Hi, I realise this is a real lack of info, but can anybody think of a simple reason that headlights, wipers, etc would interfere with the reading from a temperature sensor on an analog input? Arduino is powered from the car battery, thermistor connected:
+5 --- 4.7k resistor -- A1 -- Thermistor --- Gnd
Ground is common across the vehicle, any additional battery/alternator load anywhere in the vehicle gives an alleged increase in temperature, varying depending on the amount of load. The thermistor is a bolt in type, one wire with the bolt itself grounding via the engine etc.
The only other sensor connected at the moment is an MPX4250AP (pressure) which is connected directly to the arduino's + - and A0 pins, works with no interference.
Automotive electrical circuits are extremely noisy and the voltage is highly unstable. Always use a well filtered power adapter intended for automotive use. Also, you cannot rely on the metal frame as a ground reference, so use sensors with a separate ground lead and connect that to the Arduino ground.
Thanks. Must also point out that this is an old, completely mechanical diesel, no electronic ignition etc but yes, I think I'll have to resort to an isolated temp sensor.
What is wrong with grounding the Arduino via the rest of the vehicle, out of interest?
Look up "ground loop" to learn about grounding problems, but basically, the frame has significant resistance, so there are voltage drops between different points when current is flowing.
It's ok I'm familiar with ground loops anyway. I understand what you're saying about the resistance, but the vehicle in question has a full ladder chassis and dedicated grounding straps, not a frame made of tin foil and small high impedance cables, I wouldn't expect the resistance to be the problem. I expect it is a loop problem as you mentioned, which in fairness I'd not thought about.