# Read water temp sensor from dash wiring loom

Hi!

I would like to create a small display for my car. The project is going well. The RPM/VSS/Batt reading and displaying is working but I have issue with the water temp.

Its a 20 years old hobby car and the temp gauge is analog. Sender is a simple NCT thermistor but I couldn’t know how should I connect the Arduino to the wiring loom. The gauge has 3 pin. +12 / GND / T (thermistor). I found its schematic in the manual. I’ve attached.

I think the simple pullup resistor to Arduino’s 5V is not working because the gauge has already have one to 12V.

Is it possible to read the connect resistor values from the NCT sender with Arduino while the gauge still works?

Thanks, György

What is the voltage range, referenced to ground, of the gauge output that you want to measure with the Arduino?

groundFungus:
What is the voltage range, referenced to ground, of the gauge output that you want to measure with the Arduino?

I don't know exactly. I took out the dash and gave 12V to the IGN and GND pin then I put a simple potentiometer between to the thermistor pin of dash and GND.

I found these:
9000 Ohm -> -10C
2500 Ohm -> 20C
320 Ohm -> 80C

Because the gauge powered by 12V I think the range would be between 12V and 0V. Am I right?

Is it possible to read only the resistor values independently of the gauge from the gauge thermistor pin?

Could You post a wiring diagram of the measure setup? Things look strange to me.

As it's a 3 terminal sensor I would guess it outputs voltage. I've used 2 terminal NTC resistors combined with a certain resistor and a special formula to convert the upcomming voltage to degrees.

What resistance makes the gauge read 25C? You say the gauge has 3 wires, diagram shows 4?

Hi!

Just for clarification. Maybe I wasn’t clear.

Thermistor has 2 pin. Gauge has 3 pin. Its need 12v/ground and input from thermistor.

I draw the circuit.

My questions is, how can I read the thermistor values with Arduino without changing the gauge operation?

Thanks