I have an 89 300E that blows nice, cold air through the air conditioning for
about 20 minutes then it turns warm. If I let the car sit for about 30
minutes, I'll get cold air again. My mechanic seems to think it may be
something shutting off the compressor (short, relay, etc.). Last week, I
replaced the 30amp blower motor fuse (broken) with a 40amp in-line fuse (the
only one I could find). The blower works fine now but now I have the air
conditioning problem described above. Could the fuse and AC problem be
connected? Any thoughts?
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Its under the hood right where the heater hoses go into the cabin. It
has an electrical connector on the top. Here's a link to one in a
W126: http://articles.mbz.org/hvac/plumbing/monovalve /
That one is actually between the firewalls, but mine is not. So you
may have to look around for it, but you should be able to find it by
following the heater hoses into the cabin.
I thought that mine might be going bad the other day, but it was
another fuse that blew. It was the fuse that controlled the monovalve.
(fuse 14 in mine) So you might want to check and clean all your other
fuses as well. I say clean, because after I bought my car recently, I
cleaned all the fuse contacts (with a pencil eraser) and it noticeably
improved many electrical things. It also made my wipers start working
On your car, you got a different setup. Go to online part store like
autohausaz.com and see the picture. Once you located it... you will see one
side comes from engine... and the other side goes to the cabin.
Grab the side that goes to the cabin... if it is warm or hot, you know it is
If I understand you correctly, the monovalve can affect both the heat and
cooling operations? I haven't used my heat in awhile, but I know that the
last time it worked perfectly.
In an earlier post, about 6 months ago, I had a problem with idling on this
1989 300E that turned out to be the O-2 sensor. I still have a moderate
stumbling and surging problem at idle, after warm-up, that was present well
before the O-2 sensor problem. Some of the guys mentioned checks of the ECU
and pointometer. One check stood out, the check of pin 21 which is related
to engine coolant temperature. Could the idle and AC problems be related in
The car is with my mechanic right now. I gave him printouts from this car
forum of both replies related to the AC problem and the ECU/pointometer
checks. Mechanics don't always like that sort of feedback and reluctantly
took it. I just wanted to point him in a good general direction and share
information from people whom I consider experts ( such as yourself) on this
Message posted via CarKB.com
In climate control system... there is two separate but interrelated
system... one is AC and the other is heat.
In summer, you turn on AC... when you set to the coldest, you are on pure AC
only... but it would be freezing cold to tolerate... so you need some heat
to temper that freezing temperature.
While your AC may be operating perfectly, if the heater monovalve is bad...
and let too much hot heat go through, your AC system will never be able to
overcome that heat and you get just a little bit cool air.
I confirmed that the hose going to the cabin from the monovalve was not hot.
This test was conducted after running the AC for about 45 minutes while
idling. The auxillary fan seems to working well. It kicked on between 105-
110 C and cooled the engine down to 100 within 5 minutes. AC seems to be
working okay today, except today it's not real cool while idling. However,
I've been letting idle in the garage for about an hour now and it hasn't
changed to warm air yet. So far, I've only noticed the problem at higway
speeds. Any more thoughts?
Of course I had the garage door completely open while idling.
I'm not sure where the expansion valve is located or how to test it. I think
this car may still have R-12 refrigerant that was serviced 4 years ago by my
Mercedes mechanic. Is there any way to check what refrigerant I have? After
I check the expansion valve, how can you check how full it is or whether it
By the way, did you see my other post concerning the potentiometer in this
car, 1989 300E? I took it to a Volkswagon mechanic (no Mercedes mechanics in
Helena, MT) yesterday for idling/surging problems. You helped me fix an O-2
sensor problem I had in December 2006. I took some of the info provided by
you, T.G., and Jens in that thread and gave it to the mechanic. It concerned
testing the ECU and potentiometer. The mechanic confirmed that the output
reading was somewhere around 56,000 Ohms, well outside the 3,600 to 4,000
range. His book matched your findings, replace the potentiometer. Is it
also called an air flow sensor? Do you agree with these findings? If so,
where can I buy one? Can I go who is fair backyard mechanic do this jon
without any special tools or should I just let him do it? This mechanis,
while not an expert on Mercedes, seems fair and very capable of doing the job.
Could the AC and potentiometer problems be linked in any way?
Thanks again for your expert help,
Okay... now I have all the information. Right now, I would suspect you are
low in refrigerant as it has been 4 years since you last checked. Whether it
is R12 or 134a... is usually by the fitting on the AC line... R12 line woud
have threaded connection... whereas converted system, they would install an
adapter on it.
I don't recall what I told you 7 months ago... why didn't you fix it back
then? Surging idle can be many things... but I would start off by checking
every vacuum hose in the engine... make sure they are tight fittings.... cut
old section out and reinsert for tighter connection.
It could be the fuel pressure regulator we were talking about... EHA...
autohausaz.com has them for $277... but only change this part if it is
Where do I look for the threaded connection? An American Car Care Center
mechanic was going to put refrigerant in it the other day and actually hooked
the device to required connections. He said pressure appeared okay, but he
did not have the AC on...he didn't know anything about Mercedes and had the
climate control on "economy". he was afraid to do anything on it and sent me
on my way. I didn't realize he had it on "economy" until I got donw the road.
