Please use the new Arduino ACTH http://priuschat.com/forums/gen-ii-...-automatic-coolant-temp-hack-better-mpgs.html First off I would like to thank jstcd for his help on explaining the circuit and helping me out. All credit goes to jstcd. My modifications are based off his original design located here http://priuschat.com/forums/gen-ii-prius-modifications/36342-another-thermistor-hack.html#post628184 jstcd and myself cannot be held responsible for any damage that may occur with the use of the ACTH. OK so I have finally got around building the Automatic Coolant Temp Hack ( ACTH for short ) a few months ago and have finalized the circuit. There are a few issues with the design that you kind of have to deal with. I'll explain them later. Here is what the circuit does when it is setup correctly. I will use the temps I have setup on mine as an example. When the ACTH is turned on and the temperature is below 105F the Prius sees the actual temperature. When the actual temperature reaches 105F the ACTH kicks on and spoofs the temp to 170F. When the actual temperature reaches the spoofed temperature the ACTH will "turn off" to let the Prius see the actual temperature. The ACTH will stay "off" when the actual temperature is above 170F. This provides over heating protection. Now if the temperature cools back down below 170F the ACTH will turn back on and spoof the temp to 170F. It will keep spoofing no matter how cold the actual temperature gets. This can be a problem for people in the winter time or very cold temps. If the ACTH is spoofing and you let the coolant temp drop below 100F you have a chance of stalling the engine if it wants to run. For standard Prius owners this should not be too much of a problem even if you P&G. This problem is more for Plug-In owners that do a lot of EV mode driving. Standard prius owners could have an issue with this if you let the car sit too long or you are doing extended glides. I have worked with jstcd on trying to make a cold temp safety but I could not get anything to work right. Since I have been using this current design for a few months now with all most no issues I have decided that for the ones that will know how to use this device it should be OK to use. If your using this in cold weather just be sure to turn off the ACTH if you think the coolant temperature is getting too cold. It does not hurt to turn it off and back on every once in a while. In fact I have been doing it a lot and have not seen any issues doing so. The other issue that you may get when using this is some error codes. I don't know why they pop up but they can be cleared with a ScanGauge or CAN-View without issue. The only time I have seen the error codes is when I start messing with the circuit. If I just left it alone and let it do it thing I would never see an error. Switching the ACTH on and off multiple times does not seem to produce the errors so I dont think anyone will have any problems with that. The only other issue is with the ambient temperatures. The transistor that controls the spoofed temperature is affected by heat and cold. In the heat the spoofed temperature will be lower and in the cold the spoofed temperature will be higher. So you may have to adjust this a few times a year. You will want it to stay above 159F or you wont be able to get into S4 and the ACTH would pretty much be useless. You don't want it to get above 176F or the car will want to keep the engine on for about another 30-60 seconds while it circulates the coolant. DONT let the spoofed temp get above 180F As long as you keep the spoofed temperature between 159F and 176F the Prius will be happy and you will too. This design has only been tested on my 2006 Prius. I'm not too sure it will work on a 2004-2005 Prius. I say this because mrbigh had issues with his design of a manual hack. http://priuschat.com/forums/gen-ii-...emperature-hack-better-mpgs-4.html#post970636 His car is a 2004 and he had to make a design change for a 2007. Installing this ACTH is very simple. All you have to do is tap into pins 18, 19, and 28 on the H15 plug. You will have to mount a switch someone on your dash so you can turn the circuit on and off. Here is the connector you will be tapping into. Its located behind the glove box. E2 in pin 28. THW in pin 18. Pin 19 is to the right of pin 18 in this picture. Here is what it really looks like. Setting the start temperature and spoof temperature is a little more difficult. In order to setup the temperatures you will have to waste some gas. You also need a ScanGauge or CAN-View to view the coolant temperature. To set the turn on temperature start the car and let it run. Get the temperature up to about 105F. Turn the ACTH on. If the temperature does not instantly change and get higher turn the 2nd trimmer POT until you see the coolant temperature change. If it still does not change turn the 1st trimmer pot all away around in the opposite direction. Once you see the temperature change to a higher value than the actual temperature you can now dial in on your turn on temp. Turn the 2nd POT in in the direction that will turn it back off. Now turn the ACTH off and make sure your actual temperature is still at 105F. Turn on the ACTH. Turn the 2nd POT until the coolant temperature changes higher again. This procedure is very very precise. It is very sensitive to the slightest movement. It is very important that you turn it as slowly as you can. the slower you can turn the POT the less you will have to adjust later. Now once the temperature changes STOP! Turn off the ACTH and let the coolant cool down. You can speed up the process by turning on the heat and the fan on high. Let the temperature drop to about 95F. Now turn off the heat. Turn on the ACTH. Wait about 5 seconds. If the temperature does not changed thats good. If it starts to spoof try the procedure again but with something higher than 105F like 110F. Now with car in park and the E brake on press the accelerator until you see the temperature start to rise. Watch it rise above 100F. Once it reaches 102F is should change to the spoofed temp. Now to set the spoof temp adjust the 1st POT until you reach between 165F and 175F. DO NOT START SPOOFING BELOW 100F OR YOUR ENGINE MAY STALL! I cannot guarantee my design will work with any other Prius besides a 2006. My guess is that it may not work in a 2004-2005 and it should work in a 2006-2009. The only way to find out is to have someone try it out. So now for some pictures. This is the final design for the breadboard. This is the final schematic design. 1st trimmer pot is for the spoof temperature. The 2nd trimmer POT is for the turn on temperature. JSTCD pointed out that you may want to use DPST switch to disconnect the pin 18 and 19 lead to the circuit just in case the circuit faild in some way. I have been using the SPST switch to kill the power from pin 18 and it seems to do just fine. Its up to you if you want to use a SPST or DPST switch. This is the top of the final circuit board design. Its not pretty but it works. This is the back of the final circuit board design. Its not pretty but it works. This is the circuit installed in a project box. its a pretty small box and you can hide it just about anywhere. It will fit nicely behind the glove box. You can see the 3 wires coming out the side. Pin 18 goes to a SPST switch, The Red wire is for power which comes from the SPST switch, Green is for pin 19, Black is for ground pin 28. The hole on the left is to adjust the spoof temperature. The hole on the right adjusts the start temperature. I have this final product installed in my car and its working just fine so far. Since its a duplicate of my breadboard design I don't think I will have any issues with it. I still have this laying on my dash so I can easily play with it if needed. I will be waiting until the weather warms up before I do permanent install behind the glove box. I have permission from jstcd to try to sell some of these. So my question to you would be, would you be interested in buying one of these ACTH knowing of the issues and responsibility involved? I'm thinking of selling them for $40 shipped to anywhere in the 48 states. The ACTH will come in a small black box. I will have pictures when I get the first one built. It will fit nicely behind the glove box where you tap the wires. You will be responsible for the wire taps, SPST switch, and wire for the switch. I will not include these items because I have no idea on what size switch you want and where you want to put it. So it up to you to purchase these items at your local Radio Shack or parts store. I wont be able to start making these for about another month. So for now I'm just interested in seeing if anyone would like to buy one. I will post more detailed installation instructions if people are interested. I'll edit this first post as needed. I'm no longer selling these because I ran out of parts.