quick question - I just filled my '89 up with freon (still R12) and though previously worked fine a month ago, my compressor now cycles on and off every few seconds. In fact, it only turns 'on' for 1/4 a second, before the clutch disengages for another 2 seconds, then it engages again?

I'm thinking I 'may' have overfilled, although the low pressure is exactly at 30.

thoughts?

 

What's the high pressure? The WSM diagram shows the pressure switch as a two-function device, cutting the compressor circuit when the High-side pressure is either too low or too high.

Also, is your 30 low side pressure measured with the compressor not running? If so, you are way undercharged. (same as "a lot has leaked out"...) The low side pressure should be about that when the system is charged, engine at 1500-2000 RPM, compressor is turning, and the blower is on 2. Your target pressure depends on ambient conditions, as does your high-side pressure.

I think if I go out and hang the gauges on the car right now, it woukd read about 75 lbs with nothing running or turned on. Stil the only way to read system conditioon with gauges is with the system running.

also--

Did you fix all the leaks, new hoses/o-rings/etc before you recharged? It's a bit unreasonable to put new freon in a system that previously leaked down, expecting it to have somehow healed itself and become gas tight on its own. This is one of the definitions of insanity, IIRC, doing the same thing over and over under identical conditions yet expecting a different result. Gotta ask, been insane this way myself in the past, etc.

 

Classic sign of undercharge. Along with the good Doctor's usual sound advice, for test purposes, you can jumper the compressor to +12V and observe the sight glass in the receiver for flow and/or bubbles.