Checking the additional air damper (Mercedes-Benz S-Class W126)

When the engine warms up, it requires an increased supply of air-fuel mixture, which is determined by the position of the throttle valve. For this purpose, an additional air valve is used to create a channel that bypasses the throttle valve. On a warm engine, the cross-section of the additional channel must be closed.

A faulty auxiliary valve may cause the engine to stall or run rough during warm-up, and may also have unregulated idle speed.

Caution: The engine must be cold.

To check, you need to perform the following operations:

Disconnect the auxiliary air damper connector.

Connect a voltmeter or a portable 12 V lamp to both contacts of the connector.

Start the engine. The voltmeter should show the battery voltage or the extension lamp should be on. Otherwise, find the wire break.

Reconnect the connector. Start the engine and let it idle.

Clamp the additional air damper hose (see arrow), after which the engine idle speed should drop.



Warm up the engine and pinch the hose again. The engine speed should remain unchanged. Otherwise, replace the additional air damper.