89 300E AC Problems

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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?
Gene
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Have you checked if your heater monovalve is good or bad? Sometime it does go bad and the AC cannot overcome the heat.
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I'm not sure where that is located or how to check it. Can you advise?
Tiger wrote:

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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 again.
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Thanks Doug,
If after checking the fuses I still have the problem, do you have a method of testing the monovalve?
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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 bad.
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Thanks,
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 some way?
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 car.
Thanks again,
Gene
Tiger wrote:

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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.
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Tiger,
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?
Gene
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Then I would say it is your expansion valve is bad.
Idling in the garage? Are you trying to poison yourself? CO poisoniung?
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Either expansion valve or that your system was not properly filled with refrigerant... too low...
Or when it has been filled... was not under vacuum before refilling.
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Tiger,
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 has vacuum.
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,
Gene
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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 leaking.
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Tiger,
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 again?
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 sensor.
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 stock.
Thanks,
Gene
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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.
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Tiger,
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 separately.
Gene
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The system need to be vacuumed for at least 30 minutes... you can borrow the vacuum pump at Autozone for free.

The problem is I don't know what part you are referring to. Maybe if you have a picture of it then I can tell you.

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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.
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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.
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All,
I'm actually diagnosing two problems with 1989 300E which could potentially be related.
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 available.
Thanks,
v/r
Gene
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