What sensor is this?
 

1979 3.0 SChttp://forums.pelicanparts.com/uploa...1723940551.jpg

Thermal time switch for the cold start injector.

Looks like 911.617.117.00 "temperature switch" to me. Part #30 on PET diagram 103-05

Aka as TTS (Thermo Time Switch) in the CSV (cold start valve) circuit, causing it to add extra fuel while cranking (wired to the yellow wire on the starter solenoid) in cold conditions. Typically only does this for a few seconds - there's also an internal heater, which opens the circuit/removing the path to ground once it gets up to the temperature rating.

Is the harness missing ?

Thanks for the ID of it. I did an ITB conversion and never looked into what that was.

Then it's not needed. Remove and plug the hole. Those suckers are over $300 new, so you might post it in the 911 Used Parts forum.

On my ITB EFI conversion, I removed that thermo switch (for the cold start injector) and replaced it with a normal temp sensor that is connected to the ECU for the ECT (engine coolant temp) input.

I found that chaincase temps lag head temps by minutes, meaning you'll still be using trim long after the heads reach 60C and you no longer need any. I re-assigned mine as "oil temp", because that's closer to what it reads.

CHT or a temp sensor in the breather tower works a lot better.

True, but I just compensated for the temp lag by adjusting the warm up enrichment to end at 120 deg F instead of 180 deg F. It works well.

You do you.

For me, readings from the chaincase sensor (it's on the right side of the left chaincase on a 930, but don't see that would make much difference) weren't registering more than a few degrees over ambient by the time the CHT reads 60C and up. So using that sensor location would run enrichment on too long. It was very obvious it was running rich.

Heads are the hottest part of the engine and their temp climbs very rapidly once engine is running. Heat transfer to oil is much slower, so this seems consistent with what spuggy describes.
CHT sensor was first introduced on 911 engines with EFI (Motronic) because oil temp was not a good indicator for fuel/air mixture during warmup.

I still use the chaincase sensor; I just datalog it as "oil temp", because it most closely resembles what the temp gauge in the car is reading.

Several have mentioned elsewhere that they have good results with a temp sensor in the breather cover for the EFI warmup cycle. Kind of wish I'd seen that before I went with the relatively expensive/sometimes hard to source Bosch CHT sensor (installed via the TK kit as i didn't want to remove a head and get it machined).

There isn't (or doesn't seem to be) a huge selection of short-reach Bosch temperature sensors, other than the VW/914 single-wire and the 911 one.

However, Ducati use, on their air-cooled EFI motors, a short reach temp sensor in the rocker cover (common to many Euro cars, dirt cheap and readily available as Beru ST088; should ideally use the Magneti Marrelli NTC calibration table rather than the Bosch one - but pretty close for this purpose).

With the benefit of 20/20 hindsight, I would probably do that, if I did it over...