Does anyone know how a 4Runner signals the “Fuel Level Warning Light” on the dashboard that the fuel level is too low? I can’t find any information in the FSM on a part that sends the signal to the warning light. The “Fuel Sender Gauge” submerged in the fuel tank must somehow do it. Maybe there is some sort of a switch on the sender that closes when the fuel level is too low to hold it open.

In November I filled up while on a trip and topped off the tank. I stopped at a hotel about 75 miles farther down the interstate. In the morning the fuel level warning light came on when I started the 4Runner even though the fuel tank was still nearly full. The fuel gauge correctly indicated the tank was almost full. Since then the warning light comes on and goes off no matter how much fuel is in the tank. Sometimes it goes on and stays on. Other times it flickers. Sometimes it works correctly for awhile. The warning light seems to come on more when the tank is full and gradually do better as the fuel level drops. It is irritating when it flickers at night. It is too visible. I initially thought the float must have lost its buoyancy and now it floats up and down aimlessly in the fuel. It can’t be the float. The fuel gauge still works correctly.

I know there are much worse things to go wrong with a vehicle but I really would like to fix this if I can.

Sorry, can't help you too much. Thought I'd give you a bump. I know there are known service bulletins on fuel senders on some 3rd gen models. Search around a bit more and I think you may find some good info.

I dont know for sure but I think when the fuel is low the sending unit provides the ground for the warning circuit. Possibly you could have a sending unit that is deteriorated or a wire chaffing somewhere and is grounding out. Since there is some coralation with the fuel level I would guess the issue to to be in the sending unit. I would agree with you on the float being ok.

I have not had any tank problems so I don't know how accessible the sender is. If you can just pull the connector on the sender the light should stay off. If it comes on at all, the problem is in the wiring between tank and dash. If it stays off, the problem is in the sender.

On my '85 BMW, the low fuel light had its own tiny float--a small brass ball at the deep end of the sender. When the fuel dropped low and reached the ball, it dropped maybe 3/8", and completed a circuit between two contacts providing ground to the lamp. You could get metallic crud shorting those little contacts. I'm guessing something similar is going on here.

I believe there is an access panel under the back seat.

So maybe there is a chance that the detector, whatever it is, has somehow become dirty. I guess it could even be moisture at the bottom of the tank completing the circuit.

It is tempting to simply drop the fuel tank, pull the fuel sender, inspect it and clean it or repair it if that’s all it needs. I would even take digital photos and share the experience with y’all. I’m reluctant to try that because it looks like a daunting task without a lift. I would be doing this at home on the garage floor or in the driveway if I try it.

I think I’m going to run a bottle of STP Super Concentrated Fuel Injector Cleaner through a tank full of fuel and see if the warning light behaves differently. I thought about running a bottle of dry gas through a tank but I don’t think that is as likely to solve anything. I’m usually stuck with crappy oxygenated gas and I understand that it has a tendency to absorb moisture and remove it from the tank more so than good gas does.

Really! I’m going to have to check that out. That sure makes inspecting the sender a lot more tempting.

Take a look at this thread on YT:

YT Thread

It appears you can access the wiring (for example to do the test I suggested) but to pull or inspect the sender you have to drop the tank. It is bolted on to the pickup tube.

Last week I read the thread that TheDurk provided a link to. In that thread Unhappy99 provided a link to another thread that had some interesting comparison photos of an original and replacement sending unit. Based on those photos I assume that the low fuel level warning signal is sent by the sending unit, it has a switch that is activated by the float and the switch is in the same cluster of electrical contacts on the sending unit that sends the signals to the fuel gauge. The FSM doesn’t appear to discuss it. It has information about the fuel gauge but not a switch or circuit for the Fuel Level Warning Light. The only info I can find is in the electrical diagrams for the warning light on the dashboard.

There is another thread that might have better photos of the sending unit but I can’t see them. I think I need to register at that site to see them but I don’t want to register there.

Today I finally filled up my tank and added a bottle of STP Super Concentrated Fuel Injector Cleaner. When I started the vehicle the Fuel Level Warning Light did not come on and it still hasn’t. This is the first time it seems to be working correctly with a full tank since the problem began two or three months ago. Hopefully the contact was somehow dirty and the fuel injector cleaner just fixed it. I hope I didn’t just jinx myself by not waiting longer to say something.