I first noticed the wiper deicer feature on my 90 560SEL. It's a pan-like item that mounts underneath the windshield where the wiper parks. Looks a little like a empty hot water bottle. You can find it by following the small heater lines near the cowl.

Problem is... if you have one fail, you must remove the windshield to replace. For me, living in a temperate climate, I'm going to bypass the feature and move on.

The system bleeds a small amount of coolant through the pan, and is controlled by a temperature switch located in the intake air plenum.

Whilst looking over the car for the leak, I noted the leak was draining from the left side of the car, directly behind the LF wheel. Looking at the engine compartment, I could find no signs of leakage, until I looked between the two firewalls and noticed that there was coolant present under the brake booster/master cylinder. I did not see any coolant on the rear firewall (where the booster is mounted).

To get a better view of the area, I needed to remove the plastic plenum and the wiper assembly. All this revealed was that most of the area was dry, especially anywhere near a coolant hose.

Since I was not getting any coolant smell out of the vents, or any moisture on the windshield, I was confident that the heater core was intact. So I concentrated on the area ahead of the firewall.

The big clue that the wiper deicer was the leak source was the fact that the heater coolant return line was getting hot, even though the heater valves were still closed and the input lines to the heater core were cool.

Since the return was hot, that meant there had to be flow in the line. I traced the heat to the exit of the deicer. No heat present on the input side. Which was as expected, the thermostat would be closed if much above freezing.

Blocking the lines to the deicer solved the leak problem, and I consider it done. Drove around for 30 miles yesterday and the car ran fine. Checked again to day, and no sign of leakage.

I refilled the water bottles and placed them in the trunk, just in case I have a problem on the way to work tomorrow..... Sometimes I miss the simplicity of the old diesels.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles