Like Mr97 said,

You need to check line pressure gauge. The name of the game is pressure.. I'm assuming this is a pre-1999 CLK320?

No flow? Okay, I do not know the cooler line set-up on a CLK. I'm assuming it has the little V6 and not the 6-Inline. On the V6 E320, the top cooler line on the radiator is the line from the transmission with hot fluid(to the cooler to get cooled). If you disconnect the top line, and attach some clear vinyl tubing to a clear clean/new painters graduated 1 Gallon paint bucket. It should take about 20 to 30 seconds to empty out two-quarts of fluid with the engine idling in park. If it takes minutes, you have a pressure problem...

Once you are done, just pour the fluid back into the car, and do only two quarts.

In your case, the pressure regulating valve/spring can be shot or broken(common), and or the pressure regulating solenoid is just getting tired, and these old first generation 722.6 transmissions are having this problem. The solenoid will not generate any kind of code unless they open(electrically), or short out, but when they loose mechanical or magnetic properties you get no codes.

The slippage codes, or adaptation codes are critical for clutch wear. If you have the first generation 722.6, these are vulnerable to K2 bearing failure. If you find large chunks of copper washers, you are doomed, and it will follow with needle bearings parts, but even with loosing needle bearing parts the gears will hold..

Here are two threads with juicy info. The first one has some good info. The second has lots of data.Read them both well, and download the attachments from the other, and read those.

E420 Transmission Control Problems

No Transmission Fluid Symptoms - Benzworld.org - Mercedes-Benz Discussion Forum

Best of luck,

Martin