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Old 10-05-2002, 10:33 PM
Posts: n/a
A/C cold, then hot, than cold???

1989, 260E, 294,000Km
Since a few weeks my A/C acts strangely.
It has been performing very well in the heat of the summer (near 100deg. at time) keeping the car nice and cool.
Now, driving at highway speed or standing still in traffic .the A/C would switch to full heat for 30 to 60 sec, then back to A/C (full blast) to cool it down. it could cycle this way once or twice an hour.
Any idea? Thanks

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Old 10-06-2002, 08:09 PM
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Old 10-06-2002, 09:46 PM
Registered User
Join Date: Dec 2000
Location: SF Bay Area
Posts: 115
I had a similar problem and it turned out to be the fuse under the hood had cracked (not blown) and was making intermittent contact, turning the AC on and off. I replaced the fuse and the AC now works perfectly.

I hope yours is as easy to fix.

'88 300E 265K
'91 300TE 155K
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Old 10-07-2002, 01:51 AM
Mike Murrell's Avatar
Registered User
Join Date: Feb 2000
Posts: 2,580

I'm not an expert, but have read enough here to be suspicious of the monovalve. These things influence heat control within the interior of the vehicle. I wonder if it's starting to act up?

Just a thought.
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
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Old 10-07-2002, 02:01 AM
Registered User
Join Date: Jun 2002
Location: Rancho Palos Verdes, CA
Posts: 9

Are you certain the temp was only at 100 C when this happened?

My 400E is purported to turn the A/C compressor on/off in 30-second bursts when the car is very, very hot. That was the last straw before cutting things off entirely.

I read that somewhere; I could find out where if you think it's relevant.

--Jim Kimble
1993 400E (104K)

"Instant gratification isn't fast enough."
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Old 10-07-2002, 02:59 AM
pmizell's Avatar
Benz Zealot
Join Date: Jun 2001
Location: Houston, Texas
Posts: 838
I think Mike is on to something. I've read here on the board that these bursts of hot and cold air could be due to a malfunctioning monovalve.

If you're lucky, your 103 engine has a repair "kit" that you can buy instead of a whole new monovalve. The later 103's don't have this luxury.

Good luck
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi
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Old 10-07-2002, 07:39 PM
Posts: n/a
Thanks for the advise.
But today, I have the reverse problem.
It was cold outside this morning (about 36degF). Once the car had reached it's normal operating temperature , the heater was functionning perfectly. The inside of the car quickly reached a comfortable temperature.
Cruising at 70MPH, the climate control switched very rapidely to very cold, as if the A/C was put at max. About 45 sec. later, it switched back to heat and temperature stabalized.
This pronlem seems to be ''vacuum ralated". It switched to cold when decelarating (coasting to about 55MPH) and switched back to heat when I resumed speed with the cruise control.

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