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92 Corolla, 90 Camry
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Discussion Starter #1 (Edited)
On my 90 V6 camry, the fuel gauge needle is slightly below or on the low side of the full line on the fuel gauge. It never goes to the middle or above the full fuel line on the gauge. However, as fuel gets used the fuel gauge needle goes down. Since new fuel gauges from toyota aren't available, I had cleaned the fuel sender nice and shiny and the resistor value is 0.2 ohms when I manually lift the fuel sender level arm to the full position when the sender is disconnected and out of the car. I have created a spreadsheet that lists the resistance values at different points full, 3/4, 1/2, 1/4, empty and below empty (2 gallon reserve usually). I'll need to locate where I put this file to post here.

I went to the junkyard only to find a 1989 model whereby the gauge was stuck on the 1/2 tank mark even though the fuel tank is empty, and when the instrument cluster is pulled out of the car it still shows half a fuel tank. I took the fuel gauge out and tore it apart to see if there was anything broken or not soldered correctly to see if it could be fixed before playing with my fuel gauge. I couldn't find anything that looked wrong. I assume the fluid (oil?) within the spherical plastic ball that the fine needle post is inserted into has evaporated some of the chemical that gives the oil within it less viscosity (less resistance to motion) thus the viscosity may have increased due to evaporation.

Since the fluid within this sphere can't be replaced with newer oil (?), I wonder if all that is needed to fix the needle problem is to remove the needle and with the power turned on in the car and the gas tank filled such that based on the resistance of the sender, it would read on the full mark (not overfilled so that the needle would go beyond full and peg it), then reposition the needle on the fuel gauge center post to a position on the full line mark. This would recalibrate or compensate for the higher viscosity of the fluid/oil within the fuel gauge plastic ball. Then one could fill up the gas tank further to see if the needle pegs out. Then watch as fuel is consumed until the 3/4, 1/2, 1/4, empty marks are reached and compare the resistance to the the spreadsheet of expected values to see if this recalibration fixes the problem and yet is still accurate. Then run to below empty to see if the calibration still holds. Note: these various resistances were measured by manually moving with the sender out and cleaned up nice and shiney and put into a spreadsheet. Thus when the fuel sender is inserted back into the tank and one is measuring the resistance as the repositioned needle hits these various fuel gauge marks during normal use, the resistance can be measured and compared to the prerecorded values in the spreadsheet to determine how close the recalibration is to the manually recorded values. Moreover, the low fuel light should come on based on the sender units low fuel light resistance heating up and causing resistance (and current to flow to turn on light bulb) since it isn't being cooled because its not sitting in the gasoline to absorb the heat. The spreadsheet values measured before doing all of this can be used as a comparison to see if the needle on the empty mark is calibrated correctly by using the low fuel light resistance along with the measure fuel sender float resistance at the empty mark.

The gauge windings are wrapped 90 degrees apart around the internal spherical ball that the pin is inserted into and upon which the fuel gauge needle is attached. I'm guessing that the magnetic fields of the two windings create a differential magnetic torque (non-mutual inductance) on the needle thats inserted into the plastic ball and that there is a metal surface inside the ball that helps move the needle based on this torque value. The two magnetic coils and their fields each may be equal or one or the other is weaker based on the current being the same or different flowing in each coil winding as the current changes within the sender unit due to the variable resistance change with fuel level. Thus, the resistors within the fuel gauge (current limit and prevent a short) and the senders variable resistance with fuel level create changing current levels in the coils that cause changing magnetic field strengths that then torgue the needle one way or the other to change the gauge reading,and that the internal oil and its viscosity smooth out needle fluctuations because of sloshing fuel in the tank caused by sender float changes. The oil viscosity also prevents the needle returning to zero when the engine is turned off since the metal plate cross section area within the spherical ball doesn't easily change due to the fluids viscosity - hence one sees the current fuel level in the tank when power turned off. Therefore, the gauge can go out of calibration just due to the weight of the needle over time because of the evaporation of chemicals in the gauges internal plastic ball evaporating that causes the viscosity to increase preventing the needle to move to the correct fuel level on the gauge.

So, I think this might work, what does everyone think about this by manually pulling the fuel gauge needle off to recalibrate it? Note: my 92 corolla doesn't have this problem since the fuel gauge drops to zero when the engine is turned off. Thus, to see the current fuel level in the corolla, one needs to turn the key to accessories to see the gauge needle move to the current fuel level before starting the car or seeing if there is even enough fuel to start it.
 

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3s-gte in a Camry?!?
'89 Camry Alltrac
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The gas gauge needle has never gone above about 7/8 full in my Alltrac since I got it in '07... So it has it worse (the sender design is a bit different in the Alltrac due to the rear tank location).

I always assumed that the needle didn't go to full due to a viscosity issue (similar to what you are suggesting) - I have never run it out of fuel though to check how low it can really go.

I wish I had something to add through - other than it would be interesting to run the gauge from an external variable resistor and see what the readings are. That would be the only way you could possibly calibrate it - know the expected resistance for half tank and reset the needle there or something like that.

-Charlie
 

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Discussion Starter #3 (Edited)
Have you disconnected the sender wire harness from the gauge and then measured the resistance into the sender when you put as much gas into the tank as possible to see if resistance is near 1 ohm or less? Even though your gauge shows 7/8 (less than half tank).

As the fuel drops to 1/4 and on empty but not below it unless you have an external fuel bottle to add some, what is the sender resistance when wire harness disconnected?

