Possible PCM problem?

I just purchased a used saturn. After 30 or so min of driving and 5 or so minutes sitting idle the vehicle started to overheat. This is what I've troubleshot so far:

1. There is no leak in the cooling system.
2. The fan does not come on, ever, even when the AC is running.
3. The AC is fully charged.
4. The fuse for the fan is not burned out.
5. No problems with the thermostat.
6. The fan works when directly wired to the battery.

I brought it in to be scanned, thinking that maybe it was an engine coolant sensor, or some other sensor issue & it came back unable to scan.

A mechanic friend of mine said it sounds like the computer is out. Here is the problem with that. Everybody seems to call this part by a different name. I've heard Powertrain Control Module, Engine Control Module, Engine Control Unit, and Engine Control Computer.

I read in a previous post that the location of the PCM for my particular make and model is on the driver's side under the dash. I removed this part and brought it into an autopart's store for replacement. The clerk behind the counter said that I had pulled the wrong unit. He claimed the unit I brought in is the Transmission Control Unit & therefore I am not eligible for the refund on the core. The part I removed specifically says "Powertrain Control Module". He claims "powertrain" means transmission. He also stated that the parts look identical, so that is also why I'm mistaken. My understanding is that "powertrain" it means engine, transmission, driveshaft & everything else that powers the vehicle.

So my question is, simply, am I right or is the clerk? The fact that everyone calls the same part by 4 different names is confusing enough, and this is the first I have heard of the term Transmission Control Unit. Also, there was only one unit mounted, not 2 identical units as claimed by the clerk. So if the clerk is right & this is the TCM, then where would the PCM be?

Your help and advice in this matter is much appreciated.