96 low fuel light not working

[eurasiaoverland]

96 RZN185

Early style fuel sender with the separate low fuel level sensor.

My low fuel light does not work. It has never come on in the 9 years I have had the vehicle. I can run the tank practically dry and it never comes on. I'm not sure at what level it should come on, but I can run the fuel down so that the needle is below the E and it needs 60+ litres to refill, so it should surely be on before then.

If I pull the inspection cover above the tank and ground the correct wire, the dashboard light comes on, so the bulb and wiring from the tank to the dashboard are fine.

I pulled the sender a few years ago (before I knew about this issue) to replace the tank pipes, filter sock, gasket etc and the tank is spotless inside, the sender looks perfect (no obvious broken or loose wires etc).

So I'm wondering if anyone else has has a similar issue, if there is a known common problem with these?

Thanks

EO

 

[2TH4IST]

If I remember correctly, on the 96's they use a thermister, which is super tempature sensitive resistor. They use fuel too cool it. So if the tank gets to low and the thermistor is exposed to air it warms up. Thus turning your fuel light on. See pic. It's the small round sensor that has one red wire going to it.

So your either not low enough on the fuel level or there is a issue in the sensor.

 

[eurasiaoverland]

Thanks - that's exactly the type of sensor I have and the process was explained to me by a member on here a few years ago.

I had infinite resistance between the ground and thermistor terminals. I guess it's either a broken/disconnected wire, or the thermistor is bad. I struggle to see how the thermistor can go bad, so most likely it's the wire and it is repairable... so I'm not sure whether to buy a new sender.

Dropping the tank is not much fun and I don't want to find that I need to wait for a part to arrive from overseas and leave the car immobile, or have to do the job twice. I guess I was hoping to find a common problem which could sway the diagnosis one way or the other.

Thanks again

EO

 

[2TH4IST]

Yeah no problem. Since you grounded the connector and the fuel light came on, you confirmed everything is working from the connector forward.

Wish it was an easy fix. One way you could test the thermistor, is to drive until your very close to empty, then pop off the fuel line and jump the fuel pump pins on the diagnostic connector in the engine bay, and drain the tank. If the light doesn't come on (leave ignition in the run position), then you know the problem is inside the tank. At that point its empty so easier to drop the tank. Either way you have to drop the tank. Luckily the 4runner still works fine if the low fuel light doesn't work. You can still drive it until parts arrive.

It's also age, how old is that thermistor?

 

[phattyduck]

I had infinite resistance between the ground and thermistor terminals. I guess it's either a broken/disconnected wire, or the thermistor is bad. I struggle to see how the thermistor can go bad, so most likely it's the wire and it is repairable... so I'm not sure whether to buy a new sender.

The thermistor will have relatively low resistance, even with the low-fuel light off - there needs to be enough current flowing through it to heat it when it comes out of the fuel. I don't remember seeing one go bad previously.

Since you know how to read a fuel gauge, it isn't the end of the world... I looked through a few wiring diagrams (even for some older Toyotas) and couldn't find a spec for the thermistor, just the main sender.

I don't think Toyota would sell that part separately. I bet you could use one from basically any Toyota though (80's or 90's), so you could source it at a junkyard locally.

-Charlie