deniss,

The port by the grill is the HIGH side on a stock 1984 300SD.

deniss,

Just so you don't think folks are holding out on ya - the high side port is in front, the low side is back by the firewall. It is easy to tell by running the system a few minutes. The low side will be cold or cool, large diameter hose, the high side almost too hot to touch when the system is working properly, warm when marginal, smaller diameter hose. It has been my experience with these cars that slow leak area problems develop in and or around the R4 compressor or the expansion valve next to the evaporator in the dash. I have fought slow leaks in this system for years. The SDL is less likely to leak but boy that compressor is expensive :eek:

OK you AC guru's - fire away....

Leathermang - you're right about safety concerns, there's no arguing that.

So after some looking in the hood, I determined that the port that's up by the radiator behind the grille must be the high-pressure port. I was at first confused by what I'm guessing are the oil cooler lines.

Let's see if I got this... The high-pressure hot gas comes out of the compressor and is routed to the top of the condenser, so the high pressure port is between the compressor and the condenser. The gas becomes high-pressure liquid in the condenser and exits from the bottom of the condenser. It flows in a smaller diameter hose that is in some parts blue in color. It goes through some kind of a can that's mounted on the chassis near the compressor. There are some wires going to that can - what is it?? Ok, from the can, the blue line goes toward the cab where the evaporator is, and then a thicker black line comes out of the firewall near the vacuum line for the brake booster and goes to the compressor. There's a service port in the thicker black line near the firewall, and that must be the low-side service port.

Right?

I must have been writing up my post while a couple people have posted with the answer. Good to know I have it figured out.

charging
 

Yes you can charge from the low side. Usually the larger hose is the low side
Get a Pressure chart For R-12 or whatever gas you may be using.
A pocket thermometer for monitoring the temperature at the vent is helpful.
Determine the pressure you need at the low side by the ambient temperature.
Refer to the chart.
Connect your can and go!
Wear safety glasses and gloves.
If you do find a leak., you will need to purge the system.
Make the repair and refill. I got an old refrigerator compressor to use as a vacuum pump ….soldered a fitting on the suction side.
There always free at the curb………..
Its great that your doing this yourself……that’s how you learn …..

Don't forget a new receiver-dryer whenever you're in there.

The can is the receiver/dryer. The wires are connected to pressure switches that 1) control the aux fan, and 2) act as a compressor cutout if the pressure of the liquid is too low (no freon).

So if your system is too low and the compressor cuts out, how do you get suction at the low-side port to fill the refrigerant?

good question!!
 

good question!!
If the system was evacuated the vacuum would draw in the gas even if the compressor was off.
Not sure ...if you could jumper the pressure switch to force the compressor on.

Deniss,
You totally missed the expansion valve in your description!
The high and low side's seperated in 2 places. The expansion valve and the compressor.
The high side is high pressure high temperature liquid. Goes through the expansion valve into the evaporator where it changes to a low pressure low temperature liquid.
This is accomplished by a portion of the high pressure high temp liquid flashing into vapor as it enters the low pressure evaporator. Hense the term Flash Gas.

When charging the vacuum will draw in enough refrigerant to activate the compressor. It will cycle on and off for a while until the pressure builds up. Do not charge with liquid. You must resist the temptation!

Also, you have your license so I probably don't have to remind you to purge your lines.

Danny

I was just describing the equipment I could spot visually under the hood. I do realize it's there :)

I realize that the compressor is intended for compressing gas. I thought that when you hook up a can of R-12 to the low-side service port, it will either be charged as gas or will become gas before it reaches the compressor...

"Connect your can and go!
Wear safety glasses and gloves."--Anthony C

Here is a good example of what I was saying about too many variables...AND that a forum is often a place where warnings don't get posted the way a formal instruction book or organized site would give.....

It would not be unusual these days for the can tapper one buys to be a side tapper... as compared to a top tap from the old days...
So one could easily think they were doing the right thing and slug the compressor or break the reed valves with liquid refrigerant by not understanding that liquid comes out of the bottom of the can if in nomal position and gas if tapped at the bottom and the can is turned upside down.... but with straight sides on most cans if one is not thinking that it matters you can do great economic harm.
Liquid can be installed on the high side ... but the compressor is not running when that is done... and if the exit side of the compressor is leaking that in itself can damage the compressor.... When installing vapor on the suction side the compressor and engine are running.
So here we go.... even though all this stuff is posted on dozens of threads on this site ( SEARCH ) ....and general AC theory is wonderfully presented on the Aircondition.com site ( to include warnings ) we are typing out the answers ( mostly partial ) on this thread...

Saftey
 

Saftey is first...
leathermang brings out a good point .......
If you turn the can upside down you will get liquid. Rightside up on the low side.
Get as much information as you can. This is a great site for that.
Hey ...give it a try. If it works great......
There may be some tools you need and that may not be cost effective.
So....You realize you may need to take it to a ....what do you call those guys that like your money.....a Porfessional???
The average charge is about $300.00 at a good shop.

No worries, I will take this information with due precautions. I am browsing on aircondition.com also to supplement what's been said here. It's a good starting point.

Thanks to everyone for explaining and contributing to this thread, even if it may be a repetition of previously written stuff.

The stuff in the archives on this forum have a huge amount of information in them INCLUDING great pictures ... and mistakes people have made.... and quirks which some models have... it just does not compare to what a short thread like this provides...
A six month or year long discussion .. where people have time to go research stuff... ask other people like their mechanics or professionals in the field... can be much more in depth on each of the different things you have asked about...