climate control doesn't cool anymore

I have a 91 PA and the system was charged with R12 2 years ago. Until a couple of weeks ago, it worked perfect. Today it didn't cool. 'auto' lights up but the clutch doesn't engage. I checked for errors but it only gives a 38 error (it has always done this even though everything worked just fine). I unhooked the negative terminal to reset the system but this also didn't work.
Of course my first thought was the system has sprung a leak, but shouldn't it give a 'low freon' or 'long term freon loss' error code when there is a leak?
What should I do next?