Should I just take the car to certified AC shop and have them try to fill
The surge problem is very minor. The surge has been there for about 5 years
and doesn't seem to affect performance. Just while I'm idling, sitting at
traffic lights. I can see the RPM bounce up and down a notch or two. It
sounds like the ECU is trying to balance fuel/air mixture. What are your
thoughts on the high potentiometer reading (56,000 ohms)?
I checked all vacuum connection...wen't through two cans of carb cleaner and
checked every possible vacuum line for leaks. I changed both fuel pumps,
filters, throttle microswitch, OVP, fuel pump relay, Beru wires, Bosch plugs,
fully sythetic Mobil 1 oil has been used from day one in this car and changed
before required interval. Car starts and accelarates very well. O-2 sensor
change fixed the major stumbling problem in December. I used Bosch OEM O-2
I think the Volkswagon mechanic is right in suggesting replacement of the
potentiometer. Should I try this first, since it will be cheaper then the
fuel pressure regulator? Where can I buy one? autohausaz.com is out of
Go to another mechanic and have him do it... He obviously don't know what he
is doing as AC is AC regarless of what car it is.
I'd suggest you change out that part first before fuel pressure regulator...
but both can be at fault.
I talked to my actual Mercedes mechanic in Colorado and thought I could the
AC conversion myself. My plan was to capture the R-12 currently in the
system in an approved container, change the fittings and put the R-134 in
myself using the intrsutions with the kits. Is there anything I should be
concerned about during this procedure?
On the potentiometer issue, is there an adjustment on that part where a guy
could put the testing device on it while adjusting it to required specs? I
checked all over, including the dealer, and you can't buy the potentiometer
Do you mean the potentiometer for the AC? (i.e. The temp control
valve.) I don't know what pot. you are talking about either.
And for recovering the refrigerant, what container are you using? I am
thinking of doing this also to repair some components. I will do it
myself if it will save me a few hundred dollars, otherwise, I'll go to
the local AC guy, and save myself the trouble.
If the A/C system cools fine, don't bother switching over to R-134.
There are kits sold in auto parts stores for the conversion. But if you
want to do it right and keep the car for the next 5-10 years, the oil
needs to be replaced too. R-12 uses mineral oil while R-134 uses either
ester or PAG. Mineral oil is much thicker than Ester/PAG. R-134
molecule is smaller than R-12 so it cannot circulate mineral oil very
well. A compressor without proper lubrication will not last long.
Also remember that R-134 does not cool as well as R-12 in a R-12 system.
Modern cars are designed with R-134 in mind.
Usually the best time to convert is when a component (e.g. compressor)
needs to be replaced. If it is the case, drain the oil when the old
compressor is out. You can immediately tell the difference between
oils. If you do not plan to service the A/C system yourself, R-12 and
R-134 costs are about the same in A/C shop (since most are labor).
I don't know what potentiometer you are referring to. There is a
potentiometer in a W201 A/C system but it does not cause any "surge". I
am not sure about W124 though it shares lots of similarities with W201.
If the "surge" is in the engine, I would assume the only possible link
is the mass-air flow sensor. Older cars may use a potentionmeter as the
flow sensor but I don't think M103 engine uses it anymore. Anyway,
potentionmeter is not adjustable. I have heard someone "repair" it by
renewing the contact surface. However, I don't, and I assume most
DIYers don't, have such capability/tools.
GeneKelly via CarKB.com wrote:
I'm actually diagnosing two problems with 1989 300E which could potentially
AC Problem: Car cools nicely for about 30 minutes then starts blowing warm
air. It blows out a damp, swampy smell when it happens. I do have R12 in
the system. Monovalve appears fine, checked under Tiger's guidance.
Compressor has always leaked a little oil. Maybe the compressor's bad or out
of oil? Not sure how to check that. Auxillary fan is working fine and AC
fuse and blower motor fuse was replaced. This leads me to my next problem.
Air Mass Meter: Checked by Volkswagon mechanic. He measured resistance at
air flow sensor position indicator (also called potentiometer by some). The
sensor position indicator is located on the front of the air mass meter.
It's black with three pins on it. I think the plug that connects to it goes
to the compressor (I believe). You can see a picture of it any of the parts
sites and pulling up the mass air flow sensor or air mass meter. When my
mechanic tested the sensor, it read off the charts (57,000 ohms instead of 3,
600 to 4,400 range). Book said to replace air flow sensor. Is there any way
to fix this without buying a complete air mass meter at $650? My car is
moderately stumbling and surging. It really doesn't seem to affect the
performance of the car except when idling.
Could these two problems (AC and idling) be related? Any more advice would
be much appreciated. So far, I've gotten some great inputs from you guys.
So please, keep them coming. I'm in Montana with very few Mercedes mechanics
Wan-ning Tan wrote:
Message posted via CarKB.com
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