35-Year-Old RUUD Package Unit Still Going Strong

Well, that is a lot of questions - lets give this a shot.
1) normally of the control board there are places for 3 or 4 taps the motor gets power from. Pending on the motor will pend on which wire is on which post. 2 of the posts are marked spare, they go no where. 1 is marked AC the other heat. The highest speed should be on AC, while pending on your case - normally the next lower speed on heat. Way back in the old days, these motors were belt drive and only had one speed, but most direct drive motors have 3 or 4 speeds.
2) never seen motor wires faded inside of a furnace - some where you should be able to identify the color and the label on the motor should state the color to speed. If not, it is a trial and error. Leave the common on and switch wires to identify high and low speeds - should be easy to hear the difference.
3) There are all sorts of foam tapes foil tape to help seal ducts and so forth. The pipes leaving the duct we use thumb gum. A product that is used by electricians and you can find it in any big box store in the electrical section. It is gray and comes in bars.
4) Taking a speed reading of the motor is not possible. As soon as you open the door up to do this, more air enters the fan and loads the motor up even more. A squirrel cage fan will move as much air as you let it. Covers off and it will over load the motor. If you have too small of a return air duct, it will starve the blower and move less air. If you have a completely plugged air filter, the fan will free wheel - draw less amps but still stay the same speed. Normal high speed on that motor should be 1075 RPM. Age has no bearing here. If the bearings are badly warn then you will see a reduction in speed, but also see the motor get hot - very hot.
5) With out the model number, you will have no luck. Inside the the furnace, either top right or top left side on the wall is a label with that info (for furnace). On the AC, there is not much to know. You have a compressor and a fan that pulls air over a coil. The compressor should have info stamped on it, other than that, the contactors are all the same and the capacitor is special to those two motors. Do not assume are is an issue because of age, My AC is from 1980 and still runs. R22 was a good refrigerant. If needed, R438a or (MO99) is a drop in replacement, no oil change needed.
Assume this is a 80%'r, 90% of air flow issues is dirty blower or restricted duct work. On a 90% unit those same items, but could also be a dirty of plugged secondary exchanger. Had to wash out my son's secondary a year ago. The air filter was not kept clean.

Hope this helps - Good Luck

You will need two items, an amp probe and a manometer. Below is a normal diagram of a motor wiring. In most cases, the Black wire is high speed, the brown wire goes to the capacitor while the White or Brown White goes to the other side of the capacitor and then to neutral. Red is normally the 2nd highest speed followed by Orange and Blue.

A manometer measures the pressure or vacuum on the duct work. You can get them cheap on line. The range you are looking for is 0" to 5" water column ("WC). You will need a small hose or tube (1/4" OD) and drill a hole in the lower section of the blower housing. You can measure the vacuum there. A -.2" to a -.6" would be normal. pending on your system. The lower the number means restrictions to the furnace. The smaller the number means the blower is not moving enough air or duct work / filter is over sized.

The Motor - take an amp reading, see which wires are drawing power. Use tape to mark an ID on wires are you are doing this. Again, the Black wire should be high speed, Red the next lower speed. Switch the wires (hot side) to the next wire and look at amp draw. You should be able to determine which wires are which speeds.

Make suer you kill power to the furnace before working on any electrical wires. This should point you in a direction to figure out what you have. In 39 year, I have never run across a blower motor that there was not either a name plate, sticker plate or an engraving on the motor with info on it. There has been some burnt up tags, but most was still readable. You can pull the blower cage out and see better. Hard to say what that requires, in most, just two screws and it pulls right out. Good Luck - Let me know how it goes.

With special tools, we can figure that out. There are a few ways - one way is to check capacity. I would have to look up the calculations, but by knowing what the dry bulb (actual temperature) and wet bulb temps are- comparing the inlet (return air) to the outlet (supply air), one can calculate BTUs of cooling and then compare with the name plate on the condenser. Sounds harder than it is. I have two probes that do this for me. You would also have to assume your cfm for this calculation unless you have the meters to read that.

Another method requires a refrigeration gauge and accurate surface thermometer. Older systems use R22, but once you know the refrigerant used in the system, you can Google a PT chart.
Start with the discharge pressure of the unit and compare with the liquid line (smaller pipe) entering the house from outside. Lets say you have R22, and the pressure is 200 psi, the temp is 86 degrees. At 200 psi condensing temp is 101 degrees (see PT chart) and you have 86, that is 15 degrees sub-cooling which is a good number, 5 degrees on a 75 degree day would indicate low on charge or dirty condenser. 20+ degrees would indicate over charged or much colder outside.
Assume it was 15 degrees.
Next check the suction line - lets say the house is warm, still 75 degrees F outside. Suction is 66 psi, the larger line is 45 degrees. 66 psi is 38 degrees from 45 would be a 7 degrees super heat. A rather low number, but normal under the conditions we set. If that number gets much lower, we have an issue. If it gets above 20 we have an issue.
The lower number would also indicate not enough air flow or over charged, pending on your expansion device. Dead give away of low air flow from the blower.

Sounds complicated? Once you do it a few times, it is the one most important things AC guys miss when installing an AC unit. That number by the way is called super heat, how far above boiling we brought the gas from a liquid. If that number hits zero, we have flood back and possible damage to the compressor. Normal by the way is 10-20 degrees, again, pending on system it could vary.
As a refrigeration tech (HVAC/R), when I work on these smaller AC systems I find that 95% of the time, the installer did not do the above - assumed all was OK as the unit cooled. We find this true from a lack of training in the HVAC area. In refrigeration, we pound this into the guys head as it is the most #1 importance in any refrigeration system.

An inexpensive gauge and surface thermometer can be found on Amazon, an entire manifold is $43, but all you need is the gauges - just make sure they are not automotive. A K type thermometer is also about $40, but do not try an infrared as they are not accurate enough.

Sounds like a project for you - Good Luck