check the no.5 or no.6 in the fuse box - if that blows or fails contact the ELR does not power up hence you get no tach signal to the gauge. I believe its the same one that also powers up the klima (dont quote me on this)

check the simple items first.

ive got an opposite problem in my 95 E300D and am ready to pull my hair now - ive got ELR powered up and everything - very new harness (less than 4 years old manufacture and less than 1 year old in commission) - but its maintaining rpm at 1000.. these MBs are funny to diagnose