The A/C on my 190DT would turn itself off at highway speeds. This happens when the Klima thinks the belt is slipping, and usually means you need a new belt. Since I hadn't serviced the belt system in quite a while, I changed the belt, tensioner and idler wheel. No change. Once the obvious mechanical problems are accounted for, it becomes much harder to diagnose. It could be the EDS computer, OVP, compressor, compressor speed sensor, engine speed sensor, Klima, or wiring. I installed my spare OVP and Klima, no change. Cleanned every connector with Deoxit. No change. That left the toughest kind of problem...figuring out why the output from the compressor speed sensor isn't matching the engine speed sensor, knowing that the repair could involve the expensive replacement of the compressor or idle computer. Before going further, I reviewed the great article by Steve Brotherton: PeachPartsWiki: Mercedes-Benz A/C Control Diagnostics He mentions something that hadn't occurred to me: the fan clutch could be slipping due to having been oiled. Now, it's been a dozen years since I installed the compressor and in the interim there have been any number of leaks to deal with: head gasket, power steering hose, front main seal, holed pan, it's a long list. So I bought a can of brake cleaner and squirted it into the gap between the a/c clutch plate and the pulley. You wouldn't think a magnetic clutch would be much affected by oil, but there ya go. Problem solved, that simple. Never would have thought of it.