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Never have been a fan of high side pressures being beyond ,the tolerated limits of the compressor.I am not saying that you have ever done that.Each system has a MAX capacity for Freon and pressure. ( you could blow the reeds in the compressor if you do exceed the amount.An expensive repair would follow if the reeds blow,since little parts of the compressor go into the system,contaminating and plugging everything up.).Most manifold gauges usually give you a visual guidline of whats tolerable.In the 60's and 70's,some A/c units in Chevys and Fords were huge 2 ton BTU units.Big enough to cool your house in the summer.The high side for those would be higher.Everything is much smaller now in the name of efficiency.Each high side range tolerance is specified for each year a car is made(as is the exact amount of freon,average 2 pounds or less , for most cars)..What you need to do is phone or ask your Ford dealer what the range of pressure is for your specific year and how much the capacity is for Freon or R134a.
The designs of A/C units change often.Case in point,GM came out with a radical variable displacement compressor for much of it's line up in the mid 90's. after R134a was introduced.There was no range on the hi and low manifold readings.It was constant.If you didn't know that it was a variable displacement model,you'd think the A/C compressor was fried,because it just kept working.It never shut off.It was their way of making an A/C compressor that would help your car save gas,since it met the demands of the HVAC system and didn't cause a load on the cars Horsepower,turning on or off.It only used enough displacement (within the compressor)to meet the demand of the system.I am sorry I don't have your specific answer to your question,but as I said your Ford service manager will give you the range.It is a specific amount.
That being said ,take the temp with a thermometer in the vent with your A/C MAX,on and windows up.If it's about 45 degrees,with the motor at 2000rpm,(put a big fan in front of your car blowing in the direction of the condenser)...you can't do better than that.Also,you are aware that R134a is less efficient than R12.They went to R134a,because,it didn't have CFC's(which is bad for the ozone layer).I don't know if Ford still uses an Orfice tube to change the pressure before the Evaporator.A simple design change will have an affect on your readings.So I am sorry,but I suggest you phone and ask your dealer.BTW,besides not having enough Freon(R134a) in the system,another culprit of cooling loss is moisture in the system.That is why I like pulling a vacuum on the system for 30 minutes,once the Freon has been removed.It dries out the system and if the vacuum dosen't hold steady,then you know there is a leak somewhere.While you are pulling a vacuum,you can dry the freon ,by recycling it.The 3 R's(recover,recycle,reuse.)...Unfortunately you need some basic equipment for that.So I am not a fan of simply adding a small can of freon to the system periodically.Just a guess,but since your system is a stock Mustang A/C unit,150psi is more than likely in the range of normal high side pressure.Higher psi would seem very high,in my opinion.But,ask your dealer.
BTW,in humid weather the length of time between cycles should decrease but each cycle will be longer.(therefore the compressor is running more often).It shouldn't affect the manifold gauge pressures too much.
This is FORD HVAC 101...Yes,GM and Chrysler use simular systems,BUT,the types of specific components are different.BTW,I am satisfied with the cold air pumping out of my Roushcharged Mustang coupe.
A convertible vehicle,HVAC system must work harder,and will seem less "cool",since the cold air escapes faster,even with the top up.I could discuss this in more detail,but that is the next level of HVAC repair.We could discuss refrigerant oil,how much oil to add after an accident destroys your condensor,etc. WARNING! IF you get your HVAC system serviced at too many different establishments,you are asking for trouble.Try an get your service at one reputable place.They will know your system and it will prevent second guessing on the part of the technician,since they know your detailed HVAC service history.I know you would bring your dog and cat to the same Vet,since you get them better care since they know your animals history.It will live longer,hopefully.Same thing for HVAC systems.
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