Bob is correct.

The compressor runs until the evap temp sensor signals "too cold" and it then shuts down. If the system is marginal, or the ambient temperature is very warm, the evap temp may never reach the point of "too cold". This is quite common on R-134 systems. I'd bet a proper R-12 system would cycle normally.

If the cabin finally gets down to the desired temperature, the compressor must cycle, otherwise the evap will freeze.

Why do you believe the compressor does not cycle? It's not always readily apparent on a 617.