going either to the CPU, on a series four, or the warning light cluster itself on a series five.

Its easy to check out what I say, just pull the plug off the alt with the key to ON.

Houpty GT

If nobody replies that means that either nobody cares or nobody has a fuel level sensor that works. I don't doubt that most of them are broken though.

sharingan 19

iTrader: (16)

I care....

Houpty GT

I was loaned a resistor board from a coworker and I found that the light is completely out at 140 ohms and fully lit at 40 ohms. I will try and do some calculations to figure this out.

If anyone wants to help with the resistance while the low fuel light is lit. On my 1991 it was the blue wire with the red marks on it that went to the thermistor.

Here is a link to the spec sheet I just found:

http://www.mouser.com/catalog/specsheets/XC-600092.pdf

Here are some formulas on Wikipedia:

http://en.wikipedia.org/wiki/Thermistor

Houpty GT

I found some sites on the internet of people who have replaced thermistors, mostly on motorcycles. Two of the sites said to go with a 1000 ohm and B=4000.

There was also mention of not filling the tank with the key in the run position because when the cold gas hits the hot thermistor it will break it. One thing I would like to know is how hot the thermistor will get. I could probally test a couple thing and find out. I think fuel ignites at 460 deg. C or so and the thermistors probally get up to about 160 deg C.

Could someone test the resistance of the thermistor when everything is cool and with the igntion off. Then test the resistance with the thermistor submerged but the ignition having been on to see how much warmer it will get in the fuel.

Houpty GT

Nobody???

Houpty GT

www.digikey.com has a lot more options for thermistors but I could still use somebodies help with determining the resistance numbers.

sharingan 19

iTrader: (16)

I thought I read somewhere that the resistance value was something like 8 ohms empty and 110-112 full...

Houpty GT

That is somewhat inline with the numbers I stated for the light earlier.

"I was loaned a resistor board from a coworker and I found that the light is completely out at 140 ohms and fully lit at 40 ohms. I will try and do some calculations to figure this out."

Temps would be very helpful for those measurements though. I would hate for the light to work in the summer and not work in the winter, or vice versa.

Houpty GT

I ordered several thermistors to try. The first one is going to be a B=3700 R=1000 thermistor. I hope it works on the first try and I hope I don't start a fire. I will have to run a test first.

I will share my results and hope others will benefit from this knowledge.

Aaron Cake

iTrader: (1)

Is it actually a thermistor? Because in the schematic it's simply shown as a set of contacts, and the physical device seems to be some kind of tiny wire that functions the same way as the coolant temp sensor (ie. it senses current through the fuel).

However if it really is a thermistor, then you will need one that can stand up to the conditions in the fuel tank. That won't be easy to find.

I can't see how it can be a thermistor though, since sensing the fuel level by the temperature will be very unreliable as it heats up a lot during operation.

HAILERS

I thought it was a thermistor. The book shows it as a variable resistor. Confusing.

Aaron Cake

iTrader: (1)

The Haynes shows it as a switch. Go figure. Either way I highly doubt it's a thermistor.

HAILERS

Not a switch for sure. The light on the dash will come on faintly initially, and as the fuel gets lower, it'll get brighter. It's probably a thermistor.

HAILERS

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I've no idea why I wrote that backwards. The Warning lights come on when the alternator is not putting out. When it's not putting out, it puts a gnd on the L wire on the back of the alternator and that is what causes the Warning lights to come on.

The thermistor reads 3K ohms when it's sitting on a bench in the backyard in thirty degree weather. As in putting a meter on its Blue wire on the connector and the other lead on the gnd wire.

Houpty GT

Test one = failure

B= 3930 R = 1000

I may have messed this one up because I tested it directly to the battery and it blew up after about 5 seconds. I need to find an equivalent resistance equal to the warning bulb and parallel resistor.

jjwalker

Just because the fuel light indicator slowly illuminates and dims doesn't mean it is a thermosensor type. Putting a capacitor and resistor into the circuit can slow the lights reaction by smoothing out the output of the sensor.

The optimal way to figure out this problem, if I am reading this all correctly, is to find out what kind of buffer circuitry is between the guage, light and sensor.

Houpty GT

Success!

The B=3700 R=1000 thermistor is functional and let me know when my gas level was low. Nor did the car explode. I am going to write a how to and post the pics I took.

The only thing I noticed that I wish I had tried is that the solder had separated from the thermistor and this may have been the only issue.

Project88Turbo

iTrader: (22)

In my experience, the easy fix for the low fuel level sensor is to spray contact cleaner in the small holes on the sensor it self. A few sprays is all it takes and it is fixed. This has worked on 3 RX-7s for me so far.

Vince

Houpty GT

I don't see any way that that would repair a thermistor. Are you talking about the gauge or the warning light? I tried contact and carb cleaner on my warning light before I knew how it worked and it did not change anything.

I tested my old thermistor and found its resistance was too high and the fault was not caused by the poor solder.

The S4's have a different fuel level sender but it looks like it has the same bullet shape capsule for the warning light.

Project88Turbo

iTrader: (22)

Yes, I am talking about the bullet shaped connector. I think that fuel varnish may have been the issue as the cars had all been sitting for sometime It worked for me.

Vince