With the engine running and power to the coil, try tapping the clutch (very carefully) with a screwdriver handle, on the face, tapping toward the rear. If it engages, your gap may be too large.

Also (perhaps first) check the electrical resistence of the clutch coil. I do not know if your ML has a thermal fuse like my E, but it is possible that you have one and it is blown. If so, it usually means that you need a new compressor.