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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