RW> RW>> So, you're just now getting around to re-charging the system, why?   
      
   RW> RW>> In today's cars, if there is no freon in the system, the compressor   
   RW> RW>> is locked out ot the system by the CPU and won't engage the clutch   
   RW> RW>> at all. This affects the cooling fans and when they come on, the   
   RW> RW>> engine runs hotter and that also affects emissions...   
      
   IT lowers the emmissions, In fact California requites a higher   
   engine temperture to LOWER the enissions   
      
   RW> TW> So you are tellng us   
   that IF the computer system sees an undercharges RW> TW> AC system the   
   radiator fan is NO longer controled by the engine block RW> TW>   
   temperature sensor??   
      
   RW>That is my experience, yes.   
      
   RW> TW> I do not believe that.   
      
   RW>Believe what you will. When the compressor on my Z28 leaked all of the   
   RW>freon and the compressor oil, the engine over heated because the radiator   
   RW>fans were not engaging. Checking the engine coolant tank, it was full. The   
   RW>engine cooled down after the vehicle began to move, but when in traffic,   
   RW>the fans didn't engage when the temp gauge got to where they usually do   
   RW>engage. Refilling the AC system, the fans began to work as usual.   
      
   RW>Dunno why they build them that way, but the above is fact and not fiction.   
      
   RW>Looking at the schematic, today's AC systems are tied to the PCM through   
   RW>sensors on the AC refrigerant pressure sensor and evaporator tempurature   
   RW>sensor. The PCM also controls the AC compressor relay (clutch) and an AC   
   RW>malfunction indicator on the dash. Tracing beyond the PCM is impossible,   
   RW>unless you have a schematic of the PCM's inner circuits. Not even factory   
   RW>manuals have that feature. Aftermarket manuals won't even go into the AC   
   RW>system, as the system can be dangerous for the ordinary mechanic to work   
   RW>on. They refer you to a certified AC technician.   
      
   RW>If you have a better explanation, I'm all ears.   
      
   Well the Engine Block switch and the AC fan control are two seperate   
   circuits and should not interfere with each other.   
      
   NOT being a Chevrolet Fan I got this from www.camaroz28.com on coolant   
   fan not runing if AC looses charge   
      
   ****   
      
   There are 7 things to consider:   
   (the first 4 are ruled out because you said that the fan worked when the   
   A/C was working.) Item 5; either the coolant temperature isn't reaching   
   238øF, or faulty wiring to the relay. Item 6; ECM fan driver may be bad.   
   unforunately the ECM will need replacing. Using an adjustable temp   
   switch (for winter/summer) would save you the trouble of buying a new   
   ECM, but you'll lose the control the ECM has for different conditions.   
      
       Power   
       Fan Fuse   
       Fan Motor   
       Fan Relay   
       Coolant Fan Switch (on head) grounds the fan relay when temperature   
       exceeds 238øF.   
       ECM grounds the fan relay at programmed temperatures.   
       There are four Fan settings in the ECM constants Table to turn the   
       fan off and on.   
       Faulty wiring   
      
   GM programs a high operating temperature for lower emissions.   
   Of the 9 binary files from ftp.diy-efi.org/pub/gmecm/bin_lib/1227165/6E   
   folder 6 had the following settings: (I don't know if they're unmodified   
   files)   
      
       Turns Fan ON (A/C ON) 108øC (226.4øF) to keep the fan on when the   
       A/C pressure is below 233psi.   
       Turns Fan OFF (A/C OFF) 107øC (224.6øF)   
       Turns Fan OFF (A/C ON) 105øC (221øF)   
       Turns Fan ON (A/C OFF) 108øC (226.4øF)   
      
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