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Discussion Starter #1
I've been meaning to post this for a while...

I had often wondered, as have others, exactly what was going on with the ZR-7 fuel gauge readings. A brief investigation was needed to understand the system in order to change the calibration to something more desirable.

The first step was to determine the resistance of the fuel sensor for different fuel levels.
The tank was drained and the bike stabilized in riding position as with a 170 lb rider aboard. The tank was slowly filled and resistance measurements taken along the way to the full tank specification of 5.8 gallons. The bike was jostled about before each resistance reading was taken to make sure the float system friction was not affecting the readings. The data is plotted in the graph below.

Notice that the sensor float does not start floating upward with the fuel until a bit more than 1 gallon(the specified reserve amount). So, the resistance of the sensor stays constant at just over 95 ohms until the tank has more than one gallon in it. Beyond that the resistance decreased fairly linearly as gas quantity increases.
 

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Fuel Gauge Calibration (Part 2/4)

The second step was to hook up different resistors to the input of the fuel gauge and tabulate the gauge reading. I used 0% as the EMPTY LINE, and 100% as the FULL LINE on the gauge. Then I used this tabulated data to plot the fuel gauge reading versus gallons of gas in the tank. See plot below.

The results closely match my experience reading the gauge while riding. Topping off the tank results in the gauge needle reading above the FULL line. When the needle hits the EMPTY line I still have over 2 gallons left. For me, the gauge needle continues to fall and just as the needle hits the peg, I hit reserve with about 1 gallon left.
 

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Fuel Gauge Calibration (Part 3/4)

The Final step was to calculate the resulting resistances with different resistors shunted across the fuel sensor lines as suggested elsewhere on this site. I have plotted the results for resistor values of 470 ohm, 330 ohm, and 270 ohm. Adding the shunt resistors lifts up the bottom end of the fuel range without changing the top end of the fuel range much.

A key thing to note is that no matter what shunt resistance is used, the needle will always reach it’s lowest reading just as reserve is hit, and then remain at that position as the last gallon of gas is burned out. If a shunt resistance much less than 270 ohm is used, the gauge needle will never reach the EMPTY line.
 

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Fuel Gauge Calibration (Part 4/4)

If it is desired to move the top end of the fuel range down so the gauge needle rests closer to the FULL line when the tank if topped off, this can be achieved by adding a small value resistor in series with the sensor wire. What I wound up using on my previous ZR-7S was a 4 ohm series resistor and a 270 ohm shunt resistor. The plot below shows the resulting gauge readings along with those for the stock, and (stock + 330 ohm shunt resistor).

In the end, it is a matter of preference as to what the rider wants to see for fuel gauge readings. Currently, I have opted to leave the gauge as stock. I know I have 5 gallons when the FULL line is hit, 3.5 gallons, when at 50%, a little over 2 gallons when the EMPTY line is hit, and reserve is hit with 1 gallon left just as the needle gets to the peg.
 

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Good stuff Bolserst.

Can you send Pics of the testing. A mock set up would be ok.

gg's can get 18 litres into the tank after going onto reserve. Therefore 4 litres should remain in the tank giving say 80kms (50 miles) before stopping and walking back to the gas station. gg did 60kms once on reserve and was twitching, when pulled up to this remote gas station. Reserve changeover and the redline just about coincide.

[email protected], ava good day
 

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Discussion Starter #6
Good stuff Bolserst.

Can you send Pics of the testing. A mock set up would be ok.

gg's can get 18 litres into the tank after going onto reserve. Therefore 4 litres should remain in the tank giving say 80kms (50 miles) before stopping and walking back to the gas station. gg did 60kms once on reserve and was twitching, when pulled up to this remote gas station. Reserve changeover and the redline just about coincide.

[email protected], ava good day
I am away from my bike on vacation right now, but can send you some pics of the test setup in early january when I return. I'll try to explain briefly in words as the setup is simple. You disconnect the fuel gauge sensor connector that resides just above the #4 carburetor. Hook up a multitester to the tank side connector to read the resistance of the fuel level sensor as you add gas.

Interesting that your stock setup hits reserve just as the needle reaches the redline...does it stay at the red line as you burn the reserve? How about when you top off the tank, does the needle rest on the Full line? or above it.
 

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I wonder how consistant the readings would be across a number of Bikes?
Maybe I should try that test over the winter.

