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)