Need help: compare 2 varistors to create a 3rd varistor

[kingoftaurus]

The story:

I have an absolutely old car I am working on, an '88 Jaguar XJS Hess & Eisenhardt version. It was a one off car that was made only in '88.

The problem I am having is that the additional wire work for the additional fuel tank done in 88 has never been accurate even off the factory floor. They took the original single fuel tank, cut it in half and added an additional tank with an additional fuel sensor.

The problem:

When the sensors are at their lowest level, the gauge reads 1/4 tank. Obviously that's a problem.

The needed solution:

I have two fuel level sensors.

Upper tank sensor range is 73ohm at it's lowest level and 3.2 at it's upper limit.

Lower tank sensor range is 167ohm to 3.2 ohm.

The upper tank will be completely empty before the lower tank starts to empty.

Therefore when the upper tank is empty, I need the gauge to read somewhere in the 1/2 tank range and most importantly, the lower tank being empty must read empty..

The fuel gauge is full at 30ohm and empty at 300ohm and half at about 80

Is there a way to use an Arduino or RPI or something else to use those two varistor inputs to create a 30-300 ohm varistor using modern tech?

I consider myself a beginner, but I can follow instructions and my soldering skills are excellent! I know my way around a multimeter and I can understand simple code.

An additional note: the sensors and gauge need to remain factory. They cannot be modified. It barely seals the fuel in as is.

We have tried new sensors, new wire and a new gauge. The system just doesn't work as designed currently.

And the voltage in the vehicle fluctuates between 11.5v to 14.4v

Please help!

[kingoftaurus]

I have been thinking about ways this could be done.

Can I use a a pwm controlled mosfet to create a path to ground that "tricks" the gauge into thinking there is anywhere from 30-300ohms?

I'm thinking that I can use the two varistor inputs into an Arduino, do a little magic with code to translate them into a variable pulsed output to control the mosfet on/off.

Perhaps this will work and perhaps there's a better way?

Still waiting on some tips.. y'all are much smarter than me

[m k]

So when both are full the resistance is a bit over 6 ohms.
Add extra 22 ohms to go near 30 ohms.

Half is X || 73 = 55 (80 - 22 extra - 3 from other sensor)
(X ohms over upper tank sensor)

Empty is Y || 167 = 223 (300 - 77)
(Y ohms over lower tank sensor)

[kingoftaurus]

I tried that.

The gauge will not read full when I add another resistor.

I'm thinking it has something to do with the fact that one gauge moves to high resistance before the other gauge starts to move?

I really don't understand why...

[m k]

Sorry about the Y resistor. it's a brain fart.

Upper tank is straight forward, there X is 220 ohms after 22 ohms.

For lower tank you must add a series resistor to reach empty.
(223-167=56 after X or 300-73-167=60)
But then you can't reach full, since the series resistor is always there.

You can add a small circuit that add this extra resistor only when needed.
But then you must maintain the half level.

Maybe easiest is to do a transistor attenuator circuit that selects lower tank operation when needed.
There lower tank sensor would adjust a bit more attenuating circuit so that 300 ohms equivalent would be finally reached.
When tank is full the circuit would be close to short, missing 3 ohms for upper tank sensor must be accounted also.
Activator can be a low enough value of upper tank sensor.

How the gauge is connected?

You said the gauge operates between 30 and 300 ohms, 80 being half.
Have you tested that by replacing tank sensors with external potentiometer?

I read that there is also a circuit for refill light.
How is that connected?

Logarithmic decade has half scale around 30% of value.
How many ohms are 1/4 and 3/4?

If you finally need an extra controller keep in mind that there is also a pump.
Floats can also stop floating, at least partially, so your controller must be adaptive.

E,
II seem to have hard times with resistor calculations.

[Eraldo]

The easiest and "most" complex way of doing this is using an arduino to measure both of the tanks resistance by themselves and doing the necessary math operation to get a fuel percentage. And then on the output you can use a digital potentiometer which is programmed by the arduino to ge the desired resistance. A 1k 8 bit digital pot would do the trick.

