You have to check in two places. You have to see if vacuum is getting to and from the switch and then to the shut off element. The lines are brown and are in the engine compartment. You will need to take to a tech if the element is faulty.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM