I think there are two possibilities about the A/C problem. I don't
believe the air mass sensor has any relation to A/C.
Possibility #1, low on freon. This should be easily identified by
checking pressure on both high and low sides when the system runs.
Possibility #2, loose serpentine belt (see description below). Check
belt condition, tensioner, and oil at the compressor clutch (also gap).
The A/C compressor has a speed sensor at its rear end. The Klima relay
compares the compressor speed and the engine speed (sensor usually at
ring gear around starter). When the speed deviation is too much (I saw
it somewhere in the MB manual saying like 20% - 30% but can't find it
anymore), it assumes the compressor is seized so it cuts off the clutch
to save the belt. Therefore, the belt tension and groove condition
should be checked. Also, if oil is on the clutch plate, this may cause
it slip so much that the compressor speed isn't up to spec. If you see
oil on the plate, degrease/wash it.
The easiest way to check if it is the speed cut-off, turn off the engine
and then restart. The speed comparison logic is reset by engine off.
If the A/C runs again after restarting engine, it is definitely the
This speed comparison appears in all MB I have seen in the 80s,
including W201, W126 and W124. I am not sure about earlier or later MBs
though MB tends to use similar design repeatly.
The air mass sensor may affect the engine speed but I would assume it
also indirectly affects the compressor speed for the same degree.
Therefore I do not believe there is any relation in these two problems.
I am surprised to know M103 engine still uses the potentionmeter. I
thought it has been replaced by the more modern thin-film type by
mid-80s. The potentionmeter is a variable resistor (like fuel level
sender) and the contact surface always wears out after many years of usage.
GeneKelly via CarKB.com wrote:
I checked the belt for wear and slack and it appeared fine. It probably had
a little more deflection then I'd like (1") but it didn't seem to be slipping
while I placed a load on it.
I also didn't notice any excessive oil on the compressor or clutch. There
was some minor leaking coming from the power steering that was dripping onto
the compressor that I cleaned. Again, it didn't appear to be causing a
problem during operation.
I checked the Klima relay for scoring, none noted. I did turn off the car
after the AC went warm and turned it back on and it still blew warm air.
Probably not the cutoff?
It's most likely low freon, wouldn't you say? I've made an appointment with
an AC shop and will let you know the result of pressure and coolant level.
Based on Tiger's comments, I'll make sure system is vacuumed for at least 1/2
On the potentiometer issue: I pulled the air cleaner off and check all hoses.
I also sprayed a can of mass air flow sensor cleaner on everything and
cleaned contacts. I didn't notice a change of engine speed while spraying or
any loose connections. I did adjust the mixture setting on the mass air flow
sensor and it did seem to improve idle. I messed with it for about an hour,
ensuring I marked the original spot and number of turns before adjusting. If
I went too far one way, it ran sluggish. The other way, and the engine ran
rough. So by ear and a test drive, I was able to improve idle slightly.
However, I think it can be better. Is ther some way to test mixture with a
gauge or device while adjusting the mixture screw? Does this adjustment
affect the potentiometer? I believe this adjustment is out of whack and is
part of the problem. Last December, I had real stumbling problems that
turned out to be a worn out O-2 sensor. With the new sensor, maybe the
mixture needs to be adjusted to specs?
Wan-ning Tan wrote:
Message posted via CarKB.com
I agree with you 100% that the belt tension may be causing the AC problem. I
replaced the tensioner and shock in 2004 through a Mercedes shop (routine
maint.). Before I replace again, I'd like to try and adjust if possible. I
checked the tension scale on the tensioner and the arrow is pointing to the
left of a line with a big notch in it. Can I adjust by loosening the bolt
(17mm) on the top part of the bracket (between water/power steering pumps),
then adjusting clockwise on the six-sided adjustment that sticks out? Or is
it a lost cause? No noise, oil, greese, or other abnormality is apparent on
the belt. Belt has five grooves in it. My owner's manual calls it a poly-v-
belt and has a belt routing diagram on page 97. Is this the same as
The only other thing that's happening along with this problem is that the SRS
like comes on randomly. I know the belt runs alot of things, could this be
causing the SRS light to come on intermitently?
I still have the surging idle problem. Any tips for adjusting the idle
mixture or should I just leave this problem to the real mechanics?
Message posted via CarKB.com
Looking at the part supplier... yes, you got an adjustable version of belt
tensioner... Your tension should be like 1/2" play...
Loosen the nut on the tensioner where your wheel pulley is... then tighten
via your tensioner adjuster on the top... brass thingy... then lock it all
It is probably all related. Loose belt cause all these problems.
I tried to tighten the belt per your instructions and tensioner adjustment
would not snug up. I replaced the belt thinking that would tighten things up
a bit, it didn't. I think the adjuster is maxed out. I've got everything
apart in my garage, any suggestions on what to do next? I took the car to an
AC shop today where they topped off the refirgerant with 1 1/4 lbs, verified
pressure was good, and observed good flow through the "eye" on AC valve.
After a short while, the compressor shut down. They felt that the auxillary
fan should be on whenever the AC is on? They also suspected that the
serpentine belt was too loose and was causing all of the problems with AC
compressor shutting off and fan not coming on. So, I attemped the job myself
and now I'm stuck.
Message posted via CarKB.com
Turns out the adjustor bolt and nut were stripped. It probably has been that
way for some time. I took out the tensioner, shock, bracket, and adjustor so
I can get a better look. I had to remove fan shroud, fan blade (not clutch),
power steering pump and pulley, and water pump pulley to get to them. It was
real knuckle buster without the right tools. I didn't break anything though.
I've got new Febi tensioner, adjustor, shock, and mounting bracket coming on
Monday. The shock was replaced three years ago by my Mercedes mechanic, but
nothing else. I think the shock went bad because the tensioner was wearing
out? The tensioner and shock both looked good when I took them out, but
since I was in there anyway I just decided to replace everything. The belt
is new and was installed exactly as I took it off and matches the diagram in
my owners manual.
Do you have any images of a properly installed tensioner with adjustment
mechanism installed? When I took the tensioner out, the adjustor fell off
the bolt so I'm not exactly sure how it goes back in. It can be put on two
ways. I looked at the grease on the parts and put them back together where
they matched up best...I think I've got it figured out, I just want to be
sure. The adjustor/nut has a left hand twist so do I go counterclockwise to
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