Having trouble maintaining heat. The vacuum control relay, whatever it is,
clicks rapidly when in this stae. It's located under the glove box with the
sampler blower motor. I suspect that this unit is bad but I don't know what
it is called. Last summer I saw it on one of the online parts stores but
didn't get one. Since then, I've lost all my links so I don't remember
which sites I've seen it on. I don't think its under the climate control
categories...I think it was list under body... It has an electrical harness
conector from the climate control unit as well as all those vacuum lines to
control all the flaps and doors.
Any one know what it's called?
What you refer to is the switchover valve assembly. It (or they) are
considered as TWO parts, one with 4 ports and the other 5 ports. I hear
that newer, improved replacement is one single unit but I have never
seen it yet.
The clicking sound means it is doing the switching (on and off), per
command from climate control unit. Why the quick clicking? Because the
potentiometer is bad. Potentiometer is a mechanical variable resistor.
It is used to tell climate control unit what angle the heat/cold
blender door is. When it goes bad, the reading goes wild which makes
climate control unit 'thinks' the door is not at the correct position so
it sends command to switchover valve trying to adjust, and it keeps
adjusting because the reading never reaches expectation.
The switchover valve may go bad (burnt) but if it makes clicking sound,
that means it is still working.
Potentiometer is buried deep in the dashboard. It's cheap part,
compared to labor (my estimation is 8-12 hours, depending on your
skill). I know the sound is annoying...
Robert Tifft wrote:
Thanks for the info.
Only now reading your resonse.
Three years ago when I got this car from the original owners ( my parents,
my Dad's 83 now), this clicking was present and heat in the cabin was
sporatic ( they lived in S. Florida, so who knows how long that was going
on!). The climate control unit was damaged at that time. 2 or more buttons
would rmain pushed in or even none at all. Heat and A/C both were difficult
I replaced the climate control unit with a rebuilt one 1.5 years ago. I was
hoping that that would take care of the erratic heat and the clicking
behaind the glove box at the vacuum unit. Nope, it didn't.
I removed the unit last spring thinking I might be able to open it and clean
possible corroded contacts. Nope... wasn't apparent that it could be opened
so I didn't force the issue. Replaced it. My "unit" is actually two
mounted together, offset horizontally.
I had discovered that there were three 190's in a local salvage yard of
differing years. I then discovered what what I thought was that part on Web
parts stores and it appeared that the replacement part was good for all
production years ( or close to it).
When I finally got to the yard this fall when the Alabama heat subsided a
bit, I found that all those 190's were gone!
You are suggesting that the switchover valve ( that name sounds familiar)
may be OK and it's a pot in the dash?
Where is that pot? How big is it? What's it called? Guess it must be near
one of the vent doors. Guess I could follow vacuum lines to locate.
(Just had to have the starter and ring gear replaced for too many $$. That
also was a pre-existing condition which had gradually gotten worse. It
embarrased my wife to drive it because she never knew what sounds it might
make when she tried to start it. I taught her to put it in gear, foot out,
and rock the car a bit to rotate the gear a bit. When viewing the worn
ring gear, no wonder it was harder to rock into a good starting place...
about 5-6 inches of teeth were worn away.)
IIRC, the 'pot' (the potentiometer, I believe that's the official name)
is inside the heater/cooling unit, which is under the center dash,
behind the push-button control (PBC). The housing also includes the
blower, which is accessed from front/firewall while other components are
accessed from dash. The housing is a big box, maybe like a 13" TV.
There is a blender door inside the housing. Whether it opens toward
heater or AC evaporator will determine the output air temperature. This
'pot' is on the same axle of the blender door so its reading tells PBC
where the door is (i.e. what the output air temp is supposed to be). I
have only seen its black-and-white picture on the manual. Cannot tell
exactly how it looks like or how big it is.
I once studied the detail because mine (84 190D) is making that quick
clicking sound. Someone else had an excellent page on the net telling
all his experience about W201 ACC system. I printed it and was not able
to find that page again. That was about 2-3 years ago. If you are
interested, give me a fax # and I can try to send the hardcopy to you,
but I cannot guarantee if it's readable after fax. When I realized how
much work it involves, I decided to wait. If you want to do it, you
must have the climate control manual (no more printed copy, only CD) and
study it well before starting.
There is one W201 in a junkyard I use often. I was planning to do some
'experiment surgery' on it regarding this 'pot' because I know mine will
eventually die. Now it is pretty cold in Maryland. I may have to wait
until spring, if the car is still there.
Robert Tifft wrote:
OK, good news. The page is found. The author is John Cacavas. Go to
google and search for "John's Mercedes-Benz page". Look for the
Troubleshooting 190 Tempmatic link. I think this is an excellent page,
The pot testing is at the end of the article.
Wan-ning Tan wrote:
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