Vent issue - Mercedes-Benz Forum
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post #1 of 4 (permalink) Old 08-13-2019, 08:49 PM Thread Starter
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Vent issue

Recently my 97 S320 intermittently started to not switch to defroster/dash vents/floor when selected. The ac and heat work, blower works, all functions on the display work, it just won't switch.
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post #2 of 4 (permalink) Old 08-14-2019, 04:43 AM
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Is your PSE pump working?

The flapper doors that control the vents all operate on the PSE vacuum system, and if that pump goes bad it can cause problems with vent selection and airflow.
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post #3 of 4 (permalink) Old 08-14-2019, 06:28 AM
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There is also a vacuum line from the engine that goes to a distribution block behind the passenger side kick panel. From there, a line goes to the vacuum control block for the HVAC system behind the glove box liner. That control block has control valving to send vacuum to the various actuators that open and close the vent damper doors. If there is a leak anywhere in the system, it can cause the issues you describe.

You can find a description of the system and a trouble shooting guide at
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post #4 of 4 (permalink) Old Today, 04:25 AM
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most probably (not necessarily) the culprit is leaking distribution block at the passenger foot well (behind the carpet and metal plate).
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