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These gauges work by measuring the voltage across a fixed resistance in series with the sender's variable resistance (look up voltage divider principle if you don't know how that works). That means that the sender has to be a resistance, not a voltage so a voltage amplifier won't work unless you can open up the instrument and add the amplifier between the fixed resistance and the gauge.

What is the recommended sender resistance for the gauge and what is the range of resistance for the engine's sender? It may be possible to add a resistor to bring the sender into the range that the gauge needs....
The unit has 6 resistance settings -500, - 250, -100, Off, 100, 250, 500...

Not sure what the resistance is for the engines temperature sender? Maybe someone else can shed some light.
 

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The resistance of the temperature sender changes with temperature. At 60 C the resistance is 104 ohms, at 85 C 44 ohms, at 110 C 20 ohms and at 120 C 16 ohms. This information from my trusty Haynes manual.
 

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If memory serves correct preset on the gauge is 100 ohms. Takes a bit for mine to come off one bar flashing, 5 minutes of driving maybe. Once the thermostat opens, seems to register well.
 
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