Troubleshooting the Mercedes-Benz
190 Tempmatic ACC Climate Control System
by John
Cacavas,
© 2009. Not to be copied without permission of the author.
Last updated February 2009. If you have any comments or suggestions, please feel free to contact me at jcacavas at gmail dot com.
I have written this article while
troubleshooting my own 1985 190D 2.2. The purpose of this guide is not to serve
as a repair manual but instead to be used to aid in isolating the problems. In
my case, my goal was to a) find the cause of why heat was always coming out of
the side and defroster vents, even when the air conditioning was on, and b)
find out why the middle, "normal" AC button was making such minimal
use of the compressor. Lastly, I also suspected that the fresh air /
recirculation system may have a fault in it. Whether or not these are your
particular symptoms are not important; I am taking the position that any one
defective part can adversely affect the whole Tempmatic
system, so the best approach is to test as many parts as possible, even if it
seemingly has nothing to do with your original symptom.
It is a complex system, and as a result I
feel I need to limit the scope of this guide. The focus will therefore be on
the electronic and vacuum circuits, and will make the assumption that your
compressor is operational. If your compressor does not operate when the top of
the 3 AC buttons is depressed along with any of the other row of air
distribution buttons other than the "0", my suggestion is to contact
George_Murphy@compuserve.com for a copy of his excellent article which covers
this topic in great detail. It outlines how the compressor and Klima relay operate, and which problems may keep the
compressor from turning on. George is also a good source for remanufactured Klima relays and compressors.
A must-have tool for troubleshooting
this system is a vacuum pump with gauge. I own a Mityvac,
and have been satisfied with it. It cost around $50 at a good auto parts store.
It will come in handy for other projects as well; a separate accessory kit can
also be purchased which allows for one-person brake bleeding. Another necessary
tool will be a good digital voltmeter, with some way of piercing a wire to take
a voltage reading through it. If your voltmeter can be equipped with clips, you
can make do by pushing a cork board pushpin through the wire and then clipping
onto it.
INTERIOR CABIN TEMPERATURE SENSOR
This is a great place to start. If this sensor is not working properly, there
is no way the system will work right. It
is located right next to the rear view mirror, and we are going to test it and
clean it of all the gunk and smoke that has blown through it all these
years. It has a two-wire plug and a hose
attached to it. The hose goes down to
the aspirator blower, and we will test that below. Pry out the light next to the rear view
mirror, and on my 1985 the way to remove the sensor is to reach in and slide
the U-shaped clip holding it in position towards the driver side. Once the clip is out, then the small plastic
grille can be carefully pried out, freeing the sensor. Now test it with an ohmmeter. 68F should yield 11.5 to 13.5k ohms, 77F
should yield 9.5k-10.5k, 86F should yield 7.5-8.5k ohms. Also,
take a look inside the hole, and give it a few blasts with some electrical
contact cleaner. Now test again with the
meter. To reassemble, push on the wire and the hose,
and line up the sensor business end with the opening in the headliner and push
the plastic grille into place. Then
secure with the U-shaped clip and replace the dome lamp.
INTERMITTENT BLOWER?
If you find that your blower is working
intermittently, in the heat mode, it is a good chance that your auxiliary water
pump mounted on the inside fender well below the radiator bottle (see photo) is
old and/or broken and drawing too much current. This causes the ACC system to
shut down. To test for this condition, unplug the aux pump (a 2 pole connector
that clips next to the pump bracket) and see if the blower begins to function
normally. If this is the problem, then keep the pump unplugged until you are
able to replace it. If this is not done, the malfunctioning pump can damage the
IC's on the pushbutton unit. The aux pump is not a critical item; it only
serves to keep the hot water running through the heater core during heavy
traffic or when the engine is otherwise idling. I paid $118 for my replacement.
However, I replaced mine when it started to leak.
Sometimes you may experience a blown blower
fuse. Mercedes-Benz had a recall on some of the 190's, and it had to do with
the fact that some blowers needed more current than the fuse could handle. The
fix is to install a fuse bridge. If your fuse keeps blowing, call Mercedes-Benz
at their 800 roadside assistance number, and they will direct you to someone
who can look up online whether your car requires this service, and if it has
been done already.
TOO MUCH AIR CONDITIONING?
Sometimes the compressor just won't shut off
while the system is operating. The first place to check is the fuse on the back
of the pushbutton unit. I provide detailed instructions below for removal of
this device. See the photo below for the exact location of this fuse.
