technical question of the day: how do gauges work?

can anyone tell me how the electrical gauges in cars work?

yes, i know about variable resistance in the sending unit or whatever, but how does a certain resistance in the sending unit cause the needle to point to a certain spot on the gauge? i mean the real, down and dirty nuts and bolts of the gauge operation?

and how would one go about changing the resistance "range" of a given gauge?

my google-fu is failing me on this, just giving me about 1000 links to pages talking about the resistance in the circuit and not how it actually makes the gauge say a certain thing..

you may be muttering to yourself "why is he asking this?".. well, it's because i want to put a gas tank from a 2002 Camaro into my 86 Camaro, and they use different ohm ranges on the sending units- the gauge in my car is 0-90 ohm, the sender in the tank is 33-270ohm... i can get a 0-90 ohm sending unit that works with the tank for a V6 car, but a part of the plan for the car down the line is an aluminum 5.3 swap with a factory ecm, which uses the fuel tank level for something or other because they like to make things overly complicated these days.. and also to make it easier to remember what to buy parts for if i ever need a new sending unit.. also, i just want to learn..

Think of them as voltage meters testing the drop across the sensor.

Depends a lot on the type of meter, but "in general", you have current* through a coil whose magnetic field pushes against a magnet. One or the other is on a pivot, allowing it - and a needle - to move.

For your problem at hand, the short answer is that you have to match the sensor to the gauge; there's no easy/cheap way around it.

*the amount of current is directly proportional to the resistance of say, the sensor in your fuel tank.

Most electric gauges are ammeters measuring current flow through a resistor. The RX-7 in some years, and prolly other cars too, used a heating element to move the needle. It was just a thermometer.

There are gauges that can be recalibrated. Mostly after market (and a little pricey)

Seems to me the "right" answer for your situation would be to use one matching gauge and sending unit, then swap them both later when you swap the motor.

But the "cheap" answer to your problem would be more like adding a resistor inline to get closer to the range the gauge is expecting, then "calibrate" the gauge by bending the needle (or moving it on its shaft).

As you can see, the answer is "it depends". A lot of modern gauges use stepper motors with a microcontroller selecting the position based upon the measured input.

Just hook it up and fill up based on mileage. It will read about 1/3 tank when full and gradually as you drive the needle will peg full when you get to about 2/3 tank.

Arduino to monitor the circuit and kick out a proper value to drive the gauge?

So you have to actually change the guts of the gauge to make it read different.. if it was as easycas soldering a different resister to it, i would have gone that route to keep my life easy and my wallet at least a little plump.. guess i'll have to find a 0-90 ohm sending unit then.

novaderrik wrote: So you have to actually change the guts of the gauge to make it read different.. if it was as easycas soldering a different resister to it, i would have gone that route to keep my life easy and my wallet at least a little plump.. guess i'll have to find a 0-90 ohm sending unit then.

I'm a software guy, not a EE, but I'm sure it's possible to build an analog circuit that will translate the gas tank sender signal into the one that the gauge will understand. It's just that it's likely to be a whole lot more complicated than one resistor, and (again) I'm not a EE. :)

As BrokenYugo says, you could certainly do it with a digital microcontroller-based approach like an Arduino. It's massive overkill and excessively complicated, but it would work. If you can just install the right sender though, that's the better choice. :)

There's a million ways to skin this cat. Does it have a low fuel warning lamp switch and a trip meter? Those ought to be fairly reliable. Otherwise, an inline potentiometer ought to let you "tune" it into the neighborhood. When the warning lamp turns on, turn the knob (or use a little screwdriver if you get a small one) until the needle is on E. Done. The gas pump will let you know when it's full.

Measure the resistance across the POT when you have it to your liking and go buy a resistor(s) to match that value and solder them inline.

I was also kind of hoping it was just a simple "change this resister" kind of a deal because i could make a little extra cash converting third gen gas gauges and selling them to other third gen owners that have upgraded to the later tanks..

If you put a 135 ohm resistor in parallel with the 33-270 ohm sending unit, you get 90 ohms when the sending unit is at 270 ohms and 26 ohms when the sending unit is at 33. Not perfect, but it's something.

For the future upgrade, keep in mind The ECU is looking at the fuel level to for evap calculations. You can lie to the ECU and give it a fixed resistor value and I'm pretty sure the ECU will be happy.

You need to adjust the offset, which is 33 ohms and the span which is 90 vs 237. IRRC, it can be done with 2 potentiometers, one to move the offset, the other to reduce the span. What I don't recall is exactly how to do it. A 'net search should get your answer.

0-90 and 230-33 are wired opposite each other, so that low resistance is empty on the one and full on the other. That is the hardest part of the deal.

In reply to oldopelguy: Can that issue be resolved by swapping the hot and tank wires?

I would be absolutely stunned if the world'o'LS swap guys hadn't scienced this out already.

Anyway, your old tank and gauge was electro-mechanical, I think the term is. What happens is a stepped down voltage, usually 12 Volts, flashed to bring the average down to 6 or so volts is fed through a bimetal strip in the gauge that bends as it heats or cools. The voltage is grounded through a variable resistor in the gas tank- the sending unit. So, you have electricity creating heat, causing the strip to bend, which moves the needle.

The new one is electronic, and it has a 5 volt low amperage feed through the tank sender which gives a voltage back to the ecu, which is programmed to move a stepper motor, which places the needle where it wants it. The two types of senders are not compatible. The old gauge will melt the new sender, and the ecu will not read the old one.

Or sumptin like that.

codrus wrote: As BrokenYugo says, you could certainly do it with a digital microcontroller-based approach like an Arduino. It's massive overkill and excessively complicated, but it would work. If you can just install the right sender though, that's the better choice. :)

IDK if I'd call using a microcontroller excessively complicated in this case, overkill yes, but overkill is cheap in this case. You can't do this with just a resistor or two in the right place, at which point the preassembled arduino board, a 7805, a driver transistor, and a few lines of code is way easier to figure out (at least in my head) than some analog level shifter circuit or whatever. That and Streetwiseguy makes a really good case for isolating the two.

Could you just take a 0-6 volt gauge and use it as a fuel gauge?