How to hook up the A/C in its simplest form? - Mercedes-Benz Forum

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post #1 of 5 (permalink) Old 06-06-2013, 03:08 PM Thread Starter
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How to hook up the A/C in its simplest form?

Question for all of you ac gurus. My 72 250/8 at one time had ac. It was missing the tension pully and I was able to find one at the local boneyard. I put a new belt and started the engine. I'm sure there is no freon left and the compressor probably needs oil. I was not able to get the compressor clutch to engage, so I ran a wire from the 12v + of the battery and touched the terminals on the compressor to engage the clutch. It worked and the compressor sounded fine did not blow up. So now I am wondering, what would it take to just wire it up so that the compressor turns on and gives full ac with no temp control? Thoughts and opinions?
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post #2 of 5 (permalink) Old 06-06-2013, 04:13 PM
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Try charging the system first. You might get lucky and everything will work. Without the pressure switch in the circuit you will end up blowing out the compressor. The reason it isn't working is probably because the pressure switch isn't being tripped due to no Freon in the system.

Try running a temporary jumper between the leads on the pressure switch (located either in-line or on the dryer bottle itself) to see if the clutch engages.
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post #3 of 5 (permalink) Old 06-06-2013, 05:24 PM Thread Starter
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Mike, thanks for the info. I jumped the pressure switch on the dryer bottle and it did NOT engage the clutch. Then, the compressor started to squeal like crazy, so I shut off the engine and removed the belt. I will have to think about if I want to get into trying to get this ac system running. Right now, I'm just thinking of tinting the windows with 20% on all sides and 5% on the back.
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post #4 of 5 (permalink) Old 06-13-2013, 08:31 AM
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I can offer you some encouragement. AC works on the incredibly simple concept that when a substance that has been compressed into a liquid is allowed to suddenly expand into a gas that change of state chills the substance. After learning that I installed AC on 3 cars that originally did not have it and those systems all work perfectly.

Your 1972 was designed for R12 refrigerant that is now still available but very expensive. A conversion to R134 solves the expense problem but requires that the system be flushed of the R12 and its oil. Many experts (I'm a self taught amateur, not an expert) also recommend use of R134-compatible hoses. R134 requires a different compressor oil than R12.

Before the system can be charged with refrigerant it must be vacuumed to remove all the air/water vapor in the system. That requires a vacuum pump that can pull 26+ inches of vacuum because at 26 inches of vacuum water boils at 70* F, allowing the vacuum pump to evacuate the water vapor in the air that is in the system. The ability of the system to hold that vacuum indefinitely after the pump is turned off will tell you whether it is tight or leaky. Leaks are common at the shaft seals of used compressors and at O-ring plumbing connections. Then it is charged through a 2-gauge manifold set that is readily available from auto parts stores.

Once the system is charged it starts shooting isicles! The output air temperature of a system that is working properly is below 40* F.

Have fun!

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post #5 of 5 (permalink) Old 06-16-2013, 10:47 AM
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OK all the above is true - but I suspect the reason your clutch does not engage is you don't have BOTH the AC switch and the FAN (labeled defroster) turned on. If you do not have them both one the clutch will not engage.

Also there is an electric switch which is activated by the vacuum door (it's directly over the drivers foot) but you can hear if that is operating - with the engine running and AC off turn the fan on. Now turn the AC on - you should hear a hiss (vacuum doors) and then the fan will turn off and a different fan will turn on. If that happens you're good on the inside - but if the fans don't switch from the defroster (buried deep inside the dash on the firewall) to the AC fan (in the center console under the radio / ash tray) then the compressor clutch will not engage. A second easy check is to remove the plastic grill on either side (only need one) of the center console or maybe just look with a flash light - you should see the squirrel cage spinning with the AC (and fan) on.

As said above don't run it without oil / refrigerant for long. Also you can get R12 fairly easily and it's not all that expensive off of eBay - I'd be more concerned that you have a leak and don't have the equipment to evacuate the system and test for leaks.

FYI - I have my '73 converted - I needed 1 new hose (some of the others have been converted over the years but some are still original) I had the dryer converted and everything else done properly - it ran about $700 with the hose being maybe $150.

The R134 works great.

Good luck.

Last edited by frankflynn; 06-16-2013 at 10:51 AM. Reason: fix a typo
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