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Author Topic: Gas gauge question
Jay Lutz
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I intend to ditch my current gauges and install an AIM data system this winter. When speedo and tach go so does the gas gauge. Anyone know what the signal to the guage is......maybe 0 to 12 VDC? I have in mind to substitute a small digital voltmeter, then calibrate it 1 gallon of fuel at a time......example- 1 gallon=1.23v, 2 gallons=2.34v.....and so on. The only important thing is to be sure there is enough fuel in the tank before you hit the track. Thanks, Jay

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I believe the fuel sender is a variable resistor, so the signal to the gauge in the cluster is a resistance value, something in the neighborhood of 0 to 90 or 100 ohms. I think there is a tap out on the back of the cluster where you can read the voltage from the gauge, but if you remove the cluster you wont have it. I believe there is a detailed thread about this in the Spec. Miata Garage.

Greg Bush Verified Driver
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The stock gauge sucks, you won't miss it. The sender is better than the gauge.

There is so much mechanical resistance in the meter movement it takes a good 5 minutes for it to get to the right spot, if it doesn't stick a little. I got tired of leaving my ignition on and forgetting to turn it off.

If you run your car almost empty for weight, its pretty easy to pump it dry between sessions and add what you need. You have a fuel test port and there is a jumper you put in the test port to run the fuel pump. Takes a minute or 2.

I add gas based on last scale weight and how long the next session is.

We are slowly working on a digital gauge that has a base calibration that is close, but can be recalibrated to your car, by you, if you wish.

The cars I have tested showed some differences that didn't look like wear or age. That makes the self cal important if you plan on running it to the last drop.

The unit will also have an AiM compatible output if you plan to log your fuel level. Keep in mind you'll only get a good number on long straights. Andrew Caddell was running a prototype early last year and gave us some good feedback. We were able to see level in roughly 0.1 gallon increments.

For me, I want the display located in the back window so I can see it when I'm adding gas. That's the only time I can do anything about fuel level, so its the only time I need to see it.

For an enduro driver, its a different story.

I can't say when it will be ready, we all have bigger fish to fry right now. While waiting for this I have been using an ohm meter to measure the sender and compare to a table of ohms vs level. There is one wire going to the sender and it is grounded to the chassis.

Check the manual but if memory serves its 3 ohms full and 110 empty. Yours may vary. They did a decent job linearizing the resistance of the sender, but its not perfect.

It works pretty well having brought both cars in within 10 pounds every time and in a few cases within 2 pounds. Yes I do have a thing about minimum weights. Coming from karts where I could never make it down to weight I really enjoy doing it now.

Hope this helps,

Greg

Jay Lutz
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So you are saying the gauge is really just an ohmeter display.....not a voltmeter? If so, why not just put 2 terminals on the dash and hook up your ohmmeter before every session (with a chart say 0gal-110ohms, 1gal-100ohms, 2 gal- 89ohms...etc? No real need to actually have a gauge if I have an ohometer in the trailer. Jay

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quote:
Originally posted by Jay Lutz:
So you are saying the gauge is really just an ohmeter display.....not a voltmeter? If so, why not just put 2 terminals on the dash and hook up your ohmmeter before every session (with a chart say 0gal-110ohms, 1gal-100ohms, 2 gal- 89ohms...etc? No real need to actually have a gauge if I have an ohometer in the trailer. Jay

Other than people in the paddock thinking you are a little off when you say you need another 33 Ohms of fuel... [Cool]

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The gauge measures voltage.

The level sensor is a variable resistor.

There is a reference (regulated) voltage supplied by the harness and a voltage dividing resistor which results in a varying voltage in the gauge.

The voltage dividing resistor is in the gauge. So the gauge's inputs are the reference voltage and the senders resistance.

There are some EEs here that can do a better job of explaining it.

If you want to get a fuel level back to your DAQ (which can oly measure voltage, not resistance), you need to either leave the gauge in and get the voltage from inside it. Or you can create your own voltage divider. There are plenty of on-line calculators to help you size the resistor.

Bear in mind that, unless you can dampen the sensors output (either using a circuit or mathmatically in the DAQ), your fuel level will be nearly useless until the car has sat still for a minute or two.

In either case, you will also have to derive a polynomial to convert the voltage into gallons. There is a function in Excel to do this for you.

If you've never done it before, this process is something of an education. I went through it with other sensors (oil p, oil t, etc) when hooking up my DAQ. I'm out of inputs in my DAQ, so I never did fuel level.

-Kyle

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http://forum.specmiata.com//cgi-bin/ultimatebb.cgi?ub...c;f=1;t=001591;p=0#000007

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Greg Bush Verified Driver
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If you decide to tinker with this please note one of the functions of the stock gauge circuit is to limit the amount of current (and energy) that can get into the fuel tank through the sender.

Please keep this in mind when designing a safe circuit. Do not just hook 12 volts to the sender. [flamed]

cam Verified Driver
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Bring back an old thread, my search did not find a thread closer to the topic I'm looking at.

My Gas gauge is always reading low to empty even when I have a full tank. Today I took the fuel pump out (after tracing all fuel related wires) and determined that the float (black rectangle) is not floating. The float looks like it is starting to disintegrate at the edges. Is there an EZ solution besides buying a whole new fuel pump with sending unit?

TIA

--------------------
"The problem with Socialism is that you eventually run out of other people's money."
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"A government big enough to give you everything you want, is strong enough to take everything you have."
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Mark Verified Driver Made Donation to Website
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You can purchase just the sending unit by itself. Its not very expensive IIRC.

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Mark
http://www.ironcanyonmotorsports.com

cam Verified Driver
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I found the root cause of my inaccurate gas gauge.

First I tested the float on the sending unit by having a glass jar of gas and setting the float in it over night. Much to my surprise, the float was still floating great. This was odd since I new the float was not acting properly in the fuel tank.

As it turns out, there are metal baffles inside the tank at various locations. One of these baffles was lose and obstruction the sending unit. I removed the baffle. This explains why the outside edges of the float are chewed up.

Thanks for the feedback and the PMs

--------------------
"The problem with Socialism is that you eventually run out of other people's money."
~Prime Minister Margaret Thatcher
"A government big enough to give you everything you want, is strong enough to take everything you have."
~Thomas Jefferson

   

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