Two things that I am aware of will cause this...One is a vacuum leak (one common source is the vacuum line that goes under the battery tray)
The other thing is the potentiometer that is in the dash.
We sell the service Factory CD repair manual for this. 201 chasis.

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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