300D heater is either off or full blast? - PeachParts Mercedes-Benz Forum

edge · #1 11-12-2009, 03:08 PM

My 83 300D heater is either off or on full blast! When the dial is on the lowest blue temp it is off but when I nudge it just a little beyond this off area on the dial it is full hot. Even with low fan, the car ends up too hot and I have to shut the heater off/on every 3 minutes. What gives, why isn't my heater gradual variable intensity? Is there something I have to replace or adjust. Thanks.

Billybob · #2 11-12-2009, 03:24 PM

Monovalve?!

Jeremy5848 · #3 11-12-2009, 03:36 PM

Check the wiring to the temperature sensor that lives in the top of the dash. If it has a broken wire or is unplugged, the CCU will be "confused."

MBeige · #4 11-12-2009, 08:11 PM

If the cold setting does the same (off or full blast) the foam hose that goes from the temp sensor to the sensor itself, behind the glove box, has disintegrated and must be replaced.

toomany MBZ · #5 11-12-2009, 08:24 PM

I had a similar problem, a new mono valve did not help.
Follow the above advice, if the problem persists, let us know.

edge · #6 11-12-2009, 11:18 PM

I'll check the wiring to the temperature sensor tomorrow in the daylight. I assume you go through the glove. I've turned off all my circle vents and only heat the floor and it's too hot. Toomany MBZ, did you fix yours?

Jeremy5848 · #7 11-12-2009, 11:50 PM

Remove the glove box and you'll be able to see both the temp sensor, its wiring, and the foam tube that is supposed to connect it to the blower but probably has disintegrated. I suspect Mercedes used foam as an anti-vibration measure; you can use foam pipe insulation or a solid piece of tubing as you prefer. The blower sucks air in from the top of the dash over the sensor and, in theory, makes the sensor respond faster to temperature changes in the cabin. The sensor has two wires.

toomany MBZ · #8 11-13-2009, 10:31 AM

edge, no. That car went to the crusher before I fixed that problem.
I will suggest pulling the CCU and check for cold solders, after you've followed the previous posters advice.

Graplr · #9 11-13-2009, 11:32 AM

Quote:

Originally Posted by edge

I'll check the wiring to the temperature sensor tomorrow in the daylight. I assume you go through the glove. I've turned off all my circle vents and only heat the floor and it's too hot. Toomany MBZ, did you fix yours?

Check the wiring AT the monovalve as well. If it is unplugged the monovalve will stay open at all times providing FULL HEAT at all times. The same situation would apply it the wire is broken somewhere. If it is plugged in (on my SD it is right on top of the metal housing for the monovalve assembly- Two wire plug) check the voltage with a multimeter. If you get voltage, you should notice a change in voltage as you spin the dial. If not then I would suspect the Push-Button Unit. If you get no voltage when it is rotated to the cool side of the dial then either a broken wire OR the PBU could be at fault.

Full heat should provide NO VOLTAGE while FULL AIR should provide 12Vs. I believe any temperature inbetween it cycles on and off.

It seems to me this full heat problem people experience is usually an unplugged monovalve.

300TD · #10 11-18-2009, 08:44 PM

Hi...I know my sensor is bad and the heat is unbearable. Anyone know the part number of have a link so I get the right part.

Thanks

Jeremy5848 · #11 11-19-2009, 12:54 AM

Quote:

Originally Posted by 300TD

Hi...I know my sensor is bad and the heat is unbearable. Anyone know the part number of have a link so I get the right part.

Thanks

EPC will have the part number and you can then buy one from your dealer. EPC

Jeremy

funola · #12 11-19-2009, 07:40 AM

If the heat is full off or full hot nothing in between and the means of temp regulation is via the monovalve, doesn't that mean the monovalve is working fine? Anyway, I had this problem with my 300D plus clicking sounds in the CCU. I took apart the temp dial to expose the potentiometer and cleaned it with brake clean, checked full range of the pot with an ohmeter to make sure I have full range of continuity, reassembled and no more clicking and I have control over the temp. YMMV

edge · #13 11-19-2009, 03:18 PM

Quote:

Originally Posted by Jeremy5848

Remove the glove box and you'll be able to see both the temp sensor, its wiring, and the foam tube that is supposed to connect it to the blower but probably has disintegrated. I suspect Mercedes used foam as an anti-vibration measure; you can use foam pipe insulation or a solid piece of tubing as you prefer. The blower sucks air in from the top of the dash over the sensor and, in theory, makes the sensor respond faster to temperature changes in the cabin. The sensor has two wires.

Ok, wires are intact to the sensor and the monovalve. I noticed that there was a rubber tube from the sensor and another tube to the right running toward the pass side under the glove. I assumed these two tubes should be connected as evidence from the nasty disintegrated foam under the glove so i went to the dealer and jury rigged a rubber tube to connect them. I also was reminded that I forced opened then vent to allow the A/C to flow through the center vents last summer so i closed the vent so heat wouldn't be coming through the center fresh air vents. We'll see how it goes over the next few days.

Jeremy5848 · #14 11-19-2009, 09:11 PM

I've never heard why the factory chose to use a foam tube in this spot; my personal guess is that it deadens sound transmission and vibration from the blower to the dash, which would otherwise act like a drumhead. Lack of that tube would mean no airflow over the sensor, making the system response slow but I don't think it would make it go to the extremes you are seeing.

edge · #15 11-20-2009, 01:29 AM

Max heat problem still there but improved somewhat. It's primarily too hot coming out of the defroster vents adjacent to the front windshield. Arghhh!

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode