I just noticed that my A/C clutch is energized as soon as I turn on my ignition and does not turn off when the "ECON" or "0" buttons are pressed on the Climate control panel.
Are there any relays that could be fused closed that I can check or do I have to repair/replace the Climate Control module?

Thanks in Advance for your help.

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Ian Read
90 300TE4MATIC
79 300SD

To start with what model car is giving the problem ?? Many possibilities.

Sorry MBDOC, the car which has the problem is a 1990 300TE 4MATIC.

Thanks

Most probable cause is the M.A.S. relay. Its behind battery under plastic cover

I reseated the M.A.S. Relay and the problem went away. Hopefully it was just the terminals. It looks like an expensive module. What other functions does this component do?
I will keep an eye on the A/C system.

Keep up the good work on your site!
Thank you.

Well that reseat fix didn't work very long. The compressor engaged again all by itself. Should I try taking the cover off the M.A.S. and resolder the components inside? Or is there another place where the problem could be generated? What is the approx. cost of one of these relays?

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Ian Read
90 300TE4MATIC
79 300SD

How about taking off the MAS relay and spraying some electrical contanct cleaner on the terminals. And be sure to look at the terminals on the reciever to see if they are pushed in and all are in good order.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

My A/C problem is back. Now the clutch is always engaged on my '90 TE 124. The clutch was erractically chattering in and out but then locked in. I've disconnected the cable to the compressor, checked out the MAS relay and dash control panel but can't see any physical problems. Are there any other relays or modules that control this A/C system? Are there any diagnostic tips out there? TIA.

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Ian Read
90 300TE4MATIC (124)
79 300SD (116)

Did I understand you to say that the A/C clutch stays engaged even when you have disconnected the feed connector? If so, then the clutch has seized and must be replaced. If what you are saying is that you disconnected the clutch feed to disengage the clutch, then there are a couple of places you can take a look at. The first question is once you have engaged the clutch with one of the selectors and you then hit the Econ or off button. if the clutch stays engaged, the a relay (M.A.S.) is hung (per Benzmac). Another possibility is that the high pressure side of the system is running above spec. This will cause the clutch to cycle rapidly. It wouldn't hurt to check those refrigerant pressures. Off spec pressures can cause all kinds of weird behavior. Also check the low pressure cutoff switch (located on sight glass block on the receiver/dryer. If this is shorted, this may cause erratic clutch operation. I offer these as possible take a look suggestions. I am sure there are many others out there who know a lot more about A/C than I do. If I were you, I'd be looking for a hung relay.

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Jeff L
1987 300e
1989 300e
1987 BMW 325

Yes Jeff, the clutch engaged while I was driving, I noticed it because of a whirring noise. It was below freezing out when this happened and I was on the ECON setting. The clutch then disengaged for a short while after my next drive but then engaged again erratically. It was still engaged when zero (off) button was pushed, so I removed the connector at the pump as a quick fix. I'll take your pointers and try checking the switch, pressure etc. I hope its not the MAS relay, since its about $470CAD to replace.

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Ian Read
90 300TE4MATIC (124)
79 300SD (116)

BTW, the A/C does function during the winter, especially in defrost mode. The air is cycled through the evaporator to essentially dry it out and dehumidify the interior of the car. So, don't be surprised if you find your A/C working in the winter. Also, another thought. If something stays engaged or a whirring sound is heard (unusual sound for A/C..Are you sure?? Take a look at the compressor and see if the clutch driven disk is turning). If it is and you've shut everything off, you have to suspect the dash control unit. Remember, looks aren't everything. This unit is moderately tough to get at an test, so maybe you should get a manual before you dive into the canyon of electrical/vacuum control components. Good Luck

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Jeff L
1987 300e
1989 300e
1987 BMW 325

[This message has been edited by jeffsr (edited 02-18-2000).]

If shaking or messing with the mas relay helps for short periods, you have found your problems. It is the compressor controller.

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Benzmac:
1981 280GE SWB
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

ian: avoid the grief, bring your car over to Cool-It located in Vancouver. They are really good and honest guys. Quality service without the dealers' cost. no i'am not a sales rep for them, just a satisfied customer. ask for pietro or steve. they are listed in the yellow pages. tell them you were referred to by edward, subway guy down Main street.

