As a operating engineer I often run in to some strange problems and I am always amazed as to what the culprit is and last week I encountered another strange problem. After several contractors were called to diagnose a 50 ton York RTU, I was called in to assess the problem, the compressor is a semi-hemetric Copeland 4DK3-2500-FSD and the problem was the compressor was cooling fine and all pressure readings and temperatures were with in factory specs., and this may last for a few hours, days or just a few minutes and then the cooling capacity would drop and the suction pressure would rise and the head pressure would drop and even the oil pressure would drop, now this had been an on going problem for months now and every contractor came to the same conclusion a bad compressor and I myself concurred with this initial assessment as it seemed that the compressor was going bad. I took all the information about the original complaint and all the readings of the contractors and took my own readings, now at this point I decided to close off the isolation valve and see if the compressor would pump down and it did pump down until it reached the 25lb low pressure cut out switch. Armed with this new information I was not ready to condemn a compressor that proved to be working, so now Im stunned so I did what I always do when a mechanical failure has the upper hand, I have a Pepsi and a good cry (LOL). I thought about it and decided that further investigation was needed so I traced the system operation and the schematics and happened to find that during the operation sequence the unloaders are timed to unload at a predetermined time as this was part of the program package. I went to the electrical connector controlling the unloader and the voltage was present at the times the compressor should be unloading and the unloader was not working. Now I ask this question has anyone else ever had this problem? I have replaced many bad unloaders but never before have I had a bad unloader give these symptoms or raise the suction pressure and lower the head pressure. This is a first for me and it goes in my journal of strange and unexplained events. It's one of those things that make you go "HMMMMM"