Main.TrailIndexPage | Engine | Time Switch

Time Switch

This component is part of Engine.


The Time Switch aka Time Relay is a thermally controlled relay that operates for ONE Second only during Engine Start.

Time Switch

Time Switch MB Part # A 001 545 16 24

Time Switch/Relay Wiring Harness MB Part # A 108 540 10 09

For a comprehensive look at how this all works together, see Engine Starting Aid Tour.


Inside of Time Relay

The Time Relay/Switch was used on some early 230sl models as a supplement to the thermo time switch.

It was available as an option on later models 250sl and 280sl to help solve starting problems. For more info see Engine Starting Aid Tour.

It is a thermally operated relay with contacts designed to operate for one second only.

One of the contacts is a bi-metallic strip with a heating coil wound on it. When 12Volts is applied for one second the bi-metallic strip bends enough to break the electric circuit through the contacts.



The Time Switch can stop working because the contacts burn or pit causing an open circuit.

A typical problem associated with this is prolonged cranking when starting.

General testing of the Time Switch for correct function involves applying 12Volts to contact #86 and earth to #31 then testing for continuity between #31 and #85.

If there is an open circuit or a very high resistance you will need to either replace the unit or service it.

Servicing the Time Switch

Relay Top and Bottom

On the pin end perimeter of the can there are several areas that have been crimped. These will need to be prised open.

Note the orientation of the connector studs for correct re-assembly.

The module comes out easily.

The relay contacts can be cleaned using a fine file or emery paper.

If the heating wires are broken you may be able to re-attach them to the lugs.

Relay Internals

Relay Internals from Top

Adjusting the time function

Time relay adjustment screw

Connect 12V to #86 and earth to #31 (terminals of the heating coil) via a Test Switch.

Connect an Multimeter/Ohmmeter across the contact terminals, #85 and earth #31. Normally there should be a circuit with very low resistance.

Apply 12V and note the time taken for the contacts to open.

Adjust the Screw ONLY if no path is made through the contacts OR the time is different from one second.

Changes must be made with great caution.

For example: on this switch, when the notch of the screw was aligned with the black dot of reference, the opening time of the circuit was 25/100 of a second (the average of 4 timings).

About 20 degrees clockwise was enough to increase the time to 95/100 of a second.

Disconnect the power immediately after the circuit is opened to avoid overheating of the small coil.

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