Thanks all. Got ramps and will do it from the under side. "Hotlanta" is a bear in the summer. Again my thanks to all the brickboarders! billn

Receiver-drier is on the high side of the system. There is nothing to do there in adding charge to the system.

The service ports on a '90 are on the rear face of the compressor. (Assuming you have the OE Diesel Kiki compressor.) I have found easiest access is from beneath (you do have ramps, don't you?). One is labeled "S" and one is labeled "D". High side is "D" and low side is "S". Charge into the low side (suction side) with compressor running.

When connecting to the fitting, first thread the hose's fitting on loosely (not enough to depress the valve core). Then open the valve at the can tap or gage set and wait until refrigerant comes out at the service port where you loosely threaded the fitting. Then tighten the fitting and close the valve. This makes sure there is only refrigerant from the can to the interior of the compressor. Air trapped in the system creates strange problems.

After connecting, start engine, engage compressor clutch, open valve, charge till no bubbles in sight glass. With R-12 the system should be relatively insensitive to small overcharges, and the total charge amount is fairly large on this vehicle, so I would go ahead and charge in increments of an entire can. You may need to set the lower portion of the can in some warm (not hot) water to raise its pressure slightly and ensure flow from the can to the system. Also you may need to raise the idle speed by finagling the throttle cable somehow (this will lower suction pressure). Keep the can upright so as not to charge liquid into the suciton side.

When done, close valve, shut down car, crawl under, screw the fitting off as quickly as possible (there will be some loss) and put the cap back on.

Good luck. It's not as difficult as is seems, but does require careful lab technique.

1990 used a receiver dryer, not an accumulator dryer. This is the same for pre-1990 200 series cars. This means that you have an expansion valve system, and hence the receiver dryer is located on the high side.
This means that you DO NOT change from the receiver dryer port! In a well working system, that port can easily exceed 200psi, and can blow up your hose and your can.

The low side (suction side) would be located at the back of your compressor, under the low side hose, this is the hose coming from the firewall, and hence from the evaporator.

Note that there is also a high side port at the back of the compressor. It is just under the high side hose, this is the one going to the front of the car, and hence going to the condenser.

Charging is done at the suction (low) side. It can be somewhat hard to get your hands down there, but it is done (been there done that)

You can monitor high side pressure to verify correct charge.

As diagnosing the problem, I would suggest you verify if indeed you are low on refrigerant before possibly over-charging the system. Does the compressor clutch engage? If not, the low pressur ecutoff switch could be reading a low charge. You can verify this by bypassing the switch. It is located at the top right of your receiver-dryer. Just conenct the two wires together and with the A/C ON the compressor clutch should engage. If it does not, then forget about adding refrigerant, you just have a bad clutch or blown fuse...

The likelyhood of low charge though is high. Also realize that the likelyhood of an intrusively large leak is high. You might want to do a leak-down test first to find a leak and repair it, before potentially wasting the R12, your call. Common problem areas on your car would be the high pressure hose and the silencer on it.

Again, charge ONLY from the suction port at the back of the compressor!

Good luck,
Greg Mustang
Montreal - Ottawa
Canada