Thermo-Time Switch

The thermo-time switch, as mounted on the Type 4 engine, is basically inaccessible (see notes below). It is under the intake air distributor, so that must be removed before you can get to the switch. The switch is mounted with one of the case bolts. It may be a bit more accessible on the Beetle.

The thermo-time switch consists of a contact to ground for the cold start valve and a bimetallic strip surrounded by a heater winding. When the engine is cold and the key is turned to "start," the bimetallic strip contacts the ground contact and allows the cold start injector to inject fuel. When the key is turned to "on," that powers the heating element inside the thermo-time switch. As the heater warms the bimetallic strip, it eventually bends away and stops grounding the cold start valve.

This all happens in 10 seconds or less. This is to keep the cold start valve from injecting too much fuel and flooding the engine. So if the engine doesn't start within the 10 seconds, the thermo-time switch will shut off the cold start valve to keep the engine from flooding for future starting attempts. When the engine is already hot, the contact in the thermo-time switch is open, preventing the cold start valve from operating. A stuck closed thermo-time switch could allow the cold start valve to operate too long during starting, causing engine flooding (but regardless, the cold start valve will still only operate when the starter is operating). If the thermo-time switch sticks open, it could keep the cold start valve from operating and cause hard cold starting.

The thermo-time switch can be tested electrically. The temperature at which the switch activates is stamped on the switch housing. You can install a test lamp in series with the wire to the thermo-time switch, disable the ignition, and crank the starter. The lamp should light but then go out in a few seconds of cranking.

The switch can also be tested for resistance between the two terminals. It should vary with temperature, being close to zero when the switch is below the temperature marked on the housing and being infinite when the switch is warm.

The below addendum was sent to me by someone who read the article. It refers to removal of the thermo-time switch from a late Bus.

From Mark Clark:

Hi Sean, Just found your web page and think its great to have this stuff on line. Thanks. I have been fixing my thermo time switch lately on my '77 Westy. It can be gotten off simply buy applying a 15/16 inch socket to the switch after removing the electrical connection. First, it helps if you remove the vacuum line going into the air box, and fuel line going to the cold start valve. I know Bentley says you have to take everything off but it just was not so for me. This could save others lots of time.