But something i didn't understand is, does the fuel gauge need a series resistor to set the level? Maybe you could just implement a constant current source to command it.  The arduino doesn't contain an out of the box dac for this but you could filter the pwm and use that instead.

If you go the microcontroller route it's going to be a bit of a wire mess and a bit annoying but in the end its feasible. It will however either need prior knowledge on electronics or a lot of suggestions from the forum if you have little experience with this.

On the other hand there might be a more elegant solution out there with less complexity but I'm not knowledgeable enough to know about it and it might not be obvious.

[m k]

I should be able to calculate resistor values, even if there are more than two.
Don't know how that affects the rest.

I've now seen page 287/323 of XJS H&E but type of the level meter is still unknown.

So the rheostat is a sending unit.
What is fuel gauge receiver 191, one end of the actual level meter?
If it's for dashboard something the actual meter can still be what ever.

Wire 190 is connected to both rheostats and 192A is said to be a ground.
So upper rheostat is from 190 to 191 and lower from 190 to ground.
That's three wires to the actual level meter.
If the meter is current driven both of these rheostats should be in series with the meter.
If so then that middle wire is not needed.
(for obvious reasons I leave out calculations how partial parallel connection may behave)

I've also seen some other pages.
I think many would have made those two tanks differently.

[MarkT]

A variable resistor is called a _rheostat_.  A varistor is something totally different, a.k.a. VDR (voltage-dependent resistor), a non-linear device typically used to protect from overvoltage spikes.

If you use only two leads of a potentiometer, then its a rheostat, or you can say variable resistor, but not varistor....

[DavidAlfa]

Might be easier to modify the cluster.
There must be a resistor somehere going to the tank potentiometers.

[m k]

Page 5-6 has clearly a different connection style.

Page 5-5 indicates,
if green/orange wire is connected to upper gauge,
that rheostats are pulling green/orange towards black/white when tanks are full.

Page 5-4 has no black/white wire.

If 5-5 black/white is 5-4 191 black + then fuel level meter can be between green/orange and GND.
Then everything would be clear, but resistances are pretty low.
Means that problems can appear quite easily, especially when time passes enough.
And since 300 ohms is needed for empty tank something has happened already.

[m k]

If connection is clearly so that +V goes through rheostats and level meter to GND you can try boost converter, adjustable regulator and an extra trimmer in series between +V and first rheostat.

Adjust boost converter so high that low battery voltage is still providing what regulator needs.

More regulator output and range widens and max needle value goes higher.
More ohms to extra trimmer and the range goes lower.

Half tank and low fuel light will be what they will be.

[m k]

If and when you have a regulator you can also try lowering the feed voltage.
There you add that missing 60 ohms series resistor and 300 ohms parallel resistor over all three series resistors.
Then your minimum resistance would be (60 + 2 * 3.2) || 300.

Is it possible that original feed voltage has been much lower?

General voltage fluctuation of 3V is not too much, especially when it's mainly above 12V.
But all current driven hifi stuff will fluctuate along, probably not a standard function of Jaguar level.
If so then there already is a regulator somewhere between battery voltage and +V of level meter feed voltage.

What kind of car electronics was a norm during '80s?

I've had a Citroen C5 Hydractive 3+ from early this millennia.
It had an electronic distance to empty counter that changed to --- when distance went under 40km.
After that half of the distance was still a sure thing.
It also had a special thing, if you drove close to empty tank the second start after refill ate some counter distance, so the actual empty was probably later than the counter indicated.
Generally I guess it was less than a decade old design, but with much electrical stuff, like hydraulic pump, it probably wasn't very general.

I've also had a Peugeot 405, it was from '96, but its stuff was generally from '70s.
It also had an immobilizer, but that's clearly a new design.
Its fuel gauge had nuances.
When refill light turned on the level in the tank was still sort of plenty.
Next level was when needle was on the empty line, then the situation was still not urgent.
Only when the needle was fully under the empty line the refilling was reaching the do it right now level.
Few times I also refilled more than the tank capacity.
So its fuel level meter from '70s was pretty stable and accurate.

[kingoftaurus]

Oh guys I took a break on this car for a little while and you guys really kept on crunching on it I really appreciate that, here is the circuit diagram for the fuel level sender and you will see that it's just as confusing as anything could ever be.


Using an Arduino to take the two resistance values to calculate and use a digital potentiometer is the route that I think would be most logical to use on this. I just don't know what a digital pot is and how to make it do resistance things digitally lol


It should definitely be noted that if I completely disconnect the low fuel circuit board, it's very clear that the low fuel circuit board in this diagram has no function whatsoever in the actual reading of the fuel level. So the green orange wire actually goes to the dash fuel level sender and that black / white wire that "goes to nowhere" is ground.

Also to be clear both fuel level senders are new and they do work and the floats are okay in them it is simply an electrical issue.

[DavidAlfa]

We need to know the resistor of the upper part.
Measure voltages in Green/Orange wire with both potentiometers in max. and min. resistance positions.
Make sure Black/White is really ground.
Might be as easy as an op-amp.

We could even detect this this dinamically at power-on by loading the gauge with 300R, reading the voltage and calculating the impedance, all within the mcu.

Then only a pwm or dac output would be needed to control the sink current, adjusting the gauge.
This circuit allows fine control of 0-45mA, I don't think the potentiometers take more current than this, but can be easily adjusted by changing R6 to a different value.
The test section can be removed if you measure the Green/Orange wire voltages.
A cheap MCU will handle this like nothing (Blue pill, etc).

[m k]

Voltage levels of green/orange are also important.

If fuel level senders are really new then good, doesn't look very old.
If they are used it's possible they have similar problems than originals.
But not exactly, so new values should differ more or less.

Wire wound resistors, that those senders probably are, have a possibility to short circuit partially for what ever reason.
There some rounds contact their side buddies.
If so then meter's range will shrink and it retain its position when gauge goes over shorted section.

Low fuel circuit is most likely a voltage thingy.
Means that you can probably test it with a trimmer.
There you connect a trimmer over car battery, use high ohms enough.
And connect center tap to green/orange that is NOT going to anywhere else than to low fuel circuit.
When ohms between center tap and ground goes high enough the light turns on, so low side emulate gauges and high side the level meter.

What kind of a project is to see behind the meter block?

If black/white is ground then the circuit can be the opposite.
You can try that earlier trimmer between battery + and normally connected green/orange.
But then you must remember that you can almost short circuit the whole chain.
If tanks are full and trimmer is zero the resistance is only that 6.4 ohms and current is 2A.
This is why green/orange voltage is important.
Current driven level meter is probably mA range.
If we assume that empty level is 5% and know that it's also 300 ohms we can draw a line to full level.

But that missing 60 ohms is disturbing, to where that should be added.
If level meter is a voltage meter that 60 ohms can be missing from the top, but it can't be alone or that 6.4 ohms and 2A is back.
If empty level green/orange voltage is just few volts then everything is fine again.

Digital potentiometer and microcontroller is not a biggie.
Arduino Nano is enough.
Arduino Nano 33 IoT has a true analog out.
ATtiny85 has only 4 analog in pins.
Upper and lower tank gauges can go to A6 and A7.
Trimmers for adjusted levels to A1-A3, ATtiny85 is too small.
PWM or analog out to level meter driver, level shifter to 12V is possibly needed.
Fuel low light is different, but it's a simple on/off thing, so any digital pin will do, maybe a level shifter is needed here also.

If there are not enough installed wires fuel gauges can remain in series.
Then ATtiny85 is also back in.

Top is not adjusted.
Others are just matching values for half tank, fuel low and bottom ohms.