John
 

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Interesting that your stock setup hits reserve just as the needle reaches the redline...does it stay at the red line as you burn the reserve? How about when you top off the tank, does the needle rest on the Full line? or above it.
No, it goes below the red line as the "reserve" fuel is used. It goes above the full line as gg squeezes the last drop of fuel into the tank....way up the neck of the filler.

[email protected], ava good day
 

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Bolserst,

Beautiful work! You must be a scientist or an engineer!
A curve on a graph is worth a thousand posts...
 

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I wonder how consistant the readings would be across a number of Bikes?
Maybe I should try that test over the winter.

John

John, knowing the standards of Japanese manufacturing, I would believe that there is very little variation from one bike to another.
 

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Discussion Starter #11
I wonder how consistant the readings would be across a number of Bikes?
Maybe I should try that test over the winter.

John
Just two data points I know....but both my 2001 and and 2002 ZR-7s read within a percent of each other.

I know the service manual specified some tolerance to the fuel sensor resistor, but I don't have it handy right now. There may also be some tolerance in how the float is mounted and/or bent.
 

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Discussion Starter #12
Bolserst,

Beautiful work! You must be a scientist or an engineer!
A curve on a graph is worth a thousand posts...
ummm....errrr....engineer, guilty as charged. :D
 

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Very well written report. Even a self-acknowledged dummy like me can make sense of it!

My fuel gauge up and died when I removed the tank and then reinstalled it. I started another tread here on that. Over the holidays I hope to futz around with it and I will report back on my successes or lack thereof.
 

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Discussion Starter #14
Test setup for measurement of Fuel Level Sensor

Good stuff Bolserst.

Can you send Pics of the testing. A mock set up would be ok.

[email protected], ava good day
Here are two pics of the setup I used for measuring the fuel level sensor resistance.
I disconnected the fuel sensor connector that resides just above the #4 carb.
Then I hooked up a digital multitester to measure the resistance for the range of fuel levels in the tank form empty, to 5.8 gallons.
 

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Discussion Starter #15 (Edited)
Calibration Without Measurement Equipment

Since several people have sent me messages asking...

Measurement of the fuel level sensor resistance is not required to calibrate your fuel gauge to read “F” when you have 5.8 gallons, and “E” when you hit reserve. I did that purely out of curiosity to see what the trend was of gauge reading versus amount of fuel in the tank. All you need is a few resistors from Radio Shack, or some other local electronics shop…no special measurement equipment necessary.

See attached picture for where to insert the Series and Shunt Calibrations resistors.
**********************************************************************
***Resistors should be 1/2 Watt or higher. Avoid 1/4 Watt or less as they may over heat***
**********************************************************************

1) Calibrate the FULL position of the fuel gauge needle.
Fill the tank with 5.8 gallons of gas. Calibration is done be inserting a resistor in series with the W/Y (white with yellow line) wire on the back of the fuel gauge. A good value to start with would be 5 ohm. Radio Shack doesn’t sell 5 ohm resistors, but you can make one by putting to 10 ohm resistors in parallel as shown in the picture.
Smaller values position the needle higher, larger values position the needle lower.
This resister value is quite sensitive, so a change of just 1 ohm makes a noticeable difference.

NOTE: See details below on getting other values by parallel combination of readily available Radio Shack Resistors.

2) Calibrate the EMPTY position of the fuel gauge needle.
It is usually desireable to have the needle hit the “E” line just as you reach the reserve fuel level. Calibration is done be inserting a resistor shunt across the W/Y and BK/Y(Black with yellow line) wires. As mentioned elsewhere, a value of 470 ohm or 330 ohm would be a good starting value. The lower the value, the higher the needle is positioned at the reserve fuel level. For me, a value of 270 ohm worked the best.
The series resistor does have some effect on the EMPTY needle reading, so determine the series resistor in step 1) first.



NOTE: It is difficult to find readily availabe resistor values below 10 ohm. But you can make any value you want by putting multiple resistors in parallel.
Here is the formula for 2 resistor in parallel:

R = 1/( 1/R1 + 1/R2)

So for example to 10 ohm resistors in parallel would give you:
1/(1/10 + 1/10) = 1/(0.2) = 5 ohm.

Or a 10 ohm and a 15 ohm in parallel would give you:
1/(1/10 + 1/15) = 1/(0.16667) = 6 ohm.

For 3 resistors in parallel:
R = 1/( 1/R1 + 1/R2 + 1/R3)

Example a parallel combination of 10 ohm, 15 ohm, & 68 ohm would give you:
1/(1/10 + 1/15 + 1/68) = 1/(0.18137) = 5.51 ohms.

