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  #1  
Old 08-20-2016, 02:00 PM
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1990 560SEL A/C Oddity

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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Old 08-20-2016, 02:19 PM
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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.
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Old 08-20-2016, 08:34 PM
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Unfortunate.

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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Old 08-22-2016, 12:39 PM
Tom Evans
 
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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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Old 08-23-2016, 03:40 PM
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Quote:
Originally Posted by Mxfrank View Post
5) Vacuum problem affecting dash pods.


This is where I would start.

Inspect the 'mono valve' too.
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Old 08-23-2016, 10:23 PM
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Heater control valve?
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Old 08-25-2016, 11:23 PM
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Not the mono valve

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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Old 08-26-2016, 10:42 AM
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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
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  #9  
Old 10-29-2016, 07:09 PM
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Finally resolved, I hope . . .

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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