TROUBLESHOOTING THE HEATER VALVE
If your symptom is too much heat, a
good place to start is the heater valve located between the 2 firewalls on the
passenger side, just behind the battery. It is a simple vacuum-operated element
which controls a gate that, when vacuum is applied, shuts off the coolant flow
into the heater box. Siphon off some coolant, remove the black "leaf
trap" that sits just underneath the windshield, and find the valve. Pull
off the vacuum pipe from its top, loosen the 2 hoses from either side, and
remove it. Check it for smooth operation, and finally put the vacuum pump onto
the element and see if it holds the vacuum. If it doesn't, it's a $25 part. It
might be a good idea to replace it anyway; some owners have noticed that over
time, the valve leaks a small amount of coolant. Also, this valve can fail internally,
although the vacuum part is ok. This
will keep the heat out of the car until the problem can be completely diagnosed
and rectified. If it is in the middle
of summer and your only concern is keeping heat out of your car, you can either
apply vacuum directly to the valve, thus keeping it closed, or you can bypass
it entirely by either blocking the flow somehow, or else run the hose in such a
way that it bypasses the heater core entirely.
So, now that you have access to the valve,
check for proper vacuum going into the valve by plugging your vacuum pump
directly onto the dark red pipe which fits onto the heater valve. Start the car and set the system to maximum
cold. Is there vacuum present? There should be, and if there isn't, then it's
time to dig deeper into the system.
Best to check the integrity of the pipe from the switchover valves., and be aware that there is a 2" rubber hose
joining the 2 pieces of dark red pipe between the heater valve and the
switchover valve.
CHECKING FOR VACUUM LEAKS
The center of the
whole system are the switchover valves. There are two of them, and all
of the Tempmatic vacuum pipes connected to the vacuum
elements terminate at either one or the other. This makes it an ideal spot to
quickly check the whole system for vacuum leaks, which can occur due to heat or
age.
Accessing the switchover valves is done by
removing the glove box, the right hand side vent, and the aspirator blower, in
that order. To remove the glove box, first pull out and unhook the small clear
rectangular light. Along the edge of the box are seven black 2-piece plugs
which hold the box in place and can be pried up, one part at a time. The box
then comes out. The right side air vent is removed by grabbing on to the grille
with a pair of needle nose pliers wrapped in a rag. Give it a tug, and it comes
right out. The frame comes out once the four plastic prongs behind the grill
are pried slightly away, allowing it to be pulled towards you. You may find
that the frame comes out easier if you work at it from behind by removing the
stereo speaker just above it.
Now we test and remove the aspirator blower.
It is cylindrical, and sits directly behind the right air vent. Its function is
to suck air past the interior air temperature sensor located in the headliner,
thus facilitating a quick response from the ACC system. If the symptom of your
system is a slow response time, this is a good place to check. There are two
ways to test the aspirator blower. The first way it to turn
the key on, and light a match in the car. Blow it out, and put the
smoking match up to the sensor, in the headliner. Does the smoke get drawn in?
Then it's ok. Or, you can also turn on the ignition and hear it run and see the
fan blades spinning. Also, sometimes the pipe leading from the blower to the
sensor can disintegrate due to age, so make sure it's in one piece. (Try
blowing through it.) Once you've checked it all out, take it out by lifting it
up from its securing clip, and unplugging the electrical connector. Work the
pipe loose, and take the blower out.
Now we maneuver the two switchover valves
into a more convenient spot for testing purposes. They clip into place to the
black bar in two places, and on my car they were also secured by means of a
black nylon tie. Cut the tie, unclip the valves, and gently take them out of
the glove box, leaving them connected.
Finally, time to begin the vacuum leak
testing process.
Testing for proper engine vacuum - Notice
that vacuum comes from the manifold (or vacuum pump in the 190D) to both
switchover valve inputs through a rubber Y. There should always be vacuum at this Y, as long as the engine is running. So let's
test to see if it's still the case. Separate the pipe from the Y, and plug the
pipe into your vacuum pump. Start the engine. Vacuum ok? If not, look for a
vacuum leak between the Y and the manifold. When done, put it all back
together. Switch off the engine. A good place to start is at the rubber hose
that joins two pieces of pipe between the manifold / vacuum pump and the Y at
the switchover valve.
Now we can test the pipes and associated
elements on the 4 port switchover valve. In the MB ACC Tempmatic
book this device is known as #11.
Port 5, on the left, has a dark red pipe, and
goes to the heater valve element. Pull the pipe off the switchover valve, put
the pipe on your vacuum pump, draw it down and see if it holds vacuum. If not,
follow the line and find the leak. It's a good practice to use a new rubber
elbow to connect the pipe to the switchover valve. These parts are cheap and
can prevent vacuum leaks later.
Ports 4 and 3, next to it, control vacuum to
the blend air flap element. Note that they are connected together with a rubber
Y. Port 4 supplies vacuum when cold is selected, and port 3 vents to atmosphere
when more heat is needed. The switchover valve receives its instructions from
the pushbutton unit. To test, remove the green pipe from the Y, and plug it
into the vacuum pump. Apply vacuum and see if it holds. If not, you need to
find the leak. It may be a bad element on top of the heater box, or a bad
rubber elbow on the element. You may need to remove the dashboard to trace the
leak unfortunately. Reassemble the Y when done.
Now we test the pipes and elements connected
to the 5 port switchover valve. In the ACC manual this device is known as #12.
Ports 10 and 9 both go to the fresh air/recirculating flap underneath the heater case. Unplug each
in turn and connect to the vacuum pump. See if they both hold vacuum. They
should. It is a 2 stage element, meaning that one pipe is for a short stroke
and the other pipe is for the long stroke.
Port 8 has a middle green / white pipe which
goes to the legroom flaps element underneath the heater case but above the
fresh air vacuum element. Remove the pipe, plug it into the vacuum pump and see
if it holds vacuum. It should.
Ports 7 and 6, on the right side of the #12
switchover valve, both go to the defroster outlet flaps element on the left
side of the heater case. This is another 2 stage element, and both should hold
vacuum.
REMOVING THE LEGROOM FLAP AND FRESH AIR /
RECIRCULATING AIR VACUUM ELEMENTS
If you find that a vacuum element is no
longer holding vacuum, you may elect to repair the old one, or just purchase a
new one. George M. sells the rubber inserts, along with instructions on how to
change them. It does not take long once the elements are out of the car.
Both the legroom flaps element and the fresh
air / recirculating air element are located behind
the radio, but the center console must come out first. It sounds more difficult
than it is; first remove the center vents using a needlenose pliers surrounded by a rag, and then,
after closing the vents first, remove the two screws holding it up to the
dash. Use an offset screwdriver for
this, or you can also use short screwdriver sockets. Then remove the two screws holding it to the
transmission tunnel on either side. Remove the panel under the steering wheel,
and then the whole unit should just move forward. Unplug the connectors to the
rocker switch, the fan switch, the two pushbutton unit connectors, the two
ashtray connectors, and anything else you see in the way, including the radio.
Move the center console out of the way so you can get to the two elements. It
is easy to remove the elements once you know the trick.
The legroom flap one needs to come out first.
The secret is that they don't just turn out. With a flashlight, look at the
bottom of the element where the bayonet-style fitting sits in the metal
bracket. One of the three bayonet catches is longer than the others, and the
extra length is a piece of flexible plastic which pops into a hole in the metal
bracket once the element is oriented correctly. This little plastic button
keeps the element from twisting or shaking out of the bracket. Knowing this,
the easiest way to remove the element is to locate the plastic button sticking
up, push it in, and turn the element out. Pull up on it the whole time to help
the button out of the hole. Once it is out of its bracket, remove the plastic
cover over the end of the actuator rod. Using a small screwdriver, gently pry
towards you the metal rod going to the legroom flaps. The actuator rod should
then just come out. Maneuver the element out of the car.
The fresh air flap element secures to its
bracket in the same way. The bad news is that the plastic button is on the
hardest to access part of the bracket. I was able to push the little button in
with my finger and pull up with the other hand. Remove the two vacuum pipes,
noting that the green/yellow pipe is on top. The actuator rod is held into the
door by a small plastic fitting. Give it a quick push and it should pop out. Once both elements were out, the first thing
I did was to file down the button to half its size so that next time they would
come out without having to worry about finding the buttons. I found that it is a good idea to test and
visually confirm proper operation of the vacuum elements before putting the
center console back into place. It is
possible to hook up all the cables and then just not screw it up into place and
watch the elements open and close as the various buttons are pressed.
THE
DEFROSTER VENT VACUUM ELEMENT
First of all, what is the function of this
part? If you were to remove the
steering wheel and instrument cluster and watch it work (and this is what I
did), this is what it would do: if you select one of the console buttons that
points air towards the windshield, the element should move in one direction,
and vice versa. All this does is route
the air up or down. You can troubleshoot
the defroster vacuum element a couple of different ways. Firstly, if you have access to the switchover
valves, see if both parts of it are holding vacuum. If not, then the rubber element has
failed. You can also check it by
removing the instrument cluster and viewing its operation as indicated
above.
