Wanted to see if anyone else was experiencing this phenomenon. My wife drives a 2000 model without the automatic climate control.

When the car is cold, but coming up to temp., any temperature selection other than just off of cold comes up as scalding hot. It is as if while the thermostat is closed, the heater is getting super heated water. Once the thermostat has opened a little, the heater operates normally. This is especially annoying of you are running a few close errands, as the heater puts out cold, scalding hot, then warm air. When you stop and get back in, it is scalding hot again and then it goes to warm. Seems I am always having to fiddle with the temperature control.

I did a 4 hour trip in it the other day, and once the engine temp came up, the heat acted just as it should the whole journey. No fluctuation at all. I am sure it was engineered for an automatic system and would be great as heat would be available so soon, but the design is flawed for manual controls.

air mixing door
 

The heating system of the ML-class is unlike most other MB's. The system is a blending system that allows hot water thru the heater at all times & blends heated air with ambient air in the heating mode. SYSTEM is like American cars & in built by Nippondenso US. Possibly air trapped in heater core is causing cold/hot flashes.

The biggest complaint with our 99 ML320 is the wonky heater controls. It is either too hot or too cold, with nothing in between. The rotary dial controls are goofy and difficult to set properly...it seems they don't appreciate the need for defrost and floor heat at the same time. Must not have tested in the frozen north.

It is so exasperating that we are considering a trade up to the 2000 ML430 and auto climate control. Damned expensive solution though!