replace thermo time switch

[Andy Antipas]

I have been try to chase down my cold start problem on my '79 911SC RoW coupe. I have James Weber's book on CIS, which is very helpful.

- CO, idle, timing at factory specs.
- Engine starts from hot pefectly.
- The warm up regulator works per spec.
- Once engine catches/starts from cold it fast idles smoothly.
- Once engine is warm it drops to 950 rpm and stays there.

I decided to check the thermo time switch. I checked the the resistance from cold and once the engine was at operating temperature and here are the results:

Thermo time switch is made by VDO and is stamped 45C/9.5s

resistance from cold:

terminal G to ground = 23.5 (30-40)
terminal W to ground = 0.5 (0)
terminal G to W = 23.5 (30-40)

resistance at operating temp

terminal G to ground = 59.2 (55-85)
terminal W to ground = 97.4 (120-160)
terminal G to W = 38.6 (55-85)

Values in (__) are from Weber's book. It looks to me that my switch is out of spec and needs to be replaced, but would like to hear from the experts?

Thanks for the help in advance!

Best,

Andy Antipas

[Paulporsche]

Very interesting, Andy.

I can't help but I'm looking forward to what others say. I'm surprised to read that some of the figures have a lower resistance than spec. It seems they would get higher w/ age.

[Gunter]

Instead of resistance-check the TTS, why not check that 12V reaches the CSV for a few seconds during cranking?
Ignition switch yellow wire to Solenoid - another yellow wire from Solenoid to Thermo Time Switch - then on to the CSV.
12V on the TTS during cranking?

[Paulporsche]

Any update on this?