Thanks, Joel, I'll try them.
Jeff, to clarify my problem, the A/C compressor was engaged on any Climate Control function setting, even 0 (OFF), until I disconnected the A/C pump connector.
I believe that the whirring sound quiets in the summer but not entirely.
I am not sure why it is so audible when the compressor was engaged but could be due to the cold outside temperature and the pump was not cycling on/off so pressure was building up.

I believe that the A/C should only engage on three settings: Defrost, Bi-Level and Normal(Cooling Mode only). Each of these settings has two modes cooling and heating.

Am I right to say that the compressor should not be engaged on OFF "0", Normal (Heating mode) or EC (Ventilation) settings?

I think the cooling mode is switching "on" and "off" properly since the centre vents above the climate control buttons direct air imtermittantly.

The centre vents should only blow air in cooling modes for Bi-Level and Normal settings and during ventilation mode on the EC setting.
This is according to my Owner's Manual.

Does the MAS relay control the heating/cooling modes or is it the Control Panel Module only?
Are there any other sensors/modules that could tell the Control Panel module that should be in cooling mode regardless of setting?
One would think that the 0 (off) setting would disengage the clutch but it doesn't.
Maybe the OFF setting only shuts off the fan, valve and coolant pump.

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Ian Read
90 300TE4MATIC (124)
79 300SD (116)

Compressor should not be engaged in the off position, unless!!! the control circuitry does not know that the off postion has been selected. Chances are that Benzmac is correct in that the MAS relay is hanging. If the blower and the doors shut down when you hit off then it is probably the relay. If everything stays on, the the control center is probably to blame. Compressors get noisy for a number of reasons. Air in the system, low oil level, defective valves, overcharged system. Usually, if everything is in order, you should only hear a click as the clutch engages, the normal noise environment under the hood is normally loud enough to cover any operating noise from the compressor. If you are hearing the compressor, then you may have another issue that will need attention. I would start with that relay. It may be expensive, but it is cheaper than replacing a compressor and a system recharge

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Jeff L
1987 300e
1989 300e
1987 BMW 325

I took my car into the local MB dealer. They said it is the MAS too. Also said that I need a A/C system tune up to R134. ($700). I think i'll call Cool-It. Problem is that it cools well and I've heard that the R134 is not that efficent. I need all the cooling power i can get. These dark brown wagons really heat up in the summer. Thanks for your help.

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Ian Read
90 300TE4MATIC (124)
79 300SD (116)

iread,

If I understood you correctly, there are no known freon leaks, it's just that the compressor is locked in most or all the time. It sounds like you already know that this is a control problem, not a refrigerant problem.

If there is no leak in the system, I wouldn't pay so much as fifteen cents to change it to R134. It sounds like it might be a slow day at your MB dealers service department. They're trying to produce some revenue.

It sounds like you and Benzmac determined the problem to be the MAS. It also sounds like this independent A/C shop which is highly reccomended is the way to go.

There are some really good Benz service departments around, but there are also some that are thinking more of producing revenue than they are thinking about SERVICING their valued customers.

I was in Calgary about three weeks ago and about a foot of snow blew in when I got there. It's hard to imagine worrying about the air conditioner. Of course, in you're case it's like you're trying to turn it off! Just teasing.

Good luck with it,


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Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in
Mercedes automobiles

I wouldn't worry about losing capacity if you should convert to R-134 in the future. I personally converted an '84 300TD dark blue wagon to R-134 and have a '91 300TE wagon (also dark blue) that had already been converted. They both cool just fine here in Mississippi where the temperature and humidity hang around 95 in the summer. The conversion is more questionable on cars that have capillary tubes instead of expansion valves and undersized condensors. The trick (as usual) is to make sure whoever does it knows what they are doing as the R-12 uses mineral oil as lubricant and R-134 uses ester-based oil. This requires that you totally flush system to remove old oil.

DISCONNECT BLUE WIRE W/RED TRACER FROM LOW PRESSURE SWITCH ON REC/DRIER,IF COMPRESSOR CONTINUES TO RUN PROBLEM IS IN M.A.S. OR WIRE IS SHORTED TO GND ! IF PROBLEM STILL EXISTS IT ORIGINATES IN PUSHBUTTONASSY.




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M.F.