My e320 did that with the AC on in stop and go traffic in the summer. After I hit a deer, which I believe knocked the carbon out of the aux fans, it never gets above 100. May want to sit in your driveway with ac on fully warm and see if your aux fans kick on.

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg