fuel gauge and sending unit

I'm reaching out here. I've asked this same question on several forums now, and no one can seem to give me an answer. I have a fuel level sending unit that is 12v. It provides a 0-5v output for the gauge (0 empty, 5 full). I would like to use it with a gauge that has the typical GM input of 0-90 Ohm (0empty, 90 full) at 12v. I've spent hours searching, but I can't seem to find anyone who has done this, and I'm not even sure it's possible. I found one (yes, only one) guy who did something similar in a battery powered Porsche. The Porsche gauge is 0-75 Ohm input. Link to site- http://cruzware.com/peter/blog/?cat=6. It's about 1/3 the way down, you'll see this- My knowledge of circuits like this isn't enough to be able to know if this would work or not. I realize the values would probably need to be changed. Can someone tell me if I'm at least headed in the right direction? And if I am, how do I go about getting the right values? Thanks.

Your transistor is a discreet device - not resistive. With the way your circuit is set up when SOC is 0 you will have only 1vdc going in to the base of the transistor which should be enough to turn it on at which point it will output a straight ground to your gauge. As your SOC voltage drops your base voltage will drop and eventually the transistor will just shut off and not pass current at all.Kevin Pierson

So... you're saying this won't work? Like I said, I don't know a whole lot about these types of things, just the basics. This is just a diagram I found online. Do you have any idea what it should look like to work?

To make it would you need to replicate the output of the correct sending unit - meaning you need a varying resistance on the output. They make "digital" potentiometers - perhaps that would work? Will be much more difficult though then a simple transistor.Kevin Pierson

It's called a transconductance amplifier - ie, voltage to resistance. FETs can be used. Chips were common too - eg LM13700, but that may need a split supply (ie, +/- 5V etc). If it is for a normal fuel gauge, I'd suggest getting the original sender. If it's for a BCM, why not use an offset or expanded-scale gauge instead (eg a series zener diode)? But that assumes the BCM merely uses voltage for a battery-capacity or SOC. If it's a "true capacity" BCM, then stick to its 0-5V output. The GM gauge should be able to be modified to sense 0-5V, though it may be a high-current meter which the 0-5V BCM output cannot drive (hence need a voltage-follower). All meters are ammeters and use resistors to convert ranges or convert voltages to current.

I spent some more time searching and found the transconductance amplifier. I was going to ask if it would be able to work. I can't go back to the original sending unit. It is a swing arm type, and I now have a fuel cell with foam, so I had to switch to a capacitive tube type sender. I would like to keep the factory gauge in the dash, but I am not opposed to converting it to work with a voltage input. Is it possible to isolate the gauge circuit from the rest of the dash, then use only the sender to drive it? If I can find out the draw of the gauge? Or am I way off base?

Capacitive senders are usually voltage output - I presume hence your 0-5V. Transconductance should work, it's a matter of finding he right conversion (circuit), ie, 0-5V to 5-90 Ohm or 90-5 Ohms etc.

Well, I was able to find a company that will make me a custom ordered sender with the 0/90Ohm range. And at a reasonable price. This will save me headache and testing, and provide a warranty. Thanks for everyone's help!