Heat/AC work. But not much air from vents when heat is on - PeachParts Mercedes-Benz Forum

Khameleon · #1 10-06-2002, 12:42 AM

I have spent the last couple days trying to determine what is wrong with my '84 300D before winter is here. During the summer my automatic climate control worked great when I had the air conditioner running. All of my vents are opening and closing as they should and there is a lot of air coming from the vents when I set the blower speed to max. As the temperature drops I have started to switch on the heat. What I have noticed when the heat is on is that the blower seems to be making lots of noise and the vents all move to the proper setting, and the air coming from the vents is hot... But the amount of air coming from the vent is minimal!! If I roll the temperature setting back to min within seconds the air is blasting thru the vents again. It does not matter which setting i use as long as the heat is on the air is minimal. When heat is on i can hear the blower and its running with just as much gusto as when its got the AC going. Any help with this is greatly apperciated as I have search this forum and read the manual but have found no clues to my problem, and i am concerned that the air coming out will not be sufficent to defrost my window.

Thanks in Advanced

MikeTangas · #2 10-06-2002, 11:21 AM

Check your monovalve, sounds like it might need a quick rebuild.

Khameleon · #3 10-06-2002, 11:49 AM

I was under the impression that if the mono valve was malfunctioning, that there would be no heat. The air temperature coming from the vents is hot. But has no volume to the air flow. Let me know if I am mistaken. Thanks

MikeTangas · #4 10-06-2002, 11:59 AM

OK, read it wrong. Thought the air coming out of the vents was only warm.

Khameleon · #5 10-07-2002, 11:51 AM

Any other ideas?

Tim51 · #6 01-09-2003, 07:14 PM

Does anyone have suggestions as to what might be the problem? I am having the exact same problem with my 84 300D. For the past few weeks if I pushed the buttons on the push button unit enough, I could eventually get hot air to blow out the vents, but that was sporadic. Now the only way to get any amount of air to blow out the vents is to turn the temp dial to MIN. Once I try to add heat, by either turning the dial to increase the temp or pushing the defrost button, minimal air comes out the vents. I can still hear the fan turning. Any suggestions? Thanks.
Tim

kerry · #7 01-09-2003, 07:21 PM

I believe there is a vacuum acutator that controls the mix of outside versus recirculating air. My guess is that in high heat, there is no outside air that should be coming in and somehow the recirculating gate is closed. When on 'min' it returns to the recirculating mode and permits full flow. I think the actuators that control this are behind the glovebox.

#8 01-09-2003, 10:07 PM

On all the 123 Cars.. if you have an air intake near the speaker on the passenger side.. or one in the center of the dash.. You will get just what you fellows are talking about if the foam section of pipe from the air intake has gone to heck... Pull the glove compartment and check the pipe running from the intake to the heater blower.. If the foam section has gone bad.... drop by your local hardware store and pick up some pipe insulation for 1/2 inch copper.. it will be about 3/4 inch inside.. cut a piece and put the two ends of the plastic tube back together with the foam.. That often solves the problem you are talking about.. If thats not it... Good luck..

Ken

psfred · #9 01-10-2003, 09:11 PM

Probably a bad vac servo for floor heat. The flap can stick "between" AC and floor, so you don't get any heat on the floor. Check the defrost vents, too -- if you have lots of air there, its a servo.

There are several -- see other postings.

The little foam tube is for the air temp sensor -- if it is bad, you get strange temp control -- teh 300TE is currently oscillating between too hot and too cold, and it isn't sucking in air at the temp sensor grille (beside the courtesy lamp in this car).

Peter

Tim51 · #10 01-13-2003, 10:32 AM

Good news!!! Problem solved.
I had a bad vacuum valve switch that controlled the diaphram actuator behind that is located behind the glove box. I was fortunate that it was easy to access and not something that is tucked way behind the dash. By process of elimination I was able to narrow it down to the vacuum valve switch. I checked the PBU, diaphram actuator, source vacuum, and electrical connection to the valve switch, all checked out fine. When I tested the switch by putting 12v to it, it was bad. No one had the part locally, so I ordered it from PartsShop. In the mean time, I ran a vacuum line from the source vacuum to the diaphram actuator to allow air to flow.
The ambient air sensor tube was also bad and fixed that too. Thanks for all the help.
Tim

SCOTTFISH · #11 01-15-2003, 03:39 PM

I have an 89 300 se and have had the brushes replaced in the blower and the armature check out to be in good condition. Sometimes the heat is pretty good, other times it blows some, but you are still cold inside. What should I check first? Should it be the hose behind glove box or mono valve or thermostat? The temp gauge usually about all the time, whether sitting idle or driving 65 mph is between 80-85. The heater air seems to be warm, but not hot likd it should with the temp dial on max. Can you help me find a solution?

Scott

gsxr · #12 01-15-2003, 03:49 PM

If airflow is too low, most likely one of the flaps is NOT closing when it should - so your airflow is NOT working properly even though it seems like it. Which, oddly enough, is exactly what Tim discovered! The defrost is a usual suspect because it defaults to full open with zero vacuum signal. It's more common to have a failed rubber diaphragm in the vacuum actuator pods, I don't often hear of the electrical valves failing. But you need to check everything, like Tim did.

BTW, if the foam tube under the dash is crumbled, there's a nice W140 water drain foam piece that is the perfect size & shape to replace it. It's wire reinforced as well so it can't collapse. I used this to fix mine. Here's a photo:

psfred · #13 01-15-2003, 07:51 PM

Scott:

Several possibilities:

Monovalve (behind the battery, I think). Usual failure, however, is to refuse to close, causes too much heat. Can also stick shut. Easy to check, just unplug it, see what happens, then apply battery voltage (carefully!) to the pins on the valve. Should be closed with no voltage and open with applied voltage (someone tell me if I have that backwards, please!). You can also remove it -- four screws that hold the cover on, pull cover off and remove valve. Torn or badly bulged bellows will cause problems here, so will having the armature stuck in the solenoid winding in the cover. Easy to replace, just make sure you get the screws back into the original thread in the plastic housing.

Second possibility is that the temp control is bad, and you don't have good control. Bad hose to the temp sensor is possible here, too, as hot air from the vents will get drawn from under the dash directly, fooling the temp controller.

Third possibility is poor heater core circulation from a bad aux pump (also by the battery, you should feel it running when head is called for), bad engine water pump, or plugged heater core.

Peter