The brief
The SL280 had crossed 120,000 km and had picked up a high-pitched squeal under load from the front of the engine. The owner had also noticed the steering felt heavier when the engine was hot, which is a telltale that the power steering pump isn't being driven properly, and the battery warning light had flickered on a couple of times, the same story from the alternator end. All of it points at the belt drive at the front of the engine running out of grip.
The accessory belt is the single ribbed belt that drives the alternator, the power steering pump, the aircon compressor and the water pump off the crankshaft. A spring-loaded tensioner keeps it tight and idler pulleys guide it. After a lot of years and a lot of kilometres the belt glazes, the tensioner spring weakens, and the belt slips, so the things it drives stop being driven properly. That's the squeal, the heavy steering and the flickering light.
The diagnosis
On the lift the belt's rib face was glazed shiny with fine surface cracks across it, the wear you'd expect on a belt that's done 120,000 km. The tensioner tested weaker than spec, with a visible wobble in its bearing, so it wasn't holding the belt at proper force, which is exactly why it slipped under load. The idler pulley is on the same age clock.
Replacing only the belt would leave a tired tensioner ready to be the next failure, with the same noise back in a few months. So this was a whole-set job: belt, tensioner and idler together.
The work
Belt tension was released, the old belt and the failed tensioner came off, and new Mercedes-spec parts went on, including a fresh idler pulley. The new belt was routed correctly through the accessory drive, and tensioned to spec.
Then the engine was run through a warm-up cycle with the aircon, the alternator load and the steering all working, to confirm the belt drive ran silent, cold and hot, under load.
A check of the steering effort when warm and the charging voltage confirmed both were back to normal.
The outcome
No squeal on start or under load, no slip when the aircon and alternator both pull, and the steering weight is back to normal when the engine's hot. The battery warning hasn't flickered since.
The SL280 went home with the front of the engine quiet again. On a roadster of this age, doing the belt drive as a set, belt and tensioner and idler together, is the right way to do it once rather than chasing the same noise back every few months as the next worn pulley gives up.