My sender is underneath rear seat and wire harness connector is there with the other harness running underneath carpet to dash. So I’d think your harness would be similarly routed.

BTW, when the harness is disconnected, the fuel gauge sees an open (infinite resistance) and thus the fuel gauge needle will be on zero. Some fuel gauges are designed to move towards zero as the senders resistance increases, and other mfg'rs do the opposite.
 

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3s-gte in a Camry?!?
'89 Camry Alltrac
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8,325 Posts
Ha, as I type this my tank is the lowest it has been in a long time - the fuel light has been on for ~5-10 miles, and the needle is approaching the empty line. I'd actually have a chance to check resistances at (basically) empty and full to see. Hmmm... The plug on the Alltrac is in the trunk area - I can't remember if it is accessible without tools, though it looks like it might be under that plastic shield:



The diagrams make no distinction between the ohm readings between the two sender units (3 full, 110 empty). Also, the fuel tanks are supposedly the same size (I doubt it, the Alltrac tank is smaller).

Funny thing, the low fuel light uses a different sensor - a thermistor that is set at a specific height in the fuel. When it is not submerged in fuel, it heats up, drops in resistance and allows the low fuel light to illuminate.

-Charlie
 

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3s-gte in a Camry?!?
'89 Camry Alltrac
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8,325 Posts
Allright... I got 104 ohms when the needle was on / just above the empty line and 6 ohms after a 15 mile drive home (it wasn't reading easily for me at the gas station). So, it all seems about right there - there might be extra resistance in the wires between the cluster and the sender, or the gauge itself could be off. The max the gauge got was still short of the full mark, and that took 3-5 minutes. After about 35 miles of driving, the tank shows about 7/8 of a tank, which is right on track (I get ~21 MPG normally).

Maybe it isn't as off as I thought?

-Charlie
 

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92 Corolla, 90 Camry
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Discussion Starter #6
Was this with the connector disconnected when you measured the resistance? I assume you filled it up and then it showed 6 ohms briefly before the gauge dropped to the 7/8 mark. When it dropped to the 7//8 th's mark was the resistance? or what is it now even though you've used a gallon?
 

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3s-gte in a Camry?!?
'89 Camry Alltrac
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8,325 Posts
Measurements were all done with the connector disconnected - it would be hard to measure and probably give invalid readings any other way.

The 6 ohm reading was after getting home (~15 miles / 3/4 gallon used). About 1 mile into the drive home after filling up the gauge did get close to the full line. The resistance readings at the gas station when full (pump auto-stop) were all over the map and I didn't have time to figure out what was going on and had to get home so I never was able to get a true full reading.

I'll try to get a resistance number tonight after the kids are in bed, which will be after ~45 miles driven on the tank.

You could just put a jumper in the two wires for the level sender (short) and see if your gauge goes all the way to full... Overly viscous fluid would only exert force on a MOVING needle and shouldn't create a static issue, so it might just take too long to get to the actual reading an the needle is already on the way back down due to using fuel while driving.

-Charlie
 

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Discussion Starter #8
The fuel sender is a big wire resistor where the float just slides along it. As the fuel contaminants accumulate on the wire resistor and the metal slider contact, the resistance goes up over time. Cleaning these up brings the resistance back down.
 

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Discussion Starter #9 (Edited)
To calculate the dc wattage at lowest resistance where heat is absorbed into the fuel do the following. The lowest resistance is chosen when the tank is full to disapate the heat when it’s the highest and as fuel goes down the resistance goes up lowering the heat/wattage because current goes down. However due to the current limiting resistors in the gauge assembly, the heat will be lower even though the sender resistance is low. This is why I think they make the full tank to be low resistance and empty high resistance so sender wire resistor won’t get to hot vs other mfgs doing the opposite ( perhaps due to patent)

current=voltage/resistance=12/6=2 amps for your case

power=voltage*current=12*2=24 watts

However, with the sender being connected to the gauge with a resistor in series (current limiter) the wattage will never really be achieved. You can measure this resistance when sender disconnected and measure across the wires going to the gauge.

Another way is to measure the current using a meter and calculate power.
 

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Discussion Starter #10 (Edited)
To check if the resistance at 7/8 is what’s expected when measured just use fractions.

expected resistance=Empty resistance-7/8(empty resistance- full resistance)

=104-7/8(104-6)=18.25 ohms

at half tank resistance is 55 ohms.
if resistance is lower due to tank being full, then of course it’s the gauge being stuck at the wrong reading.
Thus to compensate take gauge apart and pull needle off post and turn it 1/8 turn or to be on full mark and put needle back on. Knowing the resistance it should be on empty, one could wait till compensated needle change reaches the empty line and measure resistance to see how close it is to the expected value. If close you know that the problem has been fixed as best as possible.

one could get a junkyard gauge to measure everything out, adjust to see what happens and if everything good repeat for the original gauge in the car.
 

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3s-gte in a Camry?!?
'89 Camry Alltrac
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8,325 Posts
You could also adjust the angle of the sending unit arm and it would do the same thing right?
No, if the gauge is reading wrong for the correct resistance of the sender, you need to fix the gauge. Adjusting the sender (bending the arm, etc.) would make the reading at one end incorrect (depending on which way you bend the arm). Also, the tank is not 'linear' with fluid depth vs. fluid amount and the arm travels through an arc. Both of those factors mean that the sender has to be carefully calibrated from the factory:



(random internet picture, but you get the idea)

-Charlie
 
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