Low Fuel Warning

[toyofancy]

In my 1983 panel-van AE71, the Low-fuel warning sender does not work.

But I don't know how the sender actually works. :bash:

 

When I short out the wire to "frame", the orange low fuel light comes on steady.

I was told it should be flashing after a while

But when the tank is empty (bone dry) the light is not indicating and the resistance is around 40 ohm.

I like to fix the original sender... but how does it work? ;)

Does anyone know? I have not been able to find any info about the operation of low-warning light.

(The fuel quantity-sender works perfectly OK and accurate)

I hope there is a bright spark amongst you lot, as finding a spare sender is very difficult now.

 

Jonno

[TRD ke70]

if you pull the sender unit out of the tank, theres a little sensor down where the pick up is, it's the one with a single wire going to it, these are interchangeable with most other Toyota's.

just get another sending unit from a wreck and change the sensor over.

 

and no they don't flash.

 

Medicine Man or TRD could shed more light on this.

[toyofancy]

Cool, I did get another sender and soldered that in. And.. it works!

Blesssings upon you my son.

An electronics engineer told me, that the low-level sender, was nothing more than a 95 cent component from JayCar.

It apparently is a Thermistor. A device which increases resistance when warm and has a stable resistance when immersed in fluid.

So when it is outside the petrol level and exposed to air, it decreases resistance gradually and so the light gradually intensifies.

Now we all know,thank you. :)

[Fatman]

First off - I offer no warranty or accept any liability for these thoughts and modifications.

This is simply a documentation of my experiences and may be useful to others.

If you choose to use my rambling you do so at your own risk.

 

 

That Said…………………

 

 

There is in fact a thermistor located in the low-level sender, it is an NTC device (Negative Temperature Coefficient)

The resistance of a NTC device gets lower as the device is heated.

 

The low level sender is made by Panasonic Industrial the link is below:-

http://industrial.panasonic.com/www-cgi/jv...+AUD0002+0+9+WW

 

The sender is wired in series with the low fuel warning light that means that there is a small current flowing through the thermistor at all times (Ignition ON) but not enough to light the bulb. Thermistor is operated in Self Heating Mode, i.e. It is heated by its own I2R losses.

 

When the thermistor is immersed in fuel its ability to dissipate heat is 500% to 600% higher than it is in air as the fuel is thermally a reasonable conductive, as a result the resistance in the circuit remains high.

When the fuel level drops down, the device is exposed to air, once it has lost the fuels cooling effect it begins to heat up, slowly at first, but as the temperature rises the resistance goes down allowing more current more losses and more heat......... as a result of this in about 2 minutes the device reaches a resistance low enough to allow the warning light to illuminate. This gradual heating effect is why the lamp illuminates slowly? (Also a good natural filter to stop circuit blinking)

 

After many years the thermistor will burn out. For my AE82 Sedan, Toyota wanted $300 for a new complete fuel sender, including float etc.

I tried contacting Panasonic Industrial for suppliers, with no reply.

 

I decided to disassemble the low fuel sender carefully with a set of pliers; sure enough I found the toasted thermistor in side.

 

I went down to the nearest electronics store and purchased a 4.7KOhm NTC thermistor soldered it in, reassembled, installed and tested.

The low level switch now operates perfectly.

 

After the considerable amount of stuffing around that I had with this problem I thought someone else might find the thermistor value used to be of interest.

If the resistance chosen is to high the light will not illuminate, if the value is to low the thermistor will fry when exposed to air.

The trick is when testing on the bench, make sure that you put an indicator bulb in series with the gauge, otherwise the resistance of the circuit is to high and even with the right thermistor the device will still fry, you need the bulb or an equivalent resistance (about 40 Ohms) in circuit to act as a current limiter when the thermistor cascades.

 

Hope this is of help to someone.

 

Regards,

Brad

Power Systems / Instrumentation Electrical Engineer