TTS - Thermo Time Swich

[Stl-Joe]

I just solved a nagging issue with my 1969 280SL Automatic.  It would rev high (1200-1400rpms) on starting and then settle down to 700-800rpms after driving it awhile.  However, even after it was warm, when I would shift from Park to 4th (or any gear for that matter), the rpm's would jump right back up to 1200-1400 and stay there until the car was again driven a short while.  I adjusted the linkage (thank you Joe Alexander) and replaced the plugs, plug wires, points, condenser and also replaced the transistorized coil with a non-transistorized unit. None of these measures solved my problem.  A seasoned MB mechanic here in St. Louis suggested we replace the Thermo Time Switch (TTS).  It looks like an oxygen sensor and is located in the vicinity of the mechanical fuel injection system.  Voila!  This component was original and defective and causing my problem.  The car runs beautifully now and shifts much more smoothly.  This part is, apparently, hard to find and there are many similar (but not compatible) parts on the market.  Also, be prepared to pay $600-800 for this part.  So, if you need one be discerning in matching the specs of this part to your car.   

[ja17]

Interesting result! The TTS regulates the starting devices!? We're glad your engine is doing well, it just does not make sense.

[Pawel66]

Interesting result! The TTS regulates the starting devices!? We're glad your engine is doing well, it just does not make sense.

I am proud to say I had the same thought as ja17.

But then... if TTS was shot and worked in an erratic way, could it send signal to enriching devices in wrong moments, making the mixture too rich when it should not have been rich?

[BobH]

Don't forget the starting relays only receive power during cranking, so can't operate during normal running, even if the TTS provides a permanent ground if faulty, the relays won't have a +24V

[BobH]

....This part is, apparently, hard to find and there are many similar (but not compatible) parts on the market.  Also, be prepared to pay $600-800 for this part.  So, if you need one be discerning in matching the specs of this part to your car.

There is a procedure for testing these devices in the tech manual, as you say these are NLA and therefore have a considerable value, so don't discard it until you check

https://www.sl113.org/wiki/Engine/ThermoTimeSwitch

[Pawel66]

Don't forget the starting relays only receive power during cranking, so can't operate during normal running, even if the TTS provides a permanent ground if faulty, the relays won't have a +24V

You are right, I was wrong!

[bracurrie]

I'm not an expert, but doesn't the increased idle speed when cold have something to do with the distributor timing retardation being canceled until engine is warm. If TTS triggers as if engine is cold then retardation is canceled and with more timing idle is higher. There are other things that cause retardation cancellation such as RPM and the engine temperature being above, I believe, 200F.
Brad

[ja17]

I believe the distributor timing is regulated by it's own separate temperature sensor.

[Bshaunessy]

STI - JOE
  RE expect to pay $600-$800 for a new MB TTS: there is an excellent write up in tech manual that, among other things, explains all the workings / logic / wiring diagram of the entire cold start system, including the DEVEL THERMOSTAT TIME SWITCH.
  Also outlined in there is a suitable alternate TTS at much less cost and more readily available : New Bosch TTS connections Bosch # F026T03101.   I had occasion to buy and install the Bosch substitute ( available from Porsche or Volkswagen jobbers) for approx $125 usd. You need to change the electrical connection plug ( easily done) but otherwise it fits on the hole in the block AND functions as it should.