Crazy heater question, heats around town cold on the highway - PeachParts Mercedes-Benz Forum

mjname · #1 12-13-2008, 04:26 PM

Just a quick question, around town the heater works pretty well. Automatic climate control (I can't find any manual switches knobs or levers). 82 300TD The heat comes on and is toasty around town but on the highway it just blows luke warm cool. I'm wondering is there an outside cold air vent some where on the car I can just cover with card board, or manaully turn off?

I'm leaving to New Orleans tomorrow morning and am dreading the parka, gloves and hat on the freeway. Of course once I am there not really worried about the heater or the air conditioner to much, until about May, then I will be really concerned about AC. But that's in May.

Jeremy5848 · #2 12-13-2008, 04:37 PM

Sounds like your monovalve may not be opening fully, thus allowing a little hot water (but not enough) to enter the heater core. You can remove the electrical connector and four screws, then lift the monovalve out of its body (you'll lose a little bit of coolant but not enough to need to pre-drain the system) and look to see if it's all crudded up or has a corroded electrical solenoid that is blocking free travel of the plunger.

Another possibility is that the aux coolant pump is crudded up and is not allowing much hot coolant through the heater core. The fix is similar to the monovalve fix, just harder to get to.

A third possibility is that the heater core is itself partially blocked. That is a much more difficult and expensive repair so let's check the monovalve and aux pump first.

Jeremy

mjname · #3 12-13-2008, 04:57 PM

Hey thanks I will investigate that in the morning. It's night and below freezing... I think I can pull that open fairly easy in the morning and see whats whats.

Thanks
Matthew

LUVMBDiesels · #4 12-13-2008, 05:05 PM

Sounds like the monovalve to me... It is probably ripped. you can get a new insert to fix this quickly. The other thing it could be is the small fan that samples air from the cabin. There is a hose that runs from the opening to the sampler fan. If the hose crumbles then the sampler draws warmer air from under the dash and cuts the heat. Do a search on this and you will see what I mean.

mobetta · #5 12-13-2008, 07:48 PM

On a 123 with ACC III,(temp wheel on left, 5 buttons across, fan on right) there is a blend flap behind the glove box. it has a green vac line going to it.

remove the glove box, you'll want to replace the disintegrated foam tubing anyhow, and check to see if the flap is open or closed, and operating correctly.
Mine wasnt closing, ever, so I disabled it.
to permanently close it, you can simply unhook the vac line to it. That is what I did. then I melted the end of the vac line w/ a lighter and pinched it off. if I want to revert, I can simply cut 1/4" off the line and plug it back in.

also check the monovalve- it will only take a few minutes.

H-townbenzoboy · #6 12-14-2008, 12:17 AM

Quote:

Originally Posted by mobetta

Mine wasnt closing, ever, so I disabled it.
to permanently close it, you can simply unhook the vac line to it. That is what I did. then I melted the end of the vac line w/ a lighter and pinched it off. if I want to revert, I can simply cut 1/4" off the line and plug it back in.

Yikes! Going behind the dash with a lighter?

You could've performed the same task with a rubber vacuum line cap.

tangofox007 · #7 12-14-2008, 12:54 AM

Quote:

Originally Posted by mjname

The heat comes on and is toasty around town but on the highway it just blows luke warm cool.

That's a classic symptom of a torn monovalve diaphragm. A monovalve repair kit will take care of the problem.

mjname · #8 12-17-2008, 11:29 PM

hey guys thanks, I started totear into this before I hit the road and thought, **** it's freezing here in the parking lot and... **** it's freezing and if I just make a run for new orleans, it's warm there and this is far less of an issue in New Orleans in the winter than here in the winter besides I need a repair kit and can't get one of those on Sunday Morning at the end of cape cod.

So I made a run for it and the heat was good enough to get me here in one piece, I will order a repair kit and take care of it now that I know how to fix it.

thanks for all the feed back and sharing your knowledge.

redassag00 · #9 12-18-2008, 09:17 AM

Try unplugging the monovalve electrical connector.

mobetta · #10 12-18-2008, 05:31 PM

Quote:

Originally Posted by H-townbenzoboy

Yikes! Going behind the dash with a lighter?

You could've performed the same task with a rubber vacuum line cap.

but I didnt have one of those.

peeps smoke cigars in their cars all the time, and most dont start them on fire. just use common cent$.

patbob · #11 12-18-2008, 05:47 PM

Quote:

Originally Posted by mobetta

peeps smoke cigars in their cars all the time, and most dont start them on fire. just use common cent$.

They do that behind the dash in your car? Interesting

Glad to hear you got it sorted out and what your fix was.

Yes, you can use an open flame back there without disaster. Plugging it would be safer -- A BB, golf tee, pencil, aquarium valve, etc.

mjname · #12 12-19-2008, 11:39 PM

I took the monovalve off today and two things, I have that green antifreeze which I bet is wrong and I have a destroyed monovalve. I'll order a new one after the holidays and also change the coolent then too!

Thanks for all the tips.

hofler · #13 12-20-2008, 01:13 AM

Sounds like you have the exact same problem I do. I am sitting on a new monoalve myself and will probably install over the holidays. shoot me a PM and lets meet up. I live in the uptown area near Broadway and Clairborn ave. So what brings you to town? Work or pleasure. Also I can hook you up with a good mechanic if you need one for anything beyond your comfort zone.

C-ya
R

Dwood1111 · #14 12-20-2008, 01:01 PM

Funny because I too have the exact same problem. Heater blows hot around town when I am just cruising around. Anything about 55 or so it blows kinda cold, but not enough on colder nights to really help. You guys ordering new mono valves from the dealer, or somewhere else? Thanks for the post. by the way, this has been driving up a wall for the last two weeks or so.

D

mjname · #15 12-20-2008, 01:39 PM

No I will order the bosh replacement monovlave kit from the store on the site. It's 45.00 but won't put it in until I change antifreeze with the recommended type by MB.