Steve,
The fan on the condenser is triggerd by a pressure sensor. It has a cut in limit and a cut out limit for pressure. Sometimes the value is the same. If You can get the PN from Monaco or Beaver Coach Sales, you may be able to find out the pressure switch limits. If you use a set of refrigerant gauges attached to the low and high side, you will be able to see the high side pressure that are the cut in and cut out pressures. As the fan cuts in, you will see the high side pressure fall until the fan stops and then it will climb again. You can record those values for future reference, and if you can get some pressure specs for the sensor, you can compare those pressure readings to spec. A guess from my experience is the cut in will be around 240lbs on the high side.
When you shut down the engine and therefor the compressor, the high side pressure is bled off slowly through either an expansion valve or an orfice. Eventually, the high and low pressure sides equalize and it may take a few minutes for the high side pressure to drop below the condensor fan cut out limit. If it is longer than you remember, then it could be the temperature is higher causing higher pressure, the air flow with the fans has been reduced due to dirt in the condenser fins, bent fins, slower spinning fan causing less air flow, overcharge of the R134A in the system, or a partially blocked valve or orfice causing slower equalization. You may also try and "map" the condenser face with an IR gun while things are running, to see if all of the tubes have flow and are cooling properly with the fan on and off. If the pressures are appropriate, the sensor is working as it should, and the cooling is adequate, then the fact that there is a lag to fan shut down, would have me just keep an eye on it and see if it remains stable as it operates now. If you had a friend with the same coach and could perform these measurements on both it would help sort out the differences, like how long it takes to drop 100lbs of pressure from the high side once the coach is shut down. That may tell you if the orfice or valve is partially blocked....Hope this helps.
Mark,
The pressure that the system reaches is a function of outside temperature, air flow over the condenser (driving or stopped), the gas charge that the system has, and the health of the compressor. You also need to find out with a set of gauges where the pressure is running and if the fan is cutting in at the right pressure. Because the condenser fan pressure switch needs to be accurate and repeatable, so as not to damage the system by allowing too high a pressure, I would see about getting some measurements done. There are times when the outside ambient is low enough that the fan does not always turn on but at 70-80 degrees or more while stopped I would expect it to come on and cycle or stay on while you are stopped. You can have adequate cooling but still not have the R134A charge level sufficient enough to reach pressure switch cut in limits for the condenser fans meaning the system was designed for a higher charge and there will be some cooling improvement.
Most AC shops can sort these things out pretty quickly if you do not own gauges or have a friend with some. I'm an advocate of taking down readings when things are working well to characterize the system and know what has changed when things are no longer going according to plan....
Later Ed
