This is the next version I'm working on for the automatic coolant temperature hack. This version is based on Bob Wilson's MSP430 micro controller hack. NHW11 Prius Temperature Hack I took Bob's code and schematic ( with his permission ) and translated it to the Arduino. I was just going to use Bob's design but I was unable to get it to work. Since I understand the Arduino code more than the MSP430 I decided to just port everything over. What makes this different than the hardware based hack is that this hack will have both hot and cold protection. It will also keep the correct fake and start temperature no matter what the ambient temperature is since the logic is based on software and not hardware. All the connections I'm using are pretty much the same as my previous ACTH as seen here. http://priuschat.com/forums/gen-ii-...-automatic-coolant-temp-hack-better-mpgs.html The only thing I'm using different is the voltage source. Instead of taking 5v from the ECU I'm taking the switched 12v from the 12v socket. I started out using a resistor/cap like Bob's design to smooth out the PWM but decided to go with a digital pot. I have removed my original design and replaced it with this current one. If you read down through the thread and some things dont make since just skip to post 18. Post 18 is where I start with this current design. Here is the schematic for this hack. This schematic can change at any time. You can use a DPDT or DPST switch. I just had a DPDT laying around and included it in the schematic. Here it is working on the bench. The blue LED means its too cold to fake the temp. I'm currently using an Arduino Nano on the bread board just for ease of use. When I get it fully working I will be using a regular Arduino in a Arduino project box. Here is the source code for the Arduino. Version 0.11.03.30 The source code includes some serial output for diagnostics. http://stuff.jaygroh.com/priuschat/acth/aacth.zip If you have any ideas or input please feel free to post. This design and source code is completely open source. Feel free to make modifications. If you do make any changes please post about it.