I've owned this car for 3 years and it's always had the same issue with the A/C. The air conditioning is very good and almost all the time it works perfectly. But, once in awhile the outlet air will become warm for 5-6 seconds. Never less, never more. After that the blower speeds up and air turns cold again. Once the car gets back to the set temperature the blower slows down and everything is back to normal. There are days when it happens several times and days when it doesn't happen at all.

Except for those little 5-6 second bursts of warn air the system work perfectly. I love to figure out what the deal is. Any ideas? Thanks.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire

1) Low pressure. The compressor will shut down if high side pressure is too low.

2) High pressure. The compressor will shut down if high side pressure is too high.

3) Engine load cutoff. Various engine controls have various ways of detecting high load situations, and will shut the compressor for brief high power bursts, like accelerating to pass.

4) Defect in the head unit.

5) Vacuum problem affecting dash pods.

6) Bad blend flap sensor.

7) Intermittent ground.

8) Bad Klima relay (if so equipped)

9) Bad indoor air temp sensor or air sampling blower

10) Bad evaporator temp sensor

On and on....intermittents are the hardest problems to solve. First thing you want to do is get a gauge set on it and read the operating pressures. Try to keep a log of occurrences, to figure out what else is going on that might be related.

Unfortunately I know most of that but was hoping for a "magic bullet."

Some of these are unlikely. I have a manifold gauge set and the refrigerant pressures are where they should be. It was a little low in June but a couple of ounces brought it up to spec.

It will happen the same way cruisihg, idling or accelerating so engine load isn't a factor.

Thanks for the other thoughts, I'll work on those.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire

I once had a intermittent heater water control valve on a 126 model. The heater has enough capacity to over power the A/C and you will get hot air out even with the A/C turned on high. There is a replaceable cartridge in the water control valve which is much less expensive than a complete valve assembly.

Mxfrank is right. There are many things that can cut the A/C compressor off. Intermittent problems can be the worst, even for experienced mechanics.

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Tom Evans
69 280SE 91K (wish I still had it)
73 220D 98K
78 300D 134K
78 300D 185K
81 300SD 301K (still have)
82 300SD 321K (still have)
84 300D 268K
84 6.2 Diesel GMC Suburban 225K (a whole different story!)

This is where I would start.

Inspect the 'mono valve' too.

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Heater control valve?

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I have never met a man so ignorant that I couldn't learn something from him. -Galileo Galilei, physicist and astronomer (1564-1642)

This has been doing the exact same thing for .3 years. I bought the car at the start of warm weather and didn't know until fall that the car had no heat. The temperature gauge had always read a little low so I first installed a new thermostat.

That brought engine temperature up to spec and I had good heat as long as the engine speed was below about 2,000 rpm. With higher speeds the outlet air would turn cold. I changed the mono valve insert but to no avail. I finally fixed it by changing the entire mono valve unit with a new oem part. I also installed a new auxiliary water pump.

Through all that the AC always did the same 5-6 second warm outlet air thing.

I just found a post somewhere that claimed the auxiliary pump could be drawing too much current and causing the control unit to cut off the compressor. He claimed that unplugging the pump fixed it.

I'll give that a try and see.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire

Check and clean your fuses in fuse box, sounds like when the fuse for ac getting hot its losing contact then when it cools contact is good, there are at least 2-3 fuses for ac may be related

I think I finally have this resolved. Unplugging the aux water pump and cleaning all the fuses made no difference.

I bought and installed a new in-car temp sensor then removed, cleaned and reinstalled the little blower that pulls air across the sensor.

So far it hasn't pulled it's 5 seconds of warm air trick. My fingers are crossed that it won't start up again in the spring.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire