Blazer A/C Problems -- Suggestions and Help Really Appreciated

1989 S-10 Blazer 4x4 2.8L engine (replaced 4.3L). I bought this car in 2003, thought the a/c just needed charged. It was still an R12 system. I decided not to convert to R134 and I bought a product called Freeze12 and followed the directions. In a nutshell, I have replaced a leaking high pressure discharge hose. There are no leaks at all in the system. There is a full charge of Freeze12 in the system. I put on a new low pressure switch on the accumulator. The clutch engages then disengages continuously (start-stop-start-stop, etc.), like it did before I replaced this switch. If I jump the low pressure switch, the clutch runs like it should, but no cold air at any time. The belt is not squeeling, as I believe it would be if the compressor was bad. If I drive with the a/c or defroster on (whether with the jumper on or using the switch like I should), the a/c-heater fuse blows in a very short time. I can only drive with the heater or the vent on without the fuse blowing, but I get vent air coming out of the floor heater and front window defroster if I have the control switch set to Heater, and vent air coming out of the floor heater and vents if I have the control switch set Vent. No cold air blows at any time. (The air coming out of the wrong vents was a problem before I did any under-the-hood a/c work, and I previously replaced the entire heater-a/c controls on the dash, as well as the blower switch, to no avail.)

Suggestions appreciated.

Have you checked pressures on the system? That would be the first step, to see if you have enough pressure for the compressor to stay engaged. Then check pressures to see if the compressor is pumping down when it does engage. If pressure is up and the compressor is not pumping down, then I'd guess it's an electrical problem and you'd need to look for a short or maybe a bad winding in the clutch itself. If you're getting air through the vents then you should have air across the evaporator ok.

A must, You have to get gauge pressures. I don't know what it is like in your area. I will not use anything but r134a as areplacement freon. By the way Frezze 12 has propane in it. I always change accumulater and orifice when I do a Retro Fit. Hay it onlys makes it a better job. I also flush out the mineral oil and replace with ester oil. The pressure switch needs to be replaced with retro switch because pressure readings are different than R12. The high side will be higher and the low side runs a little lower.
As for vents sounds like you hook up something wrong when you work on the heater controls. No vacuum inside all the air would blow out the floor and defrost only.

1989 S-10 Blazer 4x4 2.8L engine (replaced 4.3L). I bought this car in 2003, thought the a/c just needed charged. It was still an R12 system. I decided not to convert to R134 and I bought a product called Freeze12 and followed the directions. In a nutshell, I have replaced a leaking high pressure discharge hose. There are no leaks at all in the system. There is a full charge of Freeze12 in the system. I put on a new low pressure switch on the accumulator. The clutch engages then disengages continuously (start-stop-start-stop, etc.), like it did before I replaced this switch. If I jump the low pressure switch, the clutch runs like it should, but no cold air at any time. The belt is not squeeling, as I believe it would be if the compressor was bad. If I drive with the a/c or defroster on (whether with the jumper on or using the switch like I should), the a/c-heater fuse blows in a very short time. I can only drive with the heater or the vent on without the fuse blowing, but I get vent air coming out of the floor heater and front window defroster if I have the control switch set to Heater, and vent air coming out of the floor heater and vents if I have the control switch set Vent. No cold air blows at any time. (The air coming out of the wrong vents was a problem before I did any under-the-hood a/c work, and I previously replaced the entire heater-a/c controls on the dash, as well as the blower switch, to no avail.)

Suggestions appreciated.

Did you evacuate the system (pull a vacuum) after you replaced the hose?

The air coming out of the wrong vents usually means that the vacuum line has fallen off of the 6 inch diameter black vacuum ball under the hood, or one of the vacuum lines going to the ball is disconnected. Usually, if the pressure is low enough to trip the low pressure switch, the compressor won't run at all.

Are you sure that the previous owner hadn't changed to r134a? I did the r12->r134a on a '87 vette, and had not a problem. It flowed about 6 degrees warmer at the duct outlet (52' vs. 46'), but that was expected.