TTS Yellow and Red Wires

[Funracer]

83SC. Trying to better understand my TTS/CSV.

If engine temp below 45C, and I unplug just the yellow wire to terminal G, will the red wire still provide ground for the CSV to fire during start? if so, until starter release or until engine temp reaches 45C?


Same scenario. If I just unplug the red wire, no ground will be provided to CSV so it will not fire?

My guess is Yes to both.

Thanks

[PeteKz]

Yes, yes, and yes.

As you know, the yellow wire is the start circuit, which energizes the starter solenoid, thus it only has voltage when the key is in the START position. The yellow wire also provides voltage to the CSV and TTS. The red/back wire grounds the CSV through the TTS.

When the engine is cold, and you turn the key to START, the yellow circuit energizes the CSV and a heating element in the TTS. The CSV operates because the TTS is cold and connects the red/black circuit to ground.

When the key is in START, the CSV will not squirt fuel when the TTS is warm enough that it interrupts the ground circuit. This happens if the engine itself is warm, so the TTS is warm, OR when the TTS has been warmed by the internal heating element even if the engine is still cold. That prevents the CSV from continuing to squirt too much fuel and flooding the engine, if you continue cranking it after it won't start.

If you disconnect the yellow wire to the TTS (on a cold engine), the CSV will then continue squirting fuel as long as you have the key in START, and you may flood the engine, which may cause you to CONTINUE cranking the engine and further flooding it. Be aware of that if you are experimenting.

[Funracer]

Thanks so much for the easy to understand explanation.

Regards

[Schulisco]

I think I have to add that these tests / measurements are applicable on the old pure electromechanic version of the TTS incorporating a bimetallic strip and heater element. Porsche / Bosch redesigned them some years ago as a pure electronic version. There is from a catalog available from 2017 mentioning this, available here:
https://www.bosch-classic.com/de/media/pdfs/downloads/mini_kataloge/sensoren_1/thermozeitschalter_f026t0310x_2017.pdf
The original Porsche version is not listed there. Guessing Porsche denied. The only difference I guess are the round contacts for Faston plugs in 6.3mm and 8mm diameter.
This electronic version IMO cannot be diagnosed with simple tests with a multimeter. Instead you need to build a test stand similar to the car to get it tested properly.
Here are some pictures from the internals of the old version and an excerpt from Haynes manual added with my comments in german on the contacts:





Thomas