2000 C280. No cooling effect. Pushbutton Module diagnostics shows compressor discharge pressure (#07) of 6.6 bar and temps around ambient on #03 (heater core) and #05 (evap temp).

I put my gauges on the system and I get a high side pressure of around 8 bar at idle rpm and at 3000 rpm.

On the low side I get about 6.8 bar at idle and when I raise the engine rpm to 3000 the low side pressure drops slowly to around 4.5 bar. Also, the low side pressure randomly bounces around a bit.

Both the suction lines and the compressor discharge piping is at ambient temp.

I suspect the compressor is the problem. Can anyone conform or suggest further tests?

Either a bad expansion valve OR compressor.

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Follow up. It was the expansion valve. Thanks mbdoc!

I do have one concern after opening the system. There was a black residue on the high side ports of the expansion valve. The low side ports were clean. Could this be a sign that the compressor is headed for failure. IIRC, in the past Denso compressors (and the Ford versions) had a characteristic called "black death" before failure.