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....

S-Class Guru · #7 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