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  #1  
Old 06-15-2016, 05:20 PM
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W115 AC Temp probe question

Hello,

The AC Temp Probe on my W115 broke today.It is the hollow rod that protrudes from the AC switch, I assume it should lead to the evaporator box. Now the compressor is no longer engaging. I see that the probe is hollow (broke at the base of switch). Any suggestions on reattaching it or finding a replacement for my 240d?

Reference picture from another post: (Probe is on the right side and goes out up)



Thanks for the help!
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  #2  
Old 06-16-2016, 05:56 AM
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Location: Alexandria, Virginia
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If it's broken off the control, you can't re-attach it. It's part of the thermostatic control unit.
I believe that thermostat was a US-made part and may be listed in old Murray AC parts catalogs. Otherwise, you may be able to find a non-adjustable, universal type. Main thing is, it cycles the compressor to keep your evaporator from icing up. Meanwhile, you could bypass it with a toggle switch to get your AC compressor going. But on humid days, you will probably notice the airflow dwindling as the evaporator ices up, and have to switch the compressor off manually for a few minutes and let the AC-fan de-ice it.

I had the opposite problem back in the late '80s, I'd acquired a '72 W115 220 that needed alot of TLC, including an AC compressor seal, pulley bearing, and a HP AC hose. The AC was working great during my first trip, on a warmish, but extremely humid day. I had one of those little round dial thermometers in the vent and noticed it reading about 22 degrees. (Good old R12 freon!) After about 40 minutes, I could still hear the AC-fan roaring. But the airflow had choked off to almost nothing. For the rest of my trip, if I wanted AC, I had to pull off the road for several minutes about every half hour, and run the AC-fan with the engine off until the ice melted and left a big puddle under the car. Then I could proceed for about another half-hour and repeat the process. Later I discovered someone (PO or mechanic?) had been in the dash and pulled the probe out of the evaporator-box. I found the little hole, reinserted the probe and all was back to normal. But I never had 22 degree AC again!

Happy Motoring, Mark
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Last edited by Mark DiSilvestro; 06-16-2016 at 06:35 AM.
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  #3  
Old 06-16-2016, 04:07 PM
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The only cure for this is a good used unit. These are not hard to find on Ebay.

The tube is filled with either. When they break they can be repaired but it is a massive job to do so and from a tech standpoint beyond the skill of anyone who is not set up to do this repair.

The proper term for this is a thermocouple so you might use that in the search terms when looking for a replacement.
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Old 06-16-2016, 05:42 PM
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Quote:
Originally Posted by Idle View Post
The only cure for this is a good used unit. These are not hard to find on Ebay.

The tube is filled with either. When they break they can be repaired but it is a massive job to do so and from a tech standpoint beyond the skill of anyone who is not set up to do this repair.

The proper term for this is a thermocouple so you might use that in the search terms when looking for a replacement.
The tube contains ether. It is called a "Bourdon tube". It is a component of the expansion valve.

It is not a thermocouple; a thermocouple is an electrical device.

An Idle mind is the Devil's playground.
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Old 06-16-2016, 06:43 PM
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Quote:
Originally Posted by Frank Reiner View Post
The tube contains ether. It is called a "Bourdon tube". It is a component of the expansion valve.

It is not a thermocouple; a thermocouple is an electrical device.

An Idle mind is the Devil's playground.
Yes, it's not a thermocouple. But neither is it part of the expansion-valve
(though some expansion valves DO use a bourdon-tube to regulate them)
The bourdon tube here controls the AC compressor thermostatic cycling switch.

Happy Motoring, Mark
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  #6  
Old 06-19-2016, 01:23 PM
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Quote:
Originally Posted by Mark DiSilvestro View Post
Yes, it's not a thermocouple. But neither is it part of the expansion-valve
(though some expansion valves DO use a bourdon-tube to regulate them)
The bourdon tube here controls the AC compressor thermostatic cycling switch.

Happy Motoring, Mark
I used the term thermocouple since most people know what this is. This probe is often called a capillary tube. The correct term for a device such as this tube is a manometer.

But how many people know what a manometer is?

And while it is true that many modern expansion valves are connected to the manometer this one is not. Mercedes chose to cycle the compressor as a means of regulating the A/C output temp. Modern expansion valves use the opening and closing of a valve, which is piloted by the manometer, to control the rate of expansion of the gas, using high and low pressure switches in the system to cycle the compressor.

And this device has a bourdon tube but it is inside the round body of the item and is not repairable.

The only realistic solution is to hit Ebay and pick up a working replacement. If this were a 300sl Gullwing the tube would be repaired as gauges and switches like this one are not readily available.

And there is also the solution to just install a switch to cycle your A/C compressor. You will likely freeze up your coils by doing this but they will melt off in a few minutes of the compressor being turned off.
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  #7  
Old 06-20-2016, 05:36 PM
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Hi

I've got a vacuum switch and probe from a 114 car of that vintage--let me know if you're interested--thanks


Tim Kraakevik
kraakevik@voyager.net
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  #8  
Old 06-23-2016, 08:07 PM
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Thanks for all the information! I bought a couple different thermostatic switches and got a match. I used UAC SW 6490C. Here are a couple photos, my ac is working great again 8)







Hope this helps anyone else out in the future! I paid $7 for the part.
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  #9  
Old 07-10-2018, 11:25 PM
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This was extremely helpful. Thank you for the help!
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