Heater good in town; cold on the highway - PeachParts Mercedes-Benz Forum

S-Class Guru · #1 12-15-2008, 07:47 PM

'91 300SE. Old Pearl is freezing me out in this Dallas cold snap!

I seem to have the opposite problem from all the heater posts I researched:

My heater works just fine around town; blowing warm air from the proper vents, responding to temp change requests, and maintaining a set temp perfectly.
I can turn the wheel all the way up and it gets pretty warm, along with increased fan speed. Seems to be working just as it should.

BUT.... On the freeway, the air temp starts to drop. Not all the way to outside temp, but maybe 60 degree air in 30-degree weather - Brrrrrrrrr.
If I turn the wheel all the way up, the fan increases but the air temp does not.

I checked:

1. My auxilliary water pump has been disconected for 5-6 years with no problems. But the classic issue there is poor low-speed heat, correct?
2. Engine temp is good = 85C.
3. Monovalve clicks and modulates in the garage - seems okay.
4. In-cabin sensor seems okay; I installed a new suction fan last summer, and the temp stays right on around town.
5. All the vacuum pods are working; all the vent settings do the right thing.
6. Haven't checked the heat exchanger temp sensor; but the system works great around town.

I'm thinking maybe an intermittant monovalve? Maybe the increased water pressure at high speeds forces it closed or something?

Any thoughts?

Cold in Texas,
DG

d2bernhard · #2 12-15-2008, 09:23 PM

Thermostat failure = stuck open? Happened to me in my 300E M103 motor. New 'stat and nice and warm.

duxthe1 · #3 12-15-2008, 10:57 PM

When you pull out the mono-valve you will most likely find it's diaphram torn.

pawoSD · #4 12-15-2008, 11:21 PM

Quote:

Originally Posted by duxthe1

When you pull out the mono-valve you will most likely find it's diaphram torn.

Yep, it will still "function" but with a torn diaphragm it won't be effective. I had no heat in my car at higher speeds (and very little at low speeds too) until I discovered and replaced this item. $28 later I had mega nice heat! It was AMAZING.

Go on fastlane and buy a "monovalve repair kit". Can be replaced in like 3-5 mins.

d2bernhard · #5 12-16-2008, 12:04 AM

what does your temp gauge do? Is it steady or does it cool at higher speeds?

pesuazo · #6 12-16-2008, 10:24 AM

Had the same thing happen to me, long time ago. Look at the engine temp, most likely running cool on the highway. Chances are the thermostat failed open....

Chad300tdt · #7 12-16-2008, 10:34 AM

In the original post he states that the engine temp is 85C.

If the thermostat was stuck open it would take forever to get to that temp but once there, the heater would be fine.

I agree that your monovalve diaphram is torn.

EDIT: Here's a pic of my old monovalve. This tear was all it needed to have no heat other than at a stop light and even then it was lukewarm.

S-Class Guru · #8 12-16-2008, 10:45 AM

Thanks guys. The water temp is fine, stays between 80-85 under all conditions in cool weather. Thermostat replaced last year (old one did stick open). Hint, always buy the slightly more expensive thermostat with the thicker housing!

I cranked her up in the garage this morning (30 degrees) and let her warm up.
Heater came on as it should, after about two minutes (when water temp reached 35-40).
I stuck a thermometer in the side vent, turned the temp wheel up to 85F, and let the engine run until the water reached 80C. The air temp only got to
75F. So, I think it's not doing so well at low speed either.

I think the monovalve is a good suggestion. It's been 10 years since I replaced it. But logic tells me if it's torn then it would allow hot water flow all the time, rather than impede it? But, I know these things are weird.

Heck, I'll just do the mono kit, and maybe also take out the auxillary pump motor and clean it up and fuse it.

Funny thing; my wife still refused to drive the new Caddy SRX on this cold morning. It has remote start that even preheats the seats & steering wheel. But she loves the old Benz. (maybe I should have bought 12 old Benz's instead of the Caddy - Ha).

DG

Chad300tdt · #9 12-16-2008, 10:48 AM

I believe that if you disconnect the power plug from the monovalve, you get heat all the time. Someone will correct me if I'm wrong, but I think you can test it that way without removing the monovalve to see if it has a tear.

david s poole · #10 12-16-2008, 12:28 PM

pull out monovalve and check to see if full of antifreeze above the diaphram (even if not torn)pull rubber loose and empty liquid until dry,should then operate as normal.

pawoSD · #11 12-16-2008, 12:55 PM

My M103 had a stuck thermostat when I got it, on the best days the engine might reach 65C.....interior heat still worked fine though. Now the motor runs at 87C like it should, and its nice and toasty in the car.

S-Class Guru · #12 12-16-2008, 01:43 PM

David, good to hear you're still around and fixing MB's; I thought maybe you had retired and taken up something foolish like fishing or golf.

DG

david s poole · #13 12-16-2008, 01:56 PM

i moved the shop to 10920 indian trail suite 101.

tinypanzer · #14 12-16-2008, 05:15 PM

I vote for the monovalve too. My 90 300SE was having the same problem as posted by chad300td. And, my monovalve looked exactly the same. One small tear and no heat.

I thought the same thing as you; logically the torn diaphragm would allow flow all the time right? Not really, I think what's happening is that the torn diaphragm prevents the solenoid piston from moving in and out properly, and so you get very little flow due to the fact that the piston won't unseat. It may be because coolant gets behind it thus making it difficult to move.

I was amazed at how well my climate control works after I replaced it.

-tp

S-Class Guru · #15 12-16-2008, 06:17 PM

Yeah, yeah; now I remember! Thanks for the memory jog.
I went through this about 10 years ago - the solenoid can't move the piston against the coolant trapped above it.
It's Hell to get old and forgetful, and have to retire and play golf all the time!

I'll fix it and report back.

Oh, one thing I do remember; I promised David a lunch about a year ago. I WILL get by there I promise. It's just a long drive from my house these days.

Cheers,
DG