Here is a summary of some useful values for the FULL Calibration.

Value R1 R2 R3
3.33 10 10 10
3.75 10 10 15
4.07 10 10 22
4.52 10 47 10
5.00 10 10 na
5.51 15 10 68
6.00 10 15 na
6.47 15 15 47
7.01 10 47 47
7.50 10 33 330
8.02 10 68 100
 

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Discussion Starter #16
Fuel Gauge...the inner workings

So how exactly does the fuel gauge on the ZR-7 work?
In the pics below you can see that the gauge is comprised of 2 coil windings set at 90 degrees to each other. What you can’t see is the piece of soft iron inside the coils that is attached the the shaft holding the needle.

If you hooked 12 volts up to coil #1, the piece of soft iron moves to align itself with the magnetic field of coil #1 and moves the needle to rest on the peg below “E”.

If you hooked 12 volts up to coil #2, the piece of soft iron move to align itself with the magnetic field of coil #2 and moves the needle to a position 90 degrees up-scale from the peg, up above the “F” mark.

If you hooked 12 volts up to both coil #1 & #2, the piece of soft iron move to align itself with the sum of magnetic fields from both coils and moves the needle to a position half way between “E” and “F”.

So, the fuel sensor resistance along with the two resistors mounted on the gauge vary the ratio of voltages sent to the 2 coils to get the needle to track the fuel level.

Pretty simple really. This type of gauge has been in use for nearly 100 years according to the US patent office records.


Oh, and how does the gauge needle stay put even after the ignition key is turned off? Again, very simple, friction in the needle shaft holder. This is also why the needle moves so slowly when you turn the ignition on after filling the tank.
 

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Awesome work done here.....

If it is desired to move the top end of the fuel range down so the gauge needle rests closer to the FULL line when the tank if topped off, this can be achieved by adding a small value resistor in series with the sensor wire. What I wound up using on my previous ZR-7S was a 4 ohm series resistor and a 270 ohm shunt resistor. The plot below shows the resulting gauge readings along with those for the stock, and (stock + 330 ohm shunt resistor).

In the end, it is a matter of preference as to what the rider wants to see for fuel gauge readings. Currently, I have opted to leave the gauge as stock. I know I have 5 gallons when the FULL line is hit, 3.5 gallons, when at 50%, a little over 2 gallons when the EMPTY line is hit, and reserve is hit with 1 gallon left just as the needle gets to the peg.
Like the title says, Awesome work done here, but wouldn't the simple act of re-marking "Full" and "empty" on the guage do the same trick? It sounds like "F" would simply be placed higher up and "E" would simply be placed lower down. I am NOT an engineer, just a simple diesel mechanic, who prefers to work on bikes, so I may have completely misunderstood your very thorough study. In the end, I am always looking for the simplest solution (The least goes wrong with the simple things) I really dig the methodical way you did your study. I just wonder if the stock hardware could be left alone and simply remarking the gas gauge would be effective.
 

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Discussion Starter #18
I really dig the methodical way you did your study. I just wonder if the stock hardware could be left alone and simply remarking the gas gauge would be effective.
You betcha.
Nothing wrong with filling up the tank, marking a new full line, and waiting till you hit reserve to mark a new empty line.
You can also just remember where the needle points, no marks needed :D

I can't remember which bike forum it was, but I saw some folks were having new faces plates made for their fuel gauge to match fuel level better.

Me, I'm afflicted with the disease of "needing" to figure out how/why stuff works. I wasn't sure what I would find or if there would be a way to properly calibrate the gauge. In the end I found the addition of two resistors could calibrate the gauge to the OEM face plate markings. FWIW, resistors are pretty darn reliable.

Oh, and welcome to the forum!
 

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I just wonder if the stock hardware could be left alone and simply remarking the gas gauge would be effective.

I'd say NO.

My bike can be riding around for a significant amount of time with the fuel needle on the E peg and I haven't even hit reserve yet.
 

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Discussion Starter #20
I'd say NO.

My bike can be riding around for a significant amount of time with the fuel needle on the E peg and I haven't even hit reserve yet.
Your right of course, good point. He may need to move or remove the peg to get the empty line marked. I think the peg was at the -10% on the plot earlier in this thread; I remember i had to remove it to take the data. Looks like the needle would rest on the peg for about 0.5 gallon before hitting reserve.

http://www.riderforums.com/showpost.php?p=406203&postcount=2
 
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