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1985 300D air conditioning

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monarchd's picture
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Hello Mercedes Club Members:
I've recently purchased a 1985 300D with 97,000 miles. Everything about the car is pristine. No rust, no leaks...absolutely perfect. The only set back is the air conditioning. The blower motor works just fine, however the air does not get cold. Does anyone have any suggestions about this matter? Thank you for your help! Have a great day

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The first thing to check is the Freon. With the engine running and the AC on look in the sight glass on top of the receiver/dryer located at the bottom of the right fender. It is easier with another person inside to control the throttle. Have them to hold the RPMs around 2k and watch for the Freon to bubble inside the sight glass. If no bubbles the system needs to be filled. If it has R-12 in it I suggest that you keep it that way since R-12 is colder than R-143/a. The R-12 will cost you more but if it is just low the total cost should be worth the difference in the cooling you will receive.

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D.L. SWINFORD's picture
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Cold is Cold

ryoflight1975;
I don't mean to be different than darrellws but cold is cold. In Texas you need to be certified to purchase r12 but not r134a plus it costs much less much much less.
I have a '85 300dturbo that I have converted to R134a . He has the Delco compressor.
After changing the refrigirant oil and charging with 134a; I physically can feel no difference in the output temp.
The r12 may cool to 65 degrees and the r134a to 67 degrees; can feel no difference.
As darrell replied get some help.
I always spray the condenser with water when I am charging a unit. This has been better for me than using a fan and it assures you that you will get a complete charge.
Being different again; when you see bubbles in the sight glass your unit is LOW on refrigirant. Add more until when spraying with water you see the liquid rise in the sight glass and NO bubbles. Repeat this cycle and you will asure yourself of a full charge and you will keep the head pressure down.
Happy Trails Beep Beep from thr Spiderman on the Gulf Coast, Don:D

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THANK YOU

Thank you both for a response. Just got word...it's the climate control relay. A pressure test was done and the freon is fine. Just a little over $500 for parts and labor.

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darrellws's picture
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Don, my '83 300CD with R-12 will go down to 40 degrees F with outside temps in the high 80s sometimes into the 90s. Now in TX the temps are hotter and the differential there may equal the temps in NC. I still like to keep a system using the refigerant that it was designed for....

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D.L. SWINFORD's picture
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degrees is degrees

darrell;
Degrees are degrees no matter where you are.
I was just referencing a relistic number.
Having been with GM for 20+ years and having taught refer servicing; it would be one heck of a unit that would reduce the inside temp of an auto 50% with reference to outside temp.
I understand your wanting to keep you Benz as designed; but the only difference in new units vs old is the oil and refrigarant.
Agreed that the boiling point of the two are significantly different but can you really tell the difference between -30 and -20 degrees?
There will come a time when you will be hard pressed to find R-12.
There have been cases where the 15 oz. containers have a large percent of propane.
Oh well I've rattled too much so----
Happy Trails Beep Beep from The Spiderman on The Gulf Coast, Don

Jim Grillot (not verified)
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How effectively R134a will perform in a retrofit has a great deal to do with the a/c system design parameters. If the system was designed "to the edge of the envelope," then R134a will not cool the car as well as R12. If the design parameters were more generous, with a greater factor of safety, then R134a will do okay.

As an illustration, several years ago I worked on a converted 85 123 sedan. The R4 compressor had locked up and had to be replaced. I tested the car with a 3 mile drive on a 95 degree day. The center outlet temperature was some value which was within the max-min envelope. I then drove my 85 123 wagon over the same route. Two weeks previously, I had replaced the R4 compressor and weight-charged the system. The center outlet temperature was one degree colder. Obviously, the 123 body a/c system had some factors of safety built into the design.

To throw my two cents worth in on the charging issue, watching the sight glass to determine proper refrigerant charge can be very misleading, especially with R134a. The very best way to charge a system is by weight. A system which has been weight charged and which doesn't operate within center outlet max-min temperatures has other problems which should be defined and corrected.

The second best way to charge is by using gauges with factory-supplied high side, low side and center outlet temperature data.

If anyone wants to know how to empty their system and recover the refrigerant in order to be able to weight charge it, let me know and I will provide a method.

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D.L. SWINFORD's picture
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Too True

Jim Grillet or just plain Jim,
You couldn't be more right than right.
R-134a has made every DIY mechanic a refrigiration expert.
All of your automotive parts houses as well as Walmart and the likes sell DIY kits on how to do it; right way or wrong they can do it cheap.
Weight and visible liquid measuring is the only true way to charge a completely evacuated system.
But the average joe purges his system to the atmosphere and has a unit at atmospheric pressure not 29-30 inhg.
Then he charges the unit putting 134a over the remaining 12.
What can you say; you're right.
Happy Trails Beep Beep from The Spiderman on The Gulf Coast, Don:p

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