If you determine that the defroster vacuum
element is bad, the first step is to get a replacement element. Note that there are two different varieties
of vacuum element (see photo below). The
pink and green one is the earlier version, part number 201 800 05 75, and the
black flat oval one is the later version, part number 000 800 87 75. The later one is not a replacement for the
earlier one, so if you need the earlier one, don’t buy the later one and expect
it to fit. In my particular case, I
found that the VIN cutoff number wasn’t accurate; Mercedes claims I needed the
newer style, while only the earlier style with the round bayonet mount will fit
in the car. See the photo below of the
two varieties.
To change it, remove the steering wheel
(caution if your car has an airbag) and then the instrument cluster. In order to remove the instrument cluster,
it is first necessary to unhook the speedometer cable, and that isn’t always so
easy. Try sliding out the cluster an
inch or two and then try to slide your hand back there and unscrew it. It is easier to unscrew it than to screw it
back on. I found the easiest way to
screw it back on when the time comes is to remove the bottom panel by the
pedals, and then work your hand up and behind the cluster. A bit of grease on the cable and threads
helps a lot, and also line up the cable with the speedo
before giving the cluster a push into a closer position. OK, so once the steering wheel and instrument
cluster are out of the way, the rest is easy.
Look straight ahead and you can’t miss it. Remove the element, taking care not to lose
the little white plastic connector that fits on the end of the rod, and turn it
out while prying it slightly away from the mount to override the little plastic
button which prevents it from unscrewing on its own. As long as you are at it, replace the rubber
vacuum pieces which connect the elements to the tubes. If they are old, they can crack or lose their
elasticity and create vacuum leaks of their own
TESTING THE SWITCHOVER VALVES
The switchover valves consist of several
small vacuum elements which open and close via separate electrical contacts
upon the command from the pushbutton unit, which gets its command from various
electronic sensors. Switchover valves can fail electrically as well as
mechanically. Judging by the fact that Fletcher Jones had 3 of the new-style
switchover valves in stock when I called (more on the new-style switchover
valves later), I'd bet that they replace these things quite regularly.
Now, this time around, instead of testing the
pipes, we will test the ports to see if they are providing proper vacuum. Note
that if a fault is detected here, it does not necessarily mean that the
switchover valves are bad, although the part does become suspect. Starting with
the forward switchover valve, the 4 port, #11:
Let's go back to port 5, on the front 4-port.
Remove the dark red pipe, and plug your vacuum pump onto the port. Start the
car, and turn the temp wheel to Min, meaning maximum cold. Your gauge should
register vacuum, as this will close the heater valve. If this doesn't happen,
make a note of it and put the pipe back on. Below is a picture of the new style
switchover valve, courtesy of James Meyers. Notice how all the ports on the
other two valves are incorporated into this single unit.
And also, courtesy of James,
is a diagram of how to plug in the two identical connectors. There are no
markings on the switchover valve, so this picture will tell you how to do it.
THE DREADED 190 CLICKING BEHIND THE GLOVE
BOX NOISE
Ports 4 and 3 as mentioned earlier work
together to alternately supply and vent vacuum to control the blend air flaps
to regulate temperature. This is the source of the clicking behind the glove
box which has plagued so many of us 190 owners, as the system electronics
rapidly try to set the blend air flaps to the correct spot. This noise is not
indicative of a bad switchover valve, however. It is much more likely that the
associated electronics are faulty, but I will elaborate more on this below. To
test the switchover valve, remove the Y from port 4, and see if vacuum is
present at the port after setting the AC to maximum cold. If it's ok, then plug
the gauge into port 3, and set the temp to cold. Pump in some vacuum. It should
hold. Then swing the temp dial to max heat, and see if the vacuum is vented as
the blend air flaps move accordingly. If this doesn't happen, make a note and
put it back together.
On my car, I noticed that ports 4 and 3
became very hot once upon a time, as there was burn marks surrounding the area
where the pipes plug in. Nothing concrete in that, but very suspicious!
Now let's test the rear switchover valve, the
5-port. Select maximum heat and start the car:
Ports 9 and 10 should receive vacuum when the
"0" is selected;
Port 8 should receive vacuum when the third
from left button (2 white arrows) is selected;
Ports 7 and 6 should receive vacuum when the
second from right (center vent) button is selected.
The electrical portion of the switchover
valves can also be tested, but the ACC manual shows how to do it from the
pushbutton unit connectors. It's a good idea to do it this way, as removing and
then plugging in the connectors sometimes has the ability to restore a dirty
connection. You may even want to spray the contacts with electrical contact
cleaner.
To access the pushbutton unit connectors, we
first need to drop the center console. Remove the two center vents using a rag
and needlenose pliers, and then loosen the two
Phillips screws holding the console up to the dash. Close the vent so you don't
lose a screw into the console. Once the screws are out, the whole unit pivots
down a couple of inches, and the 2 plugs are clearly visible on either side on
the back of the pushbutton unit.
To test the 4-port switchover valve in front,
we check for the presence of a short or open circuit, and it is done from the
left hand connector at the pushbutton unit. There should be 60 - 70 ohms
between contacts. Remove the left side connector. Connect the negative ohmmeter
probe to a good ground (there's a metal nut in the area) ,
and test holes 1, 2, and 4 for either a short or an open circuit, which would
indicate a definite problem with that part. (ACC manual, pp. 83.11-039/3)
The 5-port switchover valve can be checked by
using the pushbutton unit right hand connector. With the valve plugged in,
connect the negative ohmmeter probe to a good ground, and test holes 1, 2, 3,
5, and 6 for either a short or an open circuit, which would indicate a problem
with that part. (ACC manual, pp. 83.11-039/4)
The good news is that if you determine that
your switchover valves are not functioning properly, whether there is an
electrical problem and / or the vacuum doesn't seem to be routing properly,
Mercedes-Benz has redesigned the two parts into one unit. This simpler design
does not require the use of the two rubber Y's, and thus there are less parts
to leak vacuum. I have also noticed that they do not click quite as loud. I
would imagine it is also more reliable on the inside too.
TESTING THE FEEDBACK POTENTIOMETER
The final component this article will test is
the feedback potentiometer, which is an electronic device that lets the
pushbutton electrical control unit know exactly where the blend air flaps are.
Like any electromechanical device, this component is prone to wearing out,
getting dirty contacts, or going out of adjustment. It is difficult to access,
but can be tested via its wires from within the glove box. Locate the 3 wire
connector which can be found in the dash around the right side of the center
vents. Pierce the center green and red wire with the + probe of your voltmeter,
and secure the other one to a GOOD ground. A bad ground wire will yield lower
and inaccurate voltage readings., so make sure you've
got it right. (My first attempt at adjusting my potentiometer was thwarted by a
bad ground.) Start the engine, select the button for the center air vents, and
set the temp wheel to the minimum, meaning maximum cold. You should read 4.4 -
4.5 DC volts for cars up to 1986, and 2.4 - 2.7 volts for cars 1987 on. Turn
the temp dial back and forth, and watch the digital readout. If at any time
there is an open circuit, then the the pot is bad. If
the readout is in any way less than perfectly smooth, then it is dirty or
defective. Any problem in this area is more than likely going to result in the
clicking noise over at the switchover valve #11 as the erratic electronic
readings give the vacuum elements mixed and confusing signals. The picture
below shows the feedback potentiometer along with the associated vacuum
element. The 3 pin connector can be accessed easily by removing the glove box.
The vacuum element shown in the photo is an example of
one way the part can go bad; note how the metal rod can flex to one side and
can no longer accurately control the feedback potentiometer. The rubber in the
element can also disintegrate due to heat or age.
If there is a problem with your feedback potentiometer, it will be
necessary to remove the dashboard, steering wheel, and instrument cluster in
order to change it. It is a time consuming job, but not an overwhelming one if
taken step by step. The good news is that it is a good opportunity to change that
cracked console. New dashboards can be had for about $350 or so. This procedure
is beyond the scope of this article, however, but if you're considering it,
just give yourself about a week for good measure to take your time and do it
right. The 201 chassis manual gives thorough instructions on how to do this
step-by-step. It is not difficult but takes a while.
If your system seems to be getting erratic or
erroneous signals, the switchover valves just are not doing what they should,
and your feedback potentiometer checks out OK, your problem may be in the
pushbutton unit. Frequently they develop cracks in the printed circuit boards.
You may be able to determine if this is your situation by removing the box and
also the bottom. The plug it back in, and turn on the system. See if you can
get different things to happen to the ACC system while you are gently flexing
the PC board with your fingers. If this happens, then you may be able to spot
the loose solder joint which can be resoldered with a
low voltage soldering iron and some rosin core solder. Alternatively, George
Murphy can test these units inexpensively, and if there's a problem with it, he
sells rebuilt exchange units.
AFTER THE
TROUBLESHOOTING...
There is a lot to go wrong with the Tempmatic system, and hopefully this guide has given you a
way to pinpoint the area of the fault. The next step is to fix it. The best way
to approach that is to obtain the 201 Chassis manual and the 201 Tempmatic ACC manual. They make the job much easier and
tell you step-by-step how to change out the defective part once you've come to
a reasonable conclusion.