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Old 07-26-2010, 06:40 PM
Registered User
Join Date: Mar 2010
Location: Potomac, MD
Posts: 7
A/C blowing warm air 89 300E

Hi - I have been doing some simple maintainance myself on my 89 300E since I bought it in 1993. I have a full set of Original Mercedes Service Manuals (paper copy bought about 15 years ago) and they have served me well. Now the hard part, the A/C in this car was converted over to R134A about 3 years ago when my son took the car to college. Last fall the A/C stopped cooling but we didn't mind but now it is extremely hot in DC and I've been trying to fix it all day today but no luck.. Here is what I have done...
Checked the KLIMA relay and the PBU. When the PBU is pressed, the KLIMA relay sends the signal to the compressor and it comes on and stays on. I put a set of gauges on it and I checked the static pressure before starting the testing with the engine off and cold - it read 85-90 psi on both low and high side...
then at idle with A/C on, blower on high and windows open, the low side read about 20psi and the high side read about 150psi- ambient temperature at test time was about 88F. it was not cooling, so I hooked up a 12 oz can of R134a to it and it took up about most of the can... then the low side pressure went up to between 37 - 40psi and the high side went up to about 250-270psi. However still no cool air - it blows air fine its just not cold....
So what could be the problem? since the compressor is holding pressure i'm thinking there shouldn't be a leak but I'm not sure - could the pressure be too high or could it have air in the system, could it be a bad expansion valve - I don't want to take it to the mechanic yet until I get an idea of what the possible problems are - I don't want them to do unnecessary parts replacements...
Any help would be really appreciated...

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Old 07-26-2010, 11:26 PM
Registered User
Join Date: Aug 2008
Posts: 436
I'd say you need to take it to a shop-

Idle pressure of 270psi is HIGH. Could your heater control valve be bad- running full heat w/ac?
Is the aux fan running on the front of the car?

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Old 07-27-2010, 08:49 AM
Posts: n/a
I think the first thing I would do is remove the glovebox liner and remove each vac line one at a time from the switchover valve and check with an auxilliary vac source. If there are any of these lines leaking you have a bad vacuum pod. If the recirc vac pod is bad you will get 60 degree or hotter air at the vent even with a properly charged a/c.

The 134 conversion is much less than ideal in these cars, but with those pressures it should be cooling better than warm air. A thermometer in the vent for a temp reading would give us more data.
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Old 11-24-2010, 12:48 PM
Benz Mondi
Join Date: Nov 2006
Location: Sacramento area
Posts: 308
A/C blows hot

I just discovered your post while doing a search for the A/C blower repair.

Hopefully you've already solved your problem.
If you haven't, I will tell you that it could be as simple as your water valve is not working properly and not closing off the hot water from the engine. It's a cheap and simple replacement and install. It's just in front of your battery has a single plug in and two rubber tubes connected to it.

I had this happen to me and so I know the symptoms very well.


03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio
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