When should the evap fan stop?

[BryanH]

I'm guessing that I could find this info in one of my service manuals or on the boards but I'm getting lazy and I think this is faster....  

I picked up a parts machine over the weekend.  Cooling system cools but I think it's running continously (I haven't had the time to sit and watch it and it's 112F these days so there won't be many breaks).  If I turn the thermostat to OFF the compressor and condensor fan stop but the evap fan keeps running.  I randomly checked my VMC77 (which is also running outside in the heat), and even when it's compressor was off the evap fan in it was running.  This surprised me as I had assumed that the evap and condensor fans both worked off the t-stat so that they were either both on or off?

So my questions are:

1. When should the evap fan turn off?  
2. What triggers it to go on or off?  
3. Is it normal for the evap fan to be on when the compressor & condensor fan are off?

[BrianB]

#1 - As long as there is voltage to the machine, the evaporator fan should be running.

#2 - The evaporator fan should never be off.

#3 - When the thermostat detects the temperature is too low, the compressor and condensor fan kick on at the same time and shut off together once the setpoint temperature has been reached, the evaporator fan continues to run.

If you plug in a "new" machine and it keeps running and running, cooling the entire time, you more than likely need a new thermostat.

Hope this helps.

[BrianB]

In fact, I just encountered this prob of a constantly running compressor and the thermostat was the issue.

[BryanH]

Thanks Brian - I figured that the t-stat would be the issue if the compressor was running constantly... But the fact that the turning the t-stat to off shutdown the compressor and condensor but not the evap really threw me off...  Now that I know it's supposed to run as long as the power is on, I can focus on whether or not the t-stat needs replacing.