Hello, Y'all! I am having all sorts of issues with my 1999 Ford Ranger XLT 4L V6. I've replaced a list of parts on the truck over the past two months. AC Compressor, thermostat, O2 Sensors, AC Vents, Vaccum Lines, Spark plugs, Heater Control valve, repaired a freon leak, etc. The list goes on and on. We would be here all day. I got the truck for cheap and knew I would have to put some work into it. First, it started out as doing a tune-up on it. Brakes, filters, oil change, etc. Then it progressed into wanting the AC to work in the truck. I've replaced everything you can think of that is related to the AC. The defrost vents work up on the dash. But the vents that point towards you in the middle will not open. Then I thought well maybe it could be the computer in the truck. Because the truck started not wanting to start and the speed odometer started not working as well as the hot and cold gauge. Checked the ECM and it has like over 50 different codes. All related to those parts I have replaced. There's no way. All parts were new when put on and done right. The computer has to be the issue. I got one off a totaled Ranger that matches my stamp #. The ECM was checked on another ranger and does work. No codes on it. So hopefully that fixes my problems. Cause at this point I am lost.

 

1999 Ranger with 4.0l V6 will have PATS(passive anti-theft system) so you can't swap "ECM"/PCM without reprogramming
And PCM/computer is not involved with most of the things you mentioned
It just does spark and fuel stuff
And PCMs/ECMs rarely fail

The dash vents are operated by vacuum from a Vacuum reservoir in the engine bay, Defrost is the Default, no vacuum, vent
There is a hard plastic black vacuum line that runs from vacuum reservoir located at the passenger side front very low down engine bay and runs along the top of passenger side frame rail to the firewall and then thru the firewall to the Climate control Vacuum switch
Black plastic line often breaks/melts near passenger side exhaust manifold

No temp gauge on dash is usually an unplugged red/white wire on the temp SENDER on the engine
Fuel injected engines have TWO engine coolant temp units
5 volts 2 wire ECT sensor, only for computer
12 volt 1 or 2 wire Temp Sender, only for dash temp gauge, always has red/white wire, if you unplug the sender and short the red/white wire to Ground and then turn on the key, the dash temp gauge should go up to HOT, this means wire and gauge are OK, replace sender if gauge wasn't working

AC system does use an internal relay in the PCM, but it rarely fails and its easy to by pass if it does
Far more likely issue for no AC Compressor start up is a pressure switch issue
There are 2 pressure switches on the AC hoses/lines, high pressure and low pressure
Locate Unplug each of these 2 wire switches, use an OHM meter or continuity tester on each switches 2 contacts, should show Connected, 0 ohms, continuity, if not replace that switch, these are "hot swappable", you will not lose any "freon"

These switches are the GROUND for the AC clutch relay(turns on compressor), one switch wire is grounded(black/white wire), that ground passes thru that switch to the next switch, and then thru that switch to the PCM, pin 86
The dash AC switch when turned on will send 12volts to PCM, pin 41, called AC demand signal, comes from fuse 10 in Cab Fuse Box
If PCM gets that 12v signal, it closes its AC relay which GROUNDS pin 69 that is connected to AC Clutch Relay and it closes which sends 12v to the Compressor clutch, this 12v is from AC fuse in engine bay fuse box

You have to test "new" parts now-a-days, they are often bad out of the box

Post some of the codes you are getting from PCM
These are the ones for engine systems
P01XX
P02XX
P03XX

P04XX are emissions can be addressed later
P05XX, sensor issues
P06XX module issues
P07XX automatic transmission issues

P1000 means battery was unhooked, it will go away on its own

P1XXX are Ford specific codes, can be engine related or not, so include these