Monday, January 17, 2011

Fuel Gauge

The fuel gauge hasn't worked since I got the car.  The previous owner pulled the sending unit out the upper tank located in the trunk. After cleaning and reassembling the sending unit, my fuel gauge still didn't work.

I then began studying the chassis wiring diagram for the car so that I could check continuity of the wire connecting the fuel gauge to the sending unit.  I discovered something very strange:
Check out number 182 & 250.  The sending units are wired in series.

Upper, trunk located tank.
After looking at the wiring diagram, I was quite confused about this secondary sending unit.  I put my car on my dad's lift, and quickly located the second sending unit....it was in a lower tank that I didn't even realize the car had.
Notice the large "T" hose that connects the filler neck to both of the tanks.
The second sending unit is just to the right of the large hose.

The "float" had a hole in it and was filled with diesel, so I drained it and chemically welded the hole closed (super glue).  The rheostat was also lightly corroded, so I cleaned it with 1200 grit sandpaper. 

I tested the rheostat with a multimeter before reassembling it.  The resistance to ground gradually increased as the float level increased.  

I put the sending unit back in the tank, and now I have a working fuel gauge.  Praise Jesus!

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2 comments:

  1. Otto BeldenOctober 21, 2011 at 4:36 PM

    Nick,

    I have a question about 2009 Toyota Highlander wiring related to the automatic dimming of the dashboard lights controlled by the light sensor on the dash (under the windshield). I don't want the dash lights to dim, is there a way to make that stop that you know of?

    Thanks!

    - Otto Belden

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  2. NickNovember 17, 2011 at 8:24 PM

    Hello Otto,

    If your dash lights appear to be typical incandescent bulbs that I would guess that their brightness is controlled by a photo-resistor or light dependent resistor (LDR) that is most likely near your windshield (on dash or near rear-view mirror. If you find that it is wired in series with your dash lighting, then you could wire a potentiometer (rheostat, or variable resistor) in its place to adjust the brightness manually. If you want your lights to be full brightness all the time, then bypass the LDR with a wire large enough to handle the current.

    If you suspect that the gauges are illuminated with LEDs instead of incandescent bulbs, then the brightness of the light is controlled by pulse width modulation (PWM). I'm pretty sure that this would most likely be accomplished using a micro-controller.

    Let me know what you find out. Sorry it took me so long to respond.

    Nick

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