Heat only on max...monovalve? - PeachParts Mercedes-Benz Forum

pizzachef · #1 02-20-2007, 08:46 PM

I've been reading about the monovalve and have a rough idea of how it works. So my heat works when clicked into the hottest setting on the temp. wheel (auto climate control). It blows hot after driving a while and when moving, but when sitting still, it seems luke warm, may just be me though.
Anyway, if I move the temp wheel off of hot, even just a hair beyond the "click", the air slowly gets cold and even starts blowing out of the center vents (that's the part that has me puzzled).

So my question is, is the monovalve bad or the acc controller? I would assume the monovalve but the fact that air starts coming out of the center vents seems wierd. Can I check the monovalve somehow?
Thanks,
-Geoff

tangofox007 · #2 02-20-2007, 08:55 PM

Quote:

Originally Posted by pizzachef

I would assume the monovalve but the fact that air starts coming out of the center vents seems wierd. Can I check the monovalve somehow?
Thanks,
-Geoff

That sounds like it's not the monovalve. Temp control unit maybe. Or a problem in the CCU, liked bad solder joints. (I had essentially the same problem. Reflowing the CCU solder joints fixed it.) Or temp sensor problem, possibly bad hose.

kerry · #3 02-20-2007, 08:59 PM

Cool at low rpm's is an indicator of a failed aux coolant pump. Unfortunately when that pump fails, it can take out the control unit.

Jeremy5848 · #4 02-21-2007, 12:54 AM

Quote:

Originally Posted by pizzachef

....if I move the temp wheel off of hot, even just a hair beyond the "click", the air slowly gets cold and even starts blowing out of the center vents....

The HOT "click" position of the temperature wheel forces the ACC to go to maximum heat. Once out of the "click," the ACC is supposed to modulate the monovalve to reduce the heat to the temp set by the wheel. Air coming out of the center vents means the ACC is going into "cool" mode. With the temp dial set to "almost" the 'click' position, the ACC should still be in mostly heat mode. The fact that you are getting cool air (after the heater core has a few minutes to cool off) means that the ACC is not operating the monovalve correctly.

The monovalve defaults to "open," a fail-safe intended to make sure you can always defrost your windows. The 12 Volt control line from the ACC tells the monovalve to close part of the time (it pulses on/off with a variable duty cycle), thus reducing the amount of hot water allowed through the heater core. If you have a choice between "all heat" and "no heat," then the monovalve is working but the ACC is not controlling it correctly.

To be on the safe side, and to check the easy parts first, you should take the monovalve apart and make sure it's clean and moving up and down easily. There is a small chance that it is sticking and so working only "all or nothing." Since the monovalve is easier to fix than the ACC, look at it first. If it is troublesome, you can get a kit for about $40 to replace the guts.

If the monovalve seems to be OK, the problem is the ACC. Resoldering all of the contacts has worked for many forumites, including me. There are also rebuilt switch units available, unfortunately fairly expensive.

#5 02-21-2007, 06:33 AM

Pull the glove box and make sure that there is a hose in place leading from the passenger side over to the center temp sensor under the little grill on the dash.

pizzachef · #6 02-21-2007, 08:39 AM

Quote:

Originally Posted by Jeremy5848

The HOT "click" position of the temperature wheel forces the ACC to go to maximum heat. Once out of the "click," the ACC is supposed to modulate the monovalve to reduce the heat to the temp set by the wheel...

I thought this may be how it worked. If that's the case, then the monovalve must be opening and closing as you say, but it would be worth checking. That might be a spring weather job

I can pull the ACC controller pretty easily, I've got the trim off already. Are the problems soldered contacts on the back or inside?

Quote:

Originally Posted by LarryBible

Pull the glove box and make sure that there is a hose in place leading from the passenger side over to the center temp sensor under the little grill on the dash.

So that little grill is a temp sensor and not the buzzer speaker??

I've been looking at that thing wondering what it was. That's easy to check...I'll do that first. What if the hose is/isn't in place?
Thanks,
-Geoff

ImBroke · #7 02-21-2007, 10:15 AM

The foam hose definitely needs to be there. They tend to deteriorate to nothing. Maybe do a search for aspirator hose. This hose does not have to be replaced with foam, it can be plastic. I replaced mine with black foam pipe insulation, which seems to work fine.

toomany MBZ · #8 02-21-2007, 11:18 AM

To check your mono valve should take maybe 10 min. It is located on firewall just to drivers side of battery, other side of drain tube. It has a wire plugging in on top, a heater hose is visible also. Unplug, remove four hold down screws, remove and inspect. There is a rubber diaphram that deteriorates over time. The plunger should move up and down. The screen should be cleaned if necessary.

kerry · #9 02-21-2007, 11:35 AM

Simply removing the wire from the monovalve should give you full heat all the time. However, as I mentioned above, it seems as though your auxiliary water pump may have failed. You may be able to hear it kick on and off when unplugging it and detect if it has seized.

pizzachef · #10 02-21-2007, 11:42 AM

Quote:

Originally Posted by toomany MBZ

...Unplug, remove four hold down screws, remove and inspect. There is a rubber diaphram that deteriorates over time. The plunger should move up and down. The screen should be cleaned if necessary.

Oh, so not necessary to drain coolant? That makes it easier...

Quote:

Originally Posted by kerry edwards

Simply removing the wire from the monovalve should give you full heat all the time. However, as I mentioned above, it seems as though your auxiliary water pump may have failed. You may be able to hear it kick on and off when unplugging it and detect if it has seized.

I'll do some investigating. If I unplug the pump and hotwire it to the battery, I should be able to hear it turn on right?

tangofox007 · #11 02-21-2007, 12:11 PM

Don't waste your time with the monovalve. It's not the primary problem. The fact that your vent configuration changes points to a control problem. A bad monovalve will not cause your center vents to come on with the temp selector in the "heat" range.

kerry · #12 02-21-2007, 12:41 PM

Quote:

Originally Posted by pizzachef

Oh, so not necessary to drain coolant? That makes it easier...

I'll do some investigating. If I unplug the pump and hotwire it to the battery, I should be able to hear it turn on right?

Yes. It is known that a seized aux pump will take out the controller. Some people have installed fuses in line with the pump to avoid this occurrence.

LUVMBDiesels · #13 02-21-2007, 12:41 PM

I would check and replace the temp sensor hose in/under the dash. The easiest way is to pull the instrument cluster as well as to remove the glove box. Check for threads on how to remove these items. I used pipe insulation foam to do the job. the reason to use this is that the space under/inside the dash gets hot quickly and so the air being drawn into the sensor needs to be insulated from this heat. I wrapped my new pipe in duct tape to keep it from splitting and to hopefully keep it intact longer.

Good luck

toomany MBZ · #14 02-21-2007, 12:42 PM

Not necessary to drain, high point of system, you may have some coolant "escape", not enough to worry about. However, do this on a COLD engine, unless you would like a trip to the burn ward. tangofox... has a point, I replaced one as a precaution, the old one is just fine.

pizzachef · #15 02-22-2007, 07:42 AM

It was the hose in the dash. I pulled the glove box and that hose was all but disintegrated...there are two rubber hoses that end about 5" apart and they're connected by what looks like foam pipe insulation, and that pipe insulation was crumbled to bits. So I bought a length of it for $0.97. Haven't put it in yet because I have no duct tape in the house (flame suit on) but the heater worked properly when the inside of the dash was cool. But when it got hot in there, the heater would blow cool air.
Monovalve good, acc unit good (thankfully), aux pump good, foam tube gone.

Thanks for the help, this site is invaluable.
-Geoff