The cylinder head temperature sensor measures the temp from cylinder head #3 and sends it to the DME computer. The original part (911.606.405.00) got its ground from the cylinder head and had a single wire going to the DME. Apparently this method of providing ground is unreliable, so the updated part provides its ground from a second wire (964.606.405.00 though there may be a new part number which is 930.606.915.00). Interestingly, on my car (87 Carrera), there already was a second ground wire in the car (two wires running to a 2-pin connector), even though the original sensor had only one wire (1-pin connector) and didn't use the ground. This was good news, because it meant that the upgrade didn't require any new wiring.
On the left side of the engine compartment towards the back, you'll see a bracket with three wire connectors on it, the top one is for the head temp sensor. If you try disconnecting the sensor while the car is running, you'll see that it does run but very roughly (and overly rich). I have heard some differing opinions on whether a faulty sensor could cause no-start conditions, but my car it definitely will not start with the sensor unplugged. It's easy to check if you have the upgraded (964-style) part in case you're wondering.
If you disconnect the sensor at the bracket in the engine tin, you'll notice that the left side (wire to DME) has two wires going to a 2-pin connector. The right side (wire to sensor) has a 1-pin connector if it's the original part (911.606.405.00), or a 2-pin connector it's the upgraded part (964.606.405.00). The upgraded part also has a thicker wire going to the sensor because it's really two wires (additional one for ground) under the insulation instead of the single wire on the original part. [Rumor has it that this is not actually a 962 part, but a 930 part instead.]
If you are diagnosing a no-start condition and have the original sensor, you might want to test the sensor's resistance to ground with an ohmmeter. Put the positive probe on the single pin in the connector coming from the sensor (the connector on the right which is fixed to the bracket) and the negative probe on a known good ground. If the sensor is working properly, it should read in the 1-2k ohms range. If it reads zero or infinity the sensor is probably bad. Interestingly, I can't seem to get a proper resistance reading from the new sensor, but it definitely works.
Okay, here's the procedure for upgrading the sensor without using any special tools. Jack up the car (rear) and secure it on jack stands, remove the left rear wheel. Inside the wheel well you'll see an oval-shaped grommet in the engine tin with a wire coming out, then the wire runs through another grommet up higher in the wheel well (along with two other wires). Remove the lower grommet by prying it out with a screwdriver. You'll see the head temp sensor in cylinder head #3 inside the hole in the engine tin. Normally a special tool would be required to remove and replace the sensor (a slotted socket that fits over the wire), but it's not needed.
Instead, cut the wire off the old sensor (as close as possible to the sensor itself), then extract the old sensor with a deep socket. Thread the new sensor in by hand and tighten it with some needle nose pliers (it's a bitch to get in there but works). Now pry out the top grommet with a screwdriver (this was very difficult, I actually had to cut out my old one). Feed the new wire through the bracket inside the wheel well (you'll need to remove and replace the little bracket). You'll notice that there are two additional wires that run through the top grommet (back into the engine compartment) along with the head temp sensor wire. Fortunately, the new grommet is slotted so these extra wires slip in easy. Slip the two extra wires into the new grommet and feed the wire through the hole. Don't replace the grommet till you've installed the wire in the engine compartment.
You should be able to reach into the back of the engine compartment and grab the new sensor wire. You'll see that it runs around the manifold pipes and through a bracket in the middle of the engine. This bracket can be easily removed with a 4" socket extension and wrench to feed the new wire through. To install the new wire on the bracket at the left side of the engine compartment, you need to take apart the bracket. You'll notice that there are two little screws on the face of the bracket. Remove these screws and the bracket slides apart. It's easier to get at these screws if you unplug all three wires (be sure to label them appropriately). Slide the old plug out, the new one in, replace the cover of the bracket.
You should see two pins in the connector on the DME end (left side). The top pin is for the temp signal, and the bottom pin is for ground. You may want to check that you are in fact getting a proper ground by reading the bottom pin with an ohmmeter. You'll notice that the connector on the sensor end (right side in the bracket) now has two pins instead of the original single pin. To complete the job, don't forget to reinstall the top grommet in the wheel well. This part is difficult, so be prepared to let a few profanities fly ;-)
That's it, head temp sensor 101, for the DIY mechanic. Thanks to Darrin Sacks for explaining how to do this with no special tools.
Doug (87 Carrera Targa)