I have two ports the first is on a line which is at the very top of the motor. the second is on a silver colored cylinder (condenser) at the back of the motor (firewall) which port do I use to add freon. It is a 1997 Ford F150 XLT 4.6 2WD

Normally your high pressure port will be next to the silver cylinder called the condenser.

trace the 2 aluminum lines coming out of the firewall. The one that goes to the compressor has the low pressure port you are looking for. It might be on the dryer (silver cylinder).

When you push the up/down button for your top, you should hear the motor running in the trunk (car should not be running to hear the motor).If you do not hear the motor running, remove the cover by removing the four phillips head screws. You should now have access to the motor and pump assembly. There is an electrical connection the pump motor, make sure it is still connected. You can also check the reservoir and see if you are low on fluid. If you are, then purchase the fluid from your local dealer or I have also used DOT 3 Power Steering fluid as a work around. You have to remove the brass bolt on top of the pump housing (10mm) and slowly pour the fluid in. I would recommend keeping a towel or rag handy as the fill port will back up quickly. Just keep it slow and you should be okay. If fluid level is low, then you probably also have a leak somewhere, either in the lines or at the cylinder connections. To see those, remove the interior rear side panels (two screws and two plastic clips that will pull out) and inspect the floor area under the cylinders. If you find fluid on the floor then inspect the cylinder on that side and find the leak. If you are lucky, it will be a loose hose connection which you can tighten up (15mm open end wrech), if you are not lucky it will be a hose or cylinder. Hoses at my dealership are $390 each, not sure what the cylinder would cost. Good luck

Low pressure service port is located on the Accumulator ( Silver Cylinder ) where the low side A/C line domes from the compressor. Location is on the passenger side fender by your evaporator housing and blower motor housing.
Thank you for using fixya and don't over charge your system.

Low pressure line comes from the firewall to the back of the compressor.( some models from back of fire wall to accumulator to compressor)

Located on the accumulator, passenger side, large silver cylinder just in front of the firewall. The port is on the side near the top of the cylinder..

Newer vehicle come with R134 vs R12. Since the two are not compatible, R134 has larger fittings. AC system has two ports, High and Low. Low is return to AC Compressor and high is output. High port is first following line from compressor to condenser (condenser is at front of vehicle like a second radiator). Keep following line till you find high port (paint cap red). Second port is low (paint cap blue).

Basic AC system consists of Compressor pump (on engine), Condenser (front grill with radiator) and Evaporator (Firewall). Pump has two ports output (high pressure) and return (low pressure). Follow lines from pump. High side is usually between pump and condenser. Follow lines and find second port this will be low side (refill) port. Paint or replace port caps (red high, blue low). Also, refrigerant in system determines port size (R12 or R34).

Typically, when when this happens, the system is low on fluid and/or must be bled. Here are the procedures:
Lower the top to the full down position.

1. Remove the rear seat cushion, seat back and inner quarter trim panels. Pull up sharply on the seat cushion at the front about midway between the middle of the seat and the outside edges. The seat back is held in by nuts. The quarter trim panels are held in by screws I think.
2. Remove the cylinder mounting brackets and remove the cylinders from the trunnion pivot mounts.
3. Rotate the cylinders 180 degrees (the hose fittings will now face to front of the vehicle.
4. Disconnect the motor/pump retaining clips from the body.
5. Rotate and raise the motor/pump assembly so the reservoir is in the up position. The rubber filler plug will be at the top.
6. Raise motor/pump fitting to at least 2 inches above the horizontal rear seat back reinforcement.
7. Tilt the motor/pump assembly to approximately the 10 o'clock position, with the rubber filler plug still at the top CAUTION: THE MOTOR/PUMP ASSEMBLY MUST REMAIN IN THE CURRENT POSITION UNTIL THE RUBBER FILLER PLUG IS REINSTALLED IN THE Step 12.
   
8. Remove the rubber fill plug from reservoir.
9. Fill the hydraulic system by adding Dexron 2 Automatic Transmission Fluid to the bottom of the fill hole CAUTION: DO NOT OPERATE THE TOP SWITCH IN THE UP (CLOSING THE TOP) DIRECTION.
   
10. With two people, activate the top switch in the down (opening the top) position to fully retract the cylinder rods and hold for approximately 5 seconds, until the motor/pump stalls. A percolating or gurgle sound/feeling is an indication that the procedure is working.
11. Top off the motor/pump with Dexron 2 and insert the filler plug.
12. Install the motor/pump assembly into the vehicle mounting bracket.
13. Rotate the cylinders back to the original position (hose fitting rearward)
14. Install the motor/pump lead and hydraulic line clips (the hydraulic line clips are to assist in the production process) to ensure that the seat back/seat cushion do not pinch the lines. If the sticky side is no longer holding to the floor pan, the clips can be left off.
15. Ensure that the bushing is still on the trunnion pivot mount and bracket. Tap the top switch in (raising the top) to get the cylinder trunnion onto the trunnion pivot mounts, if needed.
16. Install cylinder mounting brackets.
17. Raise and lower the top to ensure proper operation.
18. Install the inner quarter panels, and the rear seat back and cushion.
19. And Just a fair warning here - you WILL spill fluid inside your car. If you manage not to, I want to know how. I cover everything with garbage bags because there have been a few instances where I accidentally hit the button the wrong way and that stuff went EVERYWHERE. Be careful and good luck.

There is no gearbox. A motor runs a pump which moves the cylinders. You are probably low on fluid.
Behind the rear seat is your motor/pump/reservoir combination.
With the top down, fill the reservoir to overflowing with ATF.
Cycle the system a few times and repeat.
Do this till the reservoir will take no more fluid.
Check the lines and cylinders for leaks. The fluid must be going somewhere.
If you replace any lines or cylinders, always do them in pairs.
Karl at Topgunwon.com