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JimFreeh 08-13-2016 09:53 PM

W140 S500 overheat, lost coolant
Whilst driving to work Friday morning, my low coolant light came on. I wasn't too concerned, figured I'd look at it after I got home.....

I left early for my chemo appt and saw a nice puddle under the front of the car. Figured I'd keep a close eye on the temp gage. Got about 2/3 of the way when the temp ran into the red zone. Pulled into a convenience store and bought a flat of bottled water. That got me to the docs. After chemo I refilled the bottles, then poured the water into the radiator. That got me home.

Since there was no smell from the vents, I assumed the heater core was sound.

An examination of the radiator and engine compartment revealed coolant under the master cylinder.

To see further, I needed to remove the cowling and the wiper assy.

Still could not identify a leak in any of the Medusas hair nest around the booster and master cylinder. I noticed that the heater return line was getting hot and that I was getting water leakage under the car.

Turns out the wiper deicer had sprung a leak, and was leaking behind the firewall into the cavity that contains the m/c.

Tied off the hoses and all is well.

BTW, the W126 chassis has this "feature" as well.


JimFreeh 08-14-2016 04:03 PM

Too much of a good thing?
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.


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