I need to repair a rheostat that is the fuel level sensor in an old car I am fixing up.
Its condition does not allow me to make any measurements of resistance.
A bit of research suggests the range is around zero to 100 ohms.
Unfortunately affordable devices are 33- 220 Ohms.
Is it possiblle to change this range to 0-100 ohms with some resistors only or woulld it need some electronics to acheieve this.

Thanks

 

There are at least 20 top-notch Fuel-Gauges available and almost all of them
are available in ~3 to ~4 standard Resistance ratings,
but trying to re-wind the original Fuel-Gauge-Sender can be an extremely frustrating task.

If You are working on something really really old, or not very popular,
I would suggest installing a Custom-made Fuel-Level-Sender, they are not all that expensive,
but will require cutting a ~1.75 inch hole in the top of your Tank,
and knowing the requirements of whatever Fuel-Gauge You are trying to use.

If You are working on an antique Car,
I would not recommend trying to make the original Fuel-Gauge work,
unless You are really into self-abuse.
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Thank your for your guidance.
Cutting a hole in the Fuel tank will require engineering certification. I am capable but not formaly qualified to do so.
Therefore this would add aleast $750 to the cost.

 

I believe that the TS is not thinking of rewinding the transmitter. TS wants to know if there is a simple circuit with resistors to bring it to the desirable range.

Yes, it can be done. However, this would introduce some nonlinearity. Fortunately, a fuel gauge function is not critical.

You can also make a. linear transformation with an active circuit.

 

I don't understand.
I think your car had a 0 to 100 ohm rheostat in the tank.
I think you can purchase 33 to 220 ohm rheostats.

If you put a 200 ohm resistor across the meter or rheostat, the 33 to 220 will become 26 to 105.
A 180 ohm resistor will make it 26 to 99 ohms.

 

The Fuel Level Sender in this Subaru is a Hitachi product. Subaru have been unable to provide any specifications.
The unit is disintegrated to the point I cannot make any measurements. On Line Subaru gurus on Facebook forums have given nothing more than guesses. I now ignore them.
It is possible to mimic the Rheostat and apply resistive loads to the line and determine the resistance for empty and the resistance for full as reported by the dash board display.
This is only part of the solution as I will not be able to purchase a Fuel Tank Rheostat to perfectly match.

MRChips talked about and active circuit as a bridge between the replacement rheostat matching the output to the range of the dash board display.

Any thoughts

TIA

 

Why are You knocking yourself out to try to salvage a trashed Subaru Fuel-Gauge-Sender ???
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Complete and utter waste of money. Bought 3 all DOA . The people who sell this rubbish make out they are doing you a favour at price 20% less than New price.

 

Sometimes it's better to go to a Dealer and pay their exorbitant mark-up.
You could have bought 2 factory replacements for the Money that You have already blown on trash.

I don't know where You live, but in the USA, there are certain Automotive-Parts that I will not go to
the "Discount-Shops" for, and will only purchase from the Dealer, or a NAPA-Parts-Store,
I especially like NAPA because if there is any problem with the parts they sell
they will have a new replacement part the next day, with no questions asked.
Yes, their prices are quite steep, but they have everything, and it all depends on how valuable your time is.
And, they don't sell junk because they know they have to back-it-up.

The alternative is basically for Hot-Rods and Race-Cars, ( and the occasional Collector-Car ),
The Aftermarket for parts for Hot-Rods and Race-Cars is usually ~95% premium-quality,
so if You don't mind doing some of your own engineering, You can purchase beautiful-bullet-proof stuff.
that's usually much better than anything that originally came on the Car.

If you're building a Hot-Rod or a Race-Car, let me know.
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I live in Australia
Subaru know of no stock nor do NAPA.
So get some value and some reliability I will need to modify something to do the job.
I have no training in electronics but have no problems making circuits.

 

It is possible to mimic the Rheostat and apply resistive loads to the line and determine the resistance for empty and the resistance for full as reported by the dash board display.

Yes, that would work.
And when you do, measure the voltage across the resistor for the full and empty values, as that will help to design a circuit if one is needed with the sender you will be able to use.