TOM WALKER wrote to ROY WITT:   
      
    RW> RW>>> So, you're just now getting around to re-charging the system,   
    RW> RW>>> why?   
      
    RW> RW>>> In today's cars, if there is no freon in the system, the   
    RW> RW>>> compressor is locked out ot the system by the CPU and won't   
    RW> RW>>> engage the clutch at all. This affects the cooling fans and   
    RW> RW>>> when they come on, the engine runs hotter and that also affects   
    RW> RW>>> emissions...   
      
    TW> IT lowers the emmissions, In fact California requites a higher   
    TW> engine temperture to LOWER the enissions   
      
   Nobody cares what CA requires, except law abiding Californians. The rest   
   of us use lower temperature thermostats in our cars. I used a 180 in my   
   Z28 when I lived in CA. Some people I knew at the time also had altered   
   their exhaust to eliminate oxygen sensors in the exhaust, in an attempt to   
   increase fuel/air ratios. Some even removed the cats...   
      
    RW> TW>> So you are tellng us   
      
    TW> that IF the computer system sees an undercharges RW> TW> AC system   
    TW> the radiator fan is NO longer controled by the engine block RW> TW>   
    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   
    RW>> the freon and the compressor oil, the engine over heated because the   
    RW>> radiator fans were not engaging. Checking the engine coolant tank,   
    RW>> it was full. The engine cooled down after the vehicle began to move,   
    RW>> but when in traffic, the fans didn't engage when the temp gauge got   
    RW>> to where they usually do engage. Refilling the AC system, the fans   
    RW>> began to work as usual.   
      
    RW>> Dunno why they build them that way, but the above is fact and not   
    RW>> fiction.   
      
    RW>> Looking at the schematic, today's AC systems are tied to the PCM   
    RW>> through sensors on the AC refrigerant pressure sensor and evaporator   
    RW>> tempurature sensor. The PCM also controls the AC compressor relay   
    RW>> (clutch) and an AC malfunction indicator on the dash. Tracing beyond   
    RW>> the PCM is impossible, unless you have a schematic of the PCM's   
    RW>> inner circuits. Not even factory manuals have that feature.   
    RW>> Aftermarket manuals won't even go into the AC system, as the system   
    RW>> can be dangerous for the ordinary mechanic to work on. They refer   
    RW>> you to a certified AC technician.   
      
    RW>> If you have a better explanation, I'm all ears.   
      
    TW> Well the Engine Block switch and the AC fan control are two seperate   
    TW> circuits and should not interfere with each other.   
      
   After consulting the factory Service Manual, I'm going to stick by my   
   statement.   
      
   /quote   
   Engine Electric Coolant Fans   
     Coolant fan operation is crucial to the proper operation of the air   
   conditioning system. The coolant fans ensure the proper amount of air flow   
   acrosss the condenser throughout the vehicle operating range.   
      
     Fan operation should be checked during any air conditioning diagnosis   
   procedure. Special attention should be given to the fan wheneve excessive   
   high side pressures are encountered.   
   /unquote   
      
    TW> NOT being a Chevrolet Fan I got this from www.camaroz28.com on   
    TW> coolant fan not runing if AC looses charge   
      
    TW> ****   
      
    TW> There are 7 things to consider:   
    TW> (the first 4 are ruled out because you said that the fan worked when   
    TW> the A/C was working.) Item 5; either the coolant temperature isn't   
    TW> reaching 238øF, or faulty wiring to the relay. Item 6; ECM fan driver   
    TW> may be bad. unforunately the ECM will need replacing. Using an   
    TW> adjustable temp switch (for winter/summer) would save you the trouble   
    TW> of buying a new ECM, but you'll lose the control the ECM has for   
    TW> different conditions.   
      
   Neither of these are what is happening. The coolant temp doesn't ever go   
   to 238F in any case. Even if you have a factory 195F thermostat, it won't   
   go that high. And, since the system works as it is supposed to work when   
   it is charged, I don't believe for a minute that there is anything wrong   
   with the system (other than a system leak).   
      
   You cannot perform a fan operation check without certain diagnostic tools,   
   as specified by GM.   
      
   The first order of business when checking the AC system is to do an engine   
   off check of AC system pressures. In this instance, the first check leads   
   to "Both pressures (high and low side) below 10psi - which calls for ADD   
   1lb of R134a, leak check system, repair leak. After performing this step,   
   if you have pressures in both sides of the system of 50psi, the system is   
   OK for the next diagnostic check.   
      
   Which is a system check with the engine running and the AC system on.   
   Since the system works as it is supposed to when the system is charged, al   
   that remains to be done is to find and repair the leak.   
      
      
            R\%/itt   
      
      
   --- GoldED+/W32 1.1.5-31012   
    * Origin: Texas Lone-Star - Texan, American, USAian  (1:387/22)