Main.TrailIndexPage | Engine | Thermo time switch
Thermo Time Switch
This component is part of the Engine. Also see the related topic Fuel Injection. For a comprehensive look at how this all works together, see Engine Starting Aid Tour.
Definition
- The Thermo Time Switch (German: Thermozeitschalter) is a temperature-controlled switch that is normally closed but opens depending on the environmental temperature.
- Part number: 001 545 92 24 - starting with engine number 000405
Function
The "thermo time switch", similar but different from the thermo switch, senses coolant temperature and provides a time interval for it's feedback to a relay.Early 230SL systems, (version ll) had both a "thermo time switch" and a "thermo switch" along with two relays and a "time switch"! The thermo time switch sensed engine coolant temperature and regulated the activation of these devices longer as coolant temperature decreased.
There are several different thermo time switches used on these W113 cars over the years of production. The main difference in function is the temperature range of when the device is actuated and how long (duration in seconds) it is actuated during the cold starting process.
Testing
BBB Passenger Cars starting 1968 - Addition March 1972 page 07-14/19:
The Thermo Time Switch can be tested on the bench with an Ohm meter by measuring the resistance between terminal "G" and ground and terminal "W" and ground.
The resistance value between "G" and ground should be:
- At a temperature below 35 degree C: about 20 Ohm.
- At a temperature above 35 degree C: about 60 Ohm.
The resistance value between "W" and ground should be:
- At a temperature below 35 degree C: 0 (zero) Ohm.
- At a temperature above 35 degree C: about 100 Ohm.
With the switch installed on the engine the same procedure can be used but both wires on terminals "G" and "W" must be disconnected!
Alternative Test
Checking the thermo-time switch (TTS) goes like this:
- The engine must be cold, the colder the better.
- Remove the distributor cap or otherwise disable the ignition system so the engine can't start.
- Connect one lead of a test light or voltmeter to the terminal on the CSV solenoid, the other to a good ground. Put it where you can see it from the driver's seat.
- Watch the light/meter, and crank the engine.
...If you DO get light or voltage reading, keep cranking until the light goes out/voltage drops to zero, but count the seconds until it does. If you got light/voltage for several seconds (more than 2 but less than 12), then the relay, wiring and TTS are ok.
If you get light/voltage for 1-2 seconds only, test the TTS as below (you car may have the extra time relay, which can confuse things at this point, but if the engine is real cold and you only get light/voltage for 1-2 seconds, then the time relay is probably working but the TTS is probably bad).
Note, it is important not to crank any longer than necessary. If you crank too long, the TTS warms up and the TTS test below is not valid. If you cranked for more than 8-12 seconds, let the car sit for 15 minutes before you do the TTS test.
...If you DO NOT get light or voltage reading, stop cranking immediately. There is a problem with the TTS, the relay, or the wiring.
To test the TTS, connect a wire from the terminal marked "W" on the TTS to a good ground. DO NOT connect it to the terminal marked "G", things might get real hot if you do. Crank the engine for 5-6 seconds. If you get light/voltage now for the entire time you crank, the TTS is bad. If you get no light/voltage, there is something wrong with the relay or wiring, BUT the TTS might also be bad, you just can't tell until the other problem(s) is (are) fixed.
