Where is to low port location for a 1996 E300 Air Condition System?

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locate the A/C lines along the passenger side that go from the condenser to the firewall. the low side port is smaller than the high side port. you have to look at the ports to see the difference is size and they each should have a cap screwed on them to keep dirt out of port.

Towards rear engine next to fire wall on top about center with black or blue plastic cap, a red cap is high side.

If you've never worked on an air conditioning system before, I would recommend you take it to a shop and get it checked out.
If the system is low on freon, you have a leak that would need to be fixed to keep any freon you add from leaking back out. And it could have an electrical problem instead of being low on freon.
Freon gas is minus 35 degrees when it comes out of the can.

Hello,
The low pressure port for charging the A/C system is located on the larger aluminum tube coming from the A/C compressor. It is usually located in the tube line near the accumulator. That is an alumninum cylinder about the size of a soda can or slightly larger usually located close to the firewall along the left fender side well. It will have a plastic screw on cap on it, usually stamped with an "L" on top. Your recharge hose will only connect to the low pressure side, the high pressure port is a different size valve for safety reasons. You can only get an accurate pressure reading with the engine running and A/C running at max cold. blower on high speed, after about 5 minutes of A/C operation. Temperature outside will also have an effect on the pressure reading, optimum ambient air temperature of 70-80 degrees farenheit. Directions for recharge are on your R134a refill kit, but the system does require 20-30 PSI for the compressor to kick on. If your compressor is engaging while you are refilling, you can probably do it successfully. If your pressure is extremely low and compressor does not kick on, you most likely have a leak in the system which will have to be found and fixed before you can charge the system......Mike

you need to chase it from your compressor in the engine. there are two ports, an inlet and outlet. i have a 2002 impala and it took me a few hours to find it. it was hiding under my coolant resevoir. the ports have a black screw cap on them

low pressure port is on the cannister dryer mounted next to the fire wall. high prssure port will be in a line somewhere. the adapter from your can will not fit on the high prssure

Did you check the acumulator? There is a port on it, that is the low side port. Keep in mind that if you have to recharge the system it is because other parts need service (leak in the syst) and whatever refrigerant you put in it will leak out again. The system may work temporarily or may work inefficiently. Also may sure that there is oil in syst.

look for the large ac line/hose and follow it untill u come to the small service port this is wher u charge the small ac line/hose has the large port this is the high side

Use on of those quick freon kits that come with the fill tube and gauge. It sounds like you have a slow leak and on a hot day there is enough pressure in the system to trip the switch. If thats not the problem check the wiring coming from the switch. The switch is probably near the filter/dryer. Thats the metal canister that sweats liquid on a hot day.