A/C Options... - Page 2 - PeachParts Mercedes-Benz Forum

Hit Man X · #16 03-20-2018, 12:51 PM

Solenoid blocks die with age too. Well, the solder fails and causes goofy commanded airflow.

Mikebear79 · #17 03-20-2018, 08:18 PM

Yeah, I could see that. I drove the 560SEL today all the way down to the euro pick 'n pull in South Phoenix for some random small parts. I had the climate set to 18 C the whole way and not once did it transition from center vents to defrost, which tells me the vacuum is probably fine. I must have driven a good 25 miles, and it's 81 here today. I even tried driving for a while with the recirc button pressed and still no weirdness. Maybe yesterday was just that sort of mild in-between outside temp where the system doesn't really know which to pick (a/c or heat). - Mike

Mikebear79 · #18 03-21-2018, 12:08 AM

Nope, the more I think about it, the more it doesn't make sense. If it's a mild day and the system switches between heat and a/c, it should be switching between the center vents and the floor vents, not the center vents and defrost. I bought a MityVac so maybe I'll find something out soon. I'm going across country in July and I really really really want a 100% functioning HVAC system. - Mike

Diseasel300 · #19 03-21-2018, 12:27 AM

If you have the temp wheel set in the "white" area, the CCU can shut down the center vent and redirect the bulk of the air to the side vents and "leak" to the defrost nozzle when the fan is ramped down and the heat is cycling. What it does and how it does it depends on the temperature inside the cabin and the ambient temperature outside, sensed in the intake plenum near the firewall. The same setpoint on the temp wheel will behave differently on a cool day and a hot day, despite maintaining the same temperature inside the cabin.

To test your system, stick the temp wheel firmly in the blue, say around 18˚ and see how it does. It should keep the center vent on all the time and the fan shouldn't drop to the lowest speed unless it's getting pretty cold in the cabin. When in the "blue" area, the defrost nozzle should not be open and the center vent should always be open.

Mikebear79 · #20 03-21-2018, 12:32 AM

... which is exactly what it’s doing when on 18. It stays on center vents. Again though, just seems weird that it would open up defrost on a relatively mild day. What the hell for? And I don’t recall my 300E doing that at all.

Diseasel300 · #21 03-21-2018, 12:43 AM

Whether or not the defrost vent cycles is determined by the outside ambient temperature and the position of the temp wheel. If it determines that it's cool enough outside to cause the window to fog, it'll move the vents around so that some of the air goes up the defrost nozzle to keep the window from fogging.

If the outside temp is warm enough (above 50 or so), it could be that the outside temp sensor is skewed in such a way that the CCU thinks it needs defrost.

There's a section in either the owner's manual or the FSM (I forget which) that gives a very detailed explanation of where air is sent and what conditions have to be met for it to do various things. It's a surprisingly sophisticated system, especially for 1980s technology. That's not to say that its without its own laundry list of quirks...