There are three filters
and two are very easy to replace. The third one is difficult to get to!!
Suggest that you get a copy of Maintenance Manual, part number S2389-CS1. Starting with model year 1994, all the standard maintenance items are detailed complete with how-to- get-apart sketches.
1) Recirculating air filter:
Right side under dash, turn latches 90 degs counterclockwise on filter air housing. Remove the cover to expose filter element. Replace with new element. This filter had been changed and was not too dirty, so I washed and dried it. These are very inexpensive to replace.
2) Dust filter (also known as prefilter):
Open engine hood, pull of lip seal rubber. Disconnect vacuum lines from main air flap actuators. Remove screws holding air plenum. Pull out outside temp sensor (B10/5). Lift air plenum housing up to expose dust filter on passenger side directly over the blower/regulator assembly.
Replace the filter with the appropriate version: some cars have just a plain air filter, some have a charcoal coated filter. The latter is about $180!!!
3) Activated charcoal filter (ACF):
This one is a bugger to get to! Must remove the windshield wiper assembly. Then remove the cover (need a special MB pin-spanner wrench) for the inside heater housing. Then using a vacuum pump, apply vacuum to pneumatic seal of the ACF to be able to pull it out. This one also is very expensive, $180!
Hope this helps!
Regards . . . . JimF
'94 S500 Cpe
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