Fuel gauge wandering?

Today while driving I noticed my fuel gauge wandering a lot. It was at full than pulling onto the highway it was showing half. It kept fluctuating threw out the drive.

I've owed vehicles that that gauge would fluctuate while making turns and such but I've owned this truck for 6-7 years so I think I would have noticed it before now.

Any idea of what would cause this?

 

I'm having the same issue. I guess either wire from fuel pump or fuel pump itself

 

The ballast in the fuel tank may be malfunctioning, the electronic pad may be worn out.. Sometimes you can buy the pump and ballast seperatley.

 

It is the first snowfall today so maybe something is shorting out.

To be honest I don't really care, I just found it odd that I've never noticed it before so was wondering if it just starting happening or if I was losing my mind.

 

Is the low fuel light activated by the position of the fuel needle or by a senor in the tank?

 

I have a similar problem... My gauge doesn't quite "wander" but it definitely moves weird. My gauge stays near full for a realistic time and moves down realistically, until I hit half. Then it goes FAST. Basically once it hits half, within a day of light highway driving, its down almost to the bottom. I originally assumed it's just being weird but then I became a little more concerned considering the fuel light never comes on even when the needle is sitting at the bottom of he gauge. Like Ktaco asked, is the light activated by gauge position and mine is not coming on when I'm actually low, or is my gauge just being weird and the light isn't coming on because I'm not actually as low as the gauge says..?? Sorry if thats confusing to read, making this post from my phone.

 

Hi!
Did you ever figure out the answer to this question about what activates the fuel light? I'm having a wandering gauge too. Thanks!

 

The fuel light is wired in series with a thermistor in the gas tank. The thermistor sits near the bottom of the tank. A thermistor is a resistor that changes resistance with temperature. The resistance of the thermistor decreases when it warms up and allows more current to make it to the low fuel warning bulb. When it's submerged in the gas it stays cool and doesn't allow much current to flow to the warning bulb. You'll see the warning bulb go on and off when the fuel level is near the thermistor because the gas will slosh around and sometimes cool it or not. Once the fuel level is well below the thermistor the warming light will stay on.

If the bulb isn't lighting up, then
1) the bulb is burned out
2) the thermistor wore out and stopped working
3) the car didn't have a sender gauge with the thermistor. I'm not sure is all Tacomas had it. Some Toyota models didn't have it.
4) You're not getting low enough, but it should come on with 2-3 gallons left in the tank.

The bulb and thermistor are easy enough to test with a multimeter.

Note, if you replace the bulb with an LED it won't work correctly because the resistance of the LED is too high, so it will stay on all the time. The resistance of the bulb needs to balance the thermistor as the system was originally designed.

 

Wow, thanks for the explanation. That makes sense to me. Well, I'm scared to even run low enough on gas to see if the fuel warning light will come on or not, mostly because the fuel gauge needle itself is acting so erratically and I don't want to actually run out of gas somehow. Basically, I'm gonna just put fuel in the tank after every 200 miles, and worry about this new issue later on after I tackle more pressing items, like valve gasket/tuneup job and PS whine. This is great to know though, and I will put this info in my back pocket for when the time comes to tackle this issue. Sounds like testing the thermistor requires removing the bed to access the fuel tank, huh?

 

Or finding a wiring connector somewhere and probing from there. A wiring diagram would help to know which colors go to the thermister.

 

Ok cool. Thanks, I have the FSMs handy if I need to do that.