Chevrolet Repair: 1995 camaro cooling fans, pressure transducer, head gaskets

Question
QUESTION: Wayne,
I am looking at a 1995 Camaro, 5.7, a/t, p/everything, with a cooling fan issue. While driving, the owner stated the temp gauge went way above normal, and fan finally came on when gauge near "red zone". (as per Haynes)I checked fans, both sat, checked all three relays, all sat. I checked sockets, 1 and 2 sat, 3 no power with key off (manual said test same for all 3 relays and sockets) I haven't tested temp sensor yet (got cold out)but the more I think about this, the more I think only the high temp fan came on. Does that sound right????? The owner took it to a chev dealeer, who told him he needed head gaskets, water pump, intake gaskets, etc., etc. I ran the car for 30 mins with no abnormals, other than the fan didn't come on as the temp gauge reached midway between the center and high mark.

Claude

ANSWER: Hey,

 Headgaskets?  There could be an intermittent leak that is aerating the cooling system and causing the pump to cavitate but I would think it wouldn't last a half hour of driving without overheating.
 That system uses two cooling fans, three relays, PCM, CTS and an evap probe and high pressure transducer.  It uses the three relays to series and parallel control the circuit for various modes of cooling.  You need to put a scan tool on it and watch the fan request data.  The primary fan comes on at around 108c-114c.  I can't quite remember the exact specification but that should be close.  If the gauge is running at three quarters and there is no fan running thats a problem.

Wayne


---------- FOLLOW-UP ----------

QUESTION: Wayne,
Thanks. I have been at work all day (fixin cars just a hobby at home)thinking about this and looking on web sites. My best educated guess was that one fan comes on at a set temp, and if the temp kept rising, fan 2 came on, or if the a/c was energized, it would also kick on fan 2. Would I be correct in that the CTS sends info (resistance value) to PCM which in turn connects ground for fan(s)? If true, I suspect the CTS is faulty.
Thanks again for your service, your assistance, your input and MAN, I wasn't expecting a reply that fast.

Answer
Hey,

 You are pretty close on the description of control other than...

 The first fan comes on under initial temp control from PCM.  The second speed is actually both fans run in series for a dual fan but half speed.  When the high side input is received from the air conditioning the third relay bypasses the series circuit and makes both fans turn full speed in a parallel circuit.  The pcm receives an input voltage from the voltage drop over the thermistor in the cts.  If you disconnect and jump the CTS the PCM will recieve a high input thinking the motor is running at max temp.  
 Be wary of a plugged cooling jacket in the passengers side head.  if the cts is submerged in mud it will not receive the proper temperature of the liquid.

 I can print and fax a really nice wiring diagram if you want.  Email me at waynehowie@rogers.com with the subjectline corvette cooling system schematic and I will send one in a reply.
Wayne