So been asked by a couple members here to post on I am doing my install for rear diff sti temp sensor and light on the sti cluster. Going to post as I do things(sorry just don't have time to do everything in one shot). Way I do it is not the only way of doing it, just the way I decided was best. I will be as detailed as possible. However had a hard time taking pictures under the car since I only has a few inches of clearance. I did this install on a 06 WRX Limited. I am sure its the same for all 02-07 models.
If somebody knows where I can upload the pictures so I don't accidently remove them in the future that would be great too.
Give everybody idea on cost I spent $90-100 total for everything. I got entire rear harness and new sensor from a member here.
Items you need:
Rear diff temp sensor
Rear diff temp sensor pigtail tail
Ground bracket for rear diff
Here is the wiring diagram for it. You can see sensor has 2 cables. Only 1 of these cables are actually on the sensor. The other(ground) gets attached to a separate bracket mounted on the rear diff and grounded to the chassis. So doesn't matter where you decide to ground that cable. I personally have choosen a nut under steering dash that holds all the fuses and is mounted to the big round bar.
The cable from the sensor goes directly to the cluster pin. You say Talvai but it touches DCCD controller. Yes it does. This is because when light is on it locks the center diff to help cool the rear diff. However since I have DCCDPro controller I have to just override this. Of course if you wanted to you can tap on of the DCCD cables before cluster so it sends power to it and therefore locks the diff. I am perfectly fine locking the diff myself so I going around it.
I still need to pick up this bracket. Its called Stay-earth and part number is 11086AA022 its about $2-4 from dealership new. The sensor part number has a couple not sure the differences between them if any. 27043AA011, 27043AA012, 27043AA012. These sell for about $120 new from dealership.
Step 1 is easy. Install your rear diff temp sensor. My diff is from a JDM site so I didn't have a sensor already installed. Sensor goes on top bolt location.
Step 2. Prepare your pigtail if you have whole harness. This harness is called fuel cord. Partnumber is 81803FE052. Don't know cost of it, but sure its not cheap. All you really need is the rear diff connections. So I detaped the entire harness, removed all hard plastic pieces and cut the harness. I did not want to replace my entire harness as 90% of these connections are just a pain to reach as they are above the fuel tank.
Step 3. Now that everything is detaped you removed the plastic that surround the other cables I know cut my harness so that I only had the 2 connectors I needed. If you cut this harness all the way at the end you will have enough cable to reach the backseats. From that point on you need to solder cables together to extend it.
That being said I didn't want this harness to go to waste so saving cables for future project and all the plastic I removed I wrapped around the rear diff pigtail and then taped it all with electrical tape. I did this for a few reasons. One reason was weather as I know feel comfortable that no water/snow/mud will reach the cables themselves. Other reason was so when I wiring it didn't have to worry about wires getting cut from touching metal.
Above is a picture of the rear diff harness before electrical tape and plastic pieces. Note the 2 connectors for the sensor and bracket.
Step 4: Wire cable from rear diff to inside the car. How you do this up to you. I decided I wanted to route it on driver side as its closer to cluster and I found a spot to go inside. I also choose this method because it hides the cable very well and looks almost OEM like. As you can see harness is in on sensor(other cable is tucked up top). I put the clip on the crossmember just like STi and then ran it above the rear crossmember.
Above the crossmember I clear all axles, swaybars, lateral links, etc. I then went through little openning at the end of the crossmember right by fuel tank. Sorry don't have pics as they came out all blurry. Their is a little hole on a plastic surrounding the fuel tank. I zipped tied the wire there and continue to run it along side ABS line. From there I noticed 3 hardlines however the rubber piece had a spot for a 4th that was empty. So figured perfect spot to run cable inside.
This is where I ran the cable inside. This is where backseat would be on the driver side. Note the bar that goes across, the 3 hardlines and the ABS cable. That rubber piece had a hole not being used. So poke a little hole in it and ran the cable. Now that the cable is inside, I soldered cable to make it longer to reach the front of the car and you can run it along the door sills, hiding it the entire time.