I found reference to this problem on another forum but no solutions so I
hope someone here can help. Often ( but not always ) as soon as I get up
to freeway speeds the heat stops and I get nothing but cold air coming into
the car. Once I'm back into city driving it's as hot as ever. I'm not as
familiar with the 126 heating system as I am the 116 so does the 126 have
an amp (usual suspect) behind the glove box as well or is the system
totally different? Any ideas or solutions would be great. Many thanks.
I would think the thermostat is the most likely candidate. At high
speeds, the mass of air might actually be cooling the system to below
where it should be?
Does the temperature gauge indicate a normal temp at high speed (ie 80c)?
Just a thought.
And lemme guess, the air that comes out isn't that hot, either?
That's usually a sign it's the aux. heater pump. It's right
below the monovale, pictured here:
The hoses are so long in a 126 that there's poor cicrulcation to the
heater core so they add an auxillary pump to help out.
When they fail the water going into the heater core isn't as hot
as it should be and at highway speed the flow thorugh ventilation
cools the car down more quickly and without the heater working at
full temperature it seems like it's not working.
My old pump spun when 12V was applied, but a new one fixed this
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If your engine temperature gauge stay steady on highway as well as local
driving, then I suspect the heater monovalve problem. However, I believe
your problem may be the thermostat... but this would be easily diagnosed by
the fluctuating engine temperature gauge... highway speed would be much
colder.... and in city driving... wildly swinging... going up when at the
light... go down when moving.
Had the same problem on my 300D. Turned out to be the climate control
module. Apparently it turned off the auxillary pump that puts hot water
through the heater core. Never did figure out why but you know solid state
stuff...it doesn't need a reason.
On Wed, 30 Mar 2005 05:54:06 +0000, Ernie Sparks wrote:
Just for giggles, also consider the thermostat - they can stick
in the open position as well as the closed. At high speeds,
you are shoving lots of air through the radiator, and a
thermostat that is stuck in the open position will fail to
close up as the temp of the coolant falls below ideal operating
temperatures. When that happens, you get much cooler coolant
going through your heater core, regardless of how well
any aux pump may be working. Thermostats are also way cheaper
than climate control modules.
I caught this post after the original message had expired,
so if you've already tried this, I apologize.
Typically, the coolant will cool off gradually with a thermostat that sticks
in the open position. If I understand the original post the air went from
warm (read hot) to cold almost instantly. This is exactly what happened to
me. If I shut the engine off and immediately started it again the air was
warm for a minute or so and then suddenly turned cold. Any
On Mon, 04 Apr 2005 05:10:12 +0000, Ernie Sparks wrote:
Sigh - I've got the best of both worlds right now - a climate
control module that works fine for about 20 minutes, then
decides not to blow warm air anymore AND a stuck open
(actually missing - it's a long story) thermostat.
My assumption about the "stuck" open thermostat was based
on the fact that I thought that the original post said
something about losing the warmth at high speed. When I
get on the highway at high speeds, I definitely don't
get all the warmth I'd like due to the fact that I've been
running sans thermostat for a month or two, but you're
right - it's not a sudden chill. My optimistic debugging
technique is usually to replace all parts in a system
starting with the cheapest possible part. One of these
days, I'm going to have to actually learn what I'm doing ;-)
(Bizarrely enough, my cruise-control works for about the
same length of time, then gives up the ghost)
Have you heard a clicking in your dash (you have to listen close). My
300E does this sometimes. I have to fiddle with the buttons for the
climate control. I found sometimes the AC wan't to kick on for some
on my 82 300SD it is on the pass fire wall.
you should see the input of the heater hose going to the valve.
on mine there are 4 small screws that hold the top on.
park the car facing up hill remove the 4 screws <DON'T LOSE THEM> be
nice its a plastic housing.
pull it up.
look at the rubber parts and if it has even small nick in it replace
just don't break any of the nipples
and DON'T over tighten the screws
IF YOU OPT TO JUST OPT TO REPLACE THE GUTS.
you must buy the whole valve.
replacing the whole valve is not that bad but plastic gets brittle so
watch what your leaning against
the reason for parking up hill is less coolant loss & cleaner hands
the case, minus a few cans!
if it has one.
if tour temp gage is reading 80C and your heat output drops off fast
that is i bet it.
mine would do that.
i turned the car off restarted had heat and it cooled once again.
pulled the valve apart and it had a tear in the gasket.
the coolant gets in the magnet and makes it stick.
the case, minus a few cans!
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