Why is my AC low pressure port reading too high on my Mercedes E320 1998?

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

Either you have moisture in the system or you have overcharged it.

Verify ambient temperature is above 65 degrees F.

1. Safety> - safety glasses, long sleeves and gloves, (exposure to refrigerant can cause frostbite).

2. Attach hoses to manifold> assembly, (hi, lo, and fill).

3. Remove caps from service ports, (Low press. Larger tube from evaporator to accumulator - passenger side over fender well; High Pressure - smaller tube from evaporator along firewall towards driver's side). Verify service> ports are clean.

4. Attach low press. and high press hoses to proper service ports.

5. Start car>, reset low refrig code, and turn ac on high and fan on high.

6. Read gauges and compare with chart provided with gauge set.

7. If low and high press readings are both low, then system needs recharge.

8. To recharge, attach fill hose to can of R134a and slowly open low press valve>, (NOT HIGH PRESS VALVE), until low press side reads 40 psi.

9. When flow stops and pressures stabilize close valves> and disconnect manifold from service ports and refrigerant can. Replace service port caps.

CHECK REFRIGERANT CHARGE. USING A MANIFOLD GAUGE SET. HOOK MANIFOLD GAUGE SET TO LOW PRESSURE PORT AND HIGH PRESSURE SIDE PORT. LOW SIDE PRESSURE SIDE SHOULD HAVE 25 - 36 PSI AND HIGH PRESSURE SIDE SHOULD BE 178 - 237 PSI. IF LOW SIDE PRESSURE SIDE READ BELOW 20 PSI THE LOW PRESSURE SWITCH WILL NOT TURN ON THE AC COMPRESSOR CLUTCH NEED TO ADD LITTLE MORE REFRIGERANT. IF REFRIGERANT LEVEL CORRECT CHECK AC COMPRESSOR FUSE AND RELAY.IF POWER GOING TO AC COMPRESSOR CLUTCH YOUR COMPRESSOR CLUTCH HAS OPEN WINDING. AC COMPRESSOR CLUTCH NEED REPLACING.

YOU NEED A R-134 GAUGE SET TO CHECK LOW PRESSURE SIDE AND HIGH PRESSURE SIDE.YOU ALSO CAN BUY A LITTLE CHEAPER GAUGE TO CHECK YOUR AC LOW BLUE SIDE AND HIGH RED SIDE AT ANY AUTO ZONE OR ADVANCE AUTO PARTS.WHAT YOU DO WHEN YOU GET YOUR GAUGE SET HOOK GAUGE TO LOW PRESSURE SIDE BLUE CAP PORT AND GAUGE TO HIGH SIDE PRESSURE RED CAP PORT.YOU CRANK CAR UP GET IT OPERATING TEMPERATURE.THEN TURN AC ON HIGH COOL LET RUN 3 TO 4 MINUTES.CHECK LOW SIDE GAUGE READING SHOULD BE 28 - 32 PSI. HIGH PRESSURE SIDE GAUGE SHOULD 199 - 228 PSI. THATS NORMAL CHARGE. SEE YOU CANT JUST RECHARGE AC SYSTEM UNTIL YOU CHECK IT FIRST YOU WILL DAMAGE SYSTEM .NOW YOU CAN GO TO AUTO ZONE PARTS BUY A CHARGE CAN WITH GAUGE AND HOSE.ATTACH TO IT.YOU NEED MORE THAN ONE CAN.WARNING MAKE COMPRESSOR IS RUNNING WHEN YOU TURN ON AC .BECAUSE IF COMPRESSOR NOT RUNNING CHECK FUSE AND AC COMPRESSOR RELAY.IF BOTH OKAY.YOU COULD HAVE OPEN CLUTCH COIL A FAULTY LOW PRESSURE SWITCH WILL CAUSE AC NOT TO WORK. IF LOW PRESSURE GAUGE BLUE SIDE PRESSURE IS 28 OR 30 YOU DONT NEED A CHARGE.PROBLEM SOME WHERE ELSE.BE SURE TO WEAR SAFETY GLASSES WHEN WORKING ON AC SYSTEM.

start at ac pump and follow the lines/hoses off the compressor the large line is the low side with a small service port the high side srvice port is on the small line/hose it is the large port(and they could be on the compressor

To diagnose problems, an A/C manifold gauge set is needed to read high and low side pressure readings. Avoid adding refrigerant with a simple charging kit like the ones sold at parts stores. Don’t add any stop leak, this can cause problems in the compressor, expansion valve or condenser.
Keep in mind that using an A/C gauge set and seeing BOTH high and low side readings can help in diagnosing the problem when you know what to look for. First, on a 134A system the high and low side service ports are different sizes. AC gauge sets have color coded hoses, the blue color coded hose has a connection that fits on the low side service port and the red hose has a connection that will only fit onto the high side. The yellow hose won’t hook up to anything if just checking the readings; it can be used to connect to a vacuum pump or attached to a refrigerant can or tank.

Normal readings on high and low side with AC OFF (static pressure) - Depends on outside temperature, but normally is between 80-105 PSI
Normal low side reading with AC on high speed and MAX & engine at 800-1000 RPM’s - Ranges from 25-35 PSI - Note that on many Chrysler products a normal reading on the low side may be 15-25 PSI
Normal high side reading ranges from 200-350 PSI

Don’t assume that if adding little Freon is good that adding a lot is better!  Overcharging just a little can decrease the performance of the system and possibly damage the compressor.

Both low and high side readings are lower than normal, this indicates a cars AC system is low on refrigerant and is under-charged.
If both low and high side readings are too high, this indicates an overcharged system - too much refrigerant. This also can indicate that the condenser fan is not working, is too slow or the car is overheating and heat is transferring from the radiator to the condenser.

Good luck and hope this helps, keep me posted be glad to answer any question you may have. And yes the A/C system on your car uses R143a and there is a A/C pressure switch along the low pressure line or on the accumulator, it looks like a oil pressure sensor with a two wire lead cliped on it. 

hello,good day,you have a 210 model with v 8 engine,your high pressure connection is located at front of your condenser,beside your ax.fan,then your low pressure connection is located at engine walling near your vacuum hose for brake booster,right side ,standing in front,ty

there are 2 ports one for low pressure and one for high pressure. the low pressure is bigger in diameter and is usually close to the dryer and or fender and firewall. the high pressure is near the condensor on the fron of the vehicle. if there are caps for the ports low will be blue and high will be red. low side pressure should be 35-50 psi and high pressure should be 135-150 psi