imidazol
I simply meant that charging with the can upside down will charge as a liquid. sorry for the confusion, I edited the post. I definitely agree with your choice not to use the leak selant "super refrigerant". I think the stuff is garbage. and i've never used it.
The only way that I charge my car is by recovering the old refrigerant, evacuating my system to 29.9 inches of mercury then weighing the EXACT factory charge back into the system. The reason being is that car a/c * dont use a thermostatic expansion valvre as a metering device therefore there is no reciever. The accumulator will only hold so much refrigerent. Remember one thing, if you have a leak and a low charge, your suction and discharge pressures will lower. A lower suction pressure corresponds to a lower boiling point of the refrigerant. in an undercharged system, your evaporater tempuraters will actually be lower. This will result in colder a/c for an amount of time. When the suction pressure pressure gets low enough to correspond to a 32 degree evaporater (27.8 PSI guage for 134A, and 30.1 PSI guage for R-12) your evaporator coil will freeze. Now if your car has a properly functioning low pressure cut-out, your compressor will shut off before this happens.
The point that I am trying to convey is that if your compressor clutch is engaged and running and you are still blowing out warm air, chances are your not low on refrigerant. By adding too much refrigerant you will raise your system pressures and raise your evaporator and condensor temperatures. This will overwork your compressor and add excess heat to your radiator as well. The most prominent risk in overcharging your system is the fact that your evaporator will not be able to boil off all this refrigerant, and will send liquid back to your compressor. This is why recovering all refrigerant and weighing in a factory charge is the best/easiest way to charge your system.
Also always make sure that your condensor coil is clean, and that your cooling fans are working. If the condensor is not able to reject heat, you will have extremely high head pressures and consequenlty poor a/c. a dirty condensor will exhibit many of the same symptoms as an overcharge, or a system with air/other non-condensables in it.
Just some advice, not necessarily directed to bob dillon. sorry

If not low on refrigerant, what is the most likely problem if the clutch engages but the air is uncooled?

I checked out the compressor with the dial set on "Max" and the fan set to "high" last night. The compressor is not engaging. I take this to mean the low side is discharged?

I'm running down to Harbor Freight to get a cheapie set of manifold gauges today.

Hey, besides the manifold gauges, they also have a cheapie vacuum pump for R134 for $18. Would this be a good thing to buy, too?

How good is your air compressor? I think the venturi types use lots of air. I wore out and had to replace the pistons in my air compressor while using a sandblaster, so now i try to stay away from things that put the compressor on 100% duty cycle. I used my dad'* electric vacuum pump that we have for farm equipment.

Good compressor. # HP, 40 gallon tank.

OK, I got the gauges. On a static test, engine shut down for about 20 minutes, I have 10 pounds of pressure on the low side and Zip, Zero, Nada on the high. Did I measure that right?

Assuming I did, what'* next?

bob dillon, you need to find that leak. and it probably wont be easy. I would check all fittings for oil. Bad news is that with a system at idle (and allowed to sit long enough at idle for pressures to equalize) you should read equal pressures. If you have a system that doesnt equalize after 20 min tops after being turned off it usually means that you have a restriction in your system, most likely in your orfice. However I dont believe this to be your problem, it may be a guage problem. Either way if your reading pressures that low then you have a substantial leak. Before anything can be done you must find your leak. Even if it means changing every o ring. Another problem is that if you do in fact have 0 psi in your system then the leak will equalize with the atmosphere meaning that you will have moisture in your system. To properly fix that , you would need to pull your compressor,evap,cond,and accumulator. then you would drain the oil out of each of these components; measure the amount of oil removedand replace with that exact amount of new oil. Then After having the system open you should replace the dessicant in your accumulator/dryer.(you may get away with not doing this if you pull a great vacuum with a pump that is at least 4-5 cfm. and let the pump run for a long time. following all of this you should charge with the exact amount specified.
Now if you dont want to do it right, you can repair the leak,pull a good vacuum (29.9 in.hg) and recharge accordingly. This method will most likely work but your system will probably only last for about a year.
If you have access to a great library they should have the proper literature for your car it would have all a/c specs.
If you do tear your system apart...take lots of pics

Still hoping you have an answer for this question as I believe it would be helpful for future folks with A/C issues.

Echo SSEI. The best thing that you can do is to check your pressures. Its hard to speculate without pressures. Also if your ac clutch constantly engages and disengages with the a/c on max and the blower on high and the windows rolled down and a fairly high ambient temperature, then you probably do in fact havea undercharge. If your compressor cycles with your car ideling;imagine how it will act at higher rpms. Check your pressures and report back,also if its not at least 80 degrees outside,pressures mean pretty much nothing.

I guess I was just looking for more of a "rule of thumb" answer, since you indicated that an engaged and running clutch/but no cold air seemed to point to something other than low refrigerant. I don't want to hi-jack Bob'* thread, but I had this exact condition and (before you had posted this) I did add some refrigerant (went from 27psi to 34psi low side) on an 80* day, less than 10% humidity, A/C on, blower on MAX . I have cold A/C again, and it has held for the past 48 hours, but I will continue monitoring pressures to see if they drop.