Fuel Gauge and Water temperature gauge not working

Thank you Scotch. I think I can manage most mechanical matters, but I hate to admit that I am seriously electrically challenged . I have disasembled the little voltage regulator, but I don't think I will be able to find the fault or repair it, even as it is, as you say, a simple circuit.
Do you think an adjustable voltage regulator like the one pictured in your post will work.? Many of these on Ebay all made in China

Many appear to be small enough to fit into the OEM cover.
These V/R's would need to be adjusted on the bench by connecting to a 12 volt (Battery) source and adjusting the potentiometer while measuring the output with a digital meter. I believe the OEM rating of "about 7 volts" per the manual, is to provide a voltage to the two gauges that is well below the harness voltage. This is to account for any fluctuations in the charging system so not to affect the gauge accuracy.
These two appear to be ideal.


To account for the 3-wire OEM plug, the negative IN and OUT could be Jumped (connected) on the board. thus: + IN. - IN , + OUT to the gauges. Simply unsolder the 3 terminal OEM plug and attach it to the appropriate terminals on the replacement V/R.
Perhaps someone with an electronics background can confirm all of this?
I've made these numerous times for various applications. The Blue one was for a device in my glider. The other has a Pot. so I could change the output voltage, used for an application. I didn't make the circuit-boards - simply cut out a piece of an existing board that gave me the circuit pattern I needed for the components.

Hi, to answer the question Scotch made about negative input/output: I presume that the voltage regulator we are talking about, the one widely available on ebay is based on LM2596 chip (the one with 5 legs sticking out seen on the pictures from Scotch). So taken from datasheet, its using the same ground on negative input and negative output. To verify that, you need to measure the resistance (it should be 0 Ohm) with simple ohmmeter between both negative terminals on the regulator board.
And obviously the picture. I can't live with out pictures.

Another thing, one of the pictures, the red regulator is Step UP not down regulator which in effect increases the voltage while decreases current. Not to confuse the both.

Or we could try to fix the old one.