89 eldo with ac low refrigerant warning shuts down compressor. System properly charged with r12

USA are you or Europe???? hint cars not the same.world wide.
Mercedes-Benz ( the makers here have no service manuals sold to owners) <<< makes repairs hard and expensive.
R12 OR IS CAR UPGRADED TO R134A OR R1234?

well it has 2 caps on the port. so 2 there are
there are 2 lines there, one is huge one is small
one side is called low side, and the the other is high side.
best is not touch the 300PSI (21 BAR.) SMALL LINE TUBE.
do not attach a walmart R12 can here or it will explode. in the face, OK end safety rant1, and where goggles.
the low side is about 10X LESS ABOUT 30 PSI
THE R12 RETURNS HOT AND IN GAS FORM. USING LARGE TUBE.
THE COMPRESSOR COMPRESSES THE GAS BACK TO A LIQUID STATED, AND SEND THIS LIQUID TO THE CAB EXPANSION VALVE TO GET COLD.

ONLY MESS WITH THE LARGE LINE , PORT
R12, IS FACTORY PORT,

THIS IS ORIGINAL 1999, WHAT IS THERE NOW I'VE NEVER A CLUE BLINDED IN A TEXT BOX. HERE.. MESS ONLY WITH LOW SIDE. OR BOOM,
THE HIGH SIDE CAN EVEN HIT AND EYE AND BLIND YOU.
EASY. GOOGLE ARE NECESSARY.

IN REAL SHOP THESE 2 ARE CONNECTED TO A TOOL CALLED A MANIFOLD. AND IS USED IN A SAFE MANOR.
A SKILL TECH TRIES NOT TO INTRODUCE AIR TO THE LOOP
AND DOES SO PURGING LINES IN THE MANIFOLD FIRST.
AIR IS BAD, BAD NEWS. (AND AIR IS EVERY WHERE)<LOL

failing clutch interior valve

Depends on the year. Cars in the early 90's used R12. Anything later uses R134a. The size of the charge ports will define the R134a port. An adapter is needed to fit on the charging hose. R12 requires no such adapter. Ther charging label under the hood will tell you what refrigerant is needed too. Since you failed to give enough information on what make and model car you have, a general answer for the location of the low pressure port will be given. On the compressor, the larger diameter hose connection is the low side. Connect there, or look for the drier/accumulator (Large diameter silver container near the firewall). Connect there. You will need to access the AC information label under the hood to determine how much freon to add to the system.

If your car ac is not taking in refrigerant, it could be caused by several things.
If you have recently done work on the system and you are going to fill the system the first time, you will need to pull a very deep vacuum before trying to charge in refrigerant. If you have already properly evacuated the system and having problems getting the correct charge adjusted, you may be having problem with pressure equilizing between the refrigerant source and your ac system. To get the refrigerant going again, you need to connect the charging hose on the low pressure side of the ac system and turn the fan and ac on cool. The pressure in the ac system on the low pressure side will get lower than the pressure in the refrigerant container and flow will start. When charging initially, charge liquid on the high pressure side and with the ac system running, charge gas on the low pressure side. To charge liquid, invert most container so spout is down, to charge gas, keep container right side up. Charging by gas help you to not over charge your system. Beware on most vehicle ac systems the suction and discharge connections are two different sizes and require the proper adapter. If you still having problems creating a pressure difference to move refrigerant, you can try immersing the refrigerant container in warm water or applying warm air to the exterior of the container.
Hope this helps.

R12.... In 1994 R134a was required, but some changed earlier. Yours may have been converted. If the service ports are threaded it still has R12. Hope this helps.

NOPE STOP CHARGING DONT PUT NO MORE REFRIGERANT IN SYSTEM.DO GOING TO DAMAGE NEW COMPRESSOR AND THE EXCESS PRESSURE WILL BUILD UP CAUSE THE REFRIGERANT CANISTER TO BURST EXPLODE INJURY OR FATALLY HURT YOU. BEST TAKE CAR TO A GARAGE.LET GARAGE PUMP DOWN AC SYSTEM AND CHARGE UP.BECAUSE YOU WILL DAMAGE NEW COMPRESSOR. MOST OF TIME I WILL HOT WIRE THE AC COMPRESSOR. MAKE SURE COMPRESSOR HAS 2 OUNCES OIL IN IT.IF COMPRESSOR START UP NO OIL IT WILL LOCK UP.DONT KEEP PUTTING REFRIGERANT IN THE AC SYSTEM, THE R 134A GOING TO FREEZE UP TOO MUCH MOISTURE IN SYSTEM YOU HAVE TO HAVE A VACUUM PUMP TO PUMP DOWN AC SYSTEM CHECK FOR LEAKS BEFORE CHARGING SYSTEM UP.LOW SIDE NEED TO BE 31 - 36 PSI THE HIGH SIDE SHOULD BE 165 - 205 PSI.

in order to recharge the ac you first must clear the trouble code for the ac off warning. when you get this warning the computer shuts down the compressor to keep it from running without the lubricant that is in the freon. clear the trouble code and the compressor will run and you can recharge the ac.

is your pressure sensor on the condensor working, it controls the engaging and disengaging of the compressor, if your compressor doesnt enguage it could be a bad compressor, and with my old c4 vette which was a 1992 the test for it was to run the ac on max, let the compressor try to cycle on its on, if it doesnt unplug the pressure switch on the condensor, but (WARNING: YOU MUST DO THIS EXACTLY AS STATED) plug it back in with in 5 seconds. as the pressure switch is what tells the compressor to engage, (NOT PLUGGING IT IN WITHIN 5 SECONDS COULD CAUSE YOUR SYSTEM TO BE OVER CHARGED AND EXPLODE.) . if your ac clutch does engage and your starts running the compressor with in that 5 seconds, your compressor is good. but this will also narrow it down to an either A: and over charged system or B: a bad sensor on the condensor. or C: if your compressor runs constantly, and undercharged system. Make sure to check for leaks, use some water around the two a/c system servicing ports, check the high and the low side as over time the O-rings deteriorate, and even though they have the plastic cap on them they can leak. remove the plastic caps, and put some soapy water around them and see if you get in bubbling with the system on. On my 92 the a/c use to leak down ever 30 days, i thought it was the compressor leaking during operation. i soap water tested all the connections and it was the service connections leaking. Went to autozone, replace the caps, reserviced, and went 3 years (Before selling the car) without having to service it again.

sounds like it is low on freon , R12 if it has not been converter to 134a , if it is R12 THIS REFRIGERANT IS NOW VERY EXPENSIVE ,and you might want to consier convertion kit , but now back to recharge ,it is tricky but can be done get a recharge kit of proper refrigerant hook to low presure side fill tap turn upside-down to release liquid and open valve with the car off and motor not running turn all the controls to ac high position with the motor still not running remove the neg side of battery terminal for at least 2 minutes ( now you have to be quick with this next step ) have someone in the car ready to start it on your command , hold the refrigerant can upside down and replace the negitive terminal to start the car ( you must be quick with this step )you should see the ac compressor start to turn the clutch portion on front of course the pulley always turns , what you are trying to do is get some charge into the system before the computer gets the signal from the low presure switch to turn the compressor off , repeat this processor until the compressor continues to run add the remainder on the can of refrig to system and if the compressor continues to run add another , unless you have a large leak this should get you through the winter with heat this works in many cases ,good luck

Heater Control Module