My ac is fully charged and runs great when temperature outside is under 90 degrees but only blows hot when over that compressor won't engage when its hot out. What could this be?

undercharged--overcharged--compressor failure--blown fuse/relay--mode door vent--climate control failure--

Your compressor could be faulty. Check to see if it engages when you press the AC button with the car running.

The only way to know if a vehicle's air condition is properly charged is to use refrigerant gauges to determine the high side and low side pressure while the system is running. Without gauges, there are a few things you can check:

1. With ambient temperature over 85 degrees Fahrenheit, when you first start the engine for the day, the compressor should engage and stay engaged. If it cycles off and on for more than one minute, or if the compressor fails to engage at all, there is a likelihood that the refrigerant charge is low. (If ambient temperature is under 85 degrees Fahrenheit, a small amount of cycling is normal, but the compressor should remain engaged for a longer time than it is disengaged);
2. If the compressor is not engaging at all, momentarily place a wire jumper across the two pins of the refrigerant pressure switch connector with the engine running and the system on. The compressor should engage. If it does not, the system is not working properly. If it does, there is likely insufficient refrigerant in the system.

when last did you have your car fully serviced? small problames like yours can be solved in less than 2hours trust me you need someone to take a look at your car.

The question is: why isn't the compressor clutch turning

The compressor isn't likely seized,it may leak refrigerant,
but won't be what your asking.

The clutch can be jumped to test that

If okey, then you have to diagnose the entire
A/C Electrical System

How do you know it is properly charged,you put
refrigerant gauges on the system?

2000 Nissan Altima Heater Air Condition Section HA 76 pages Here is factory service repair manual for your vehicle . Do you know how to do testing on these system ? Automotive electrical system testing ? How to read a wiring diagram ? When hot outside is the inlet pipe to evaporator freezing up ? Do you know where this is , under the hood in the engine compartment at the fire wall . What are gauge readings , Is the compressor engaging ? Were any switches replaced ? Ambient temp. , thermal transmitter switch . pressure etc...

It all depends on the outside (ambient) air temperature. Your A/C low pressure switch is designed to turn the compressor off at 24 Psi. and turn it back on at 45 Psi. Most systems will normally run at between 25 and 40 Psi when the system is running on HIGH blower and "MAX" or "RECIRCULATE". A system that is fully charged may run at 25 Psi when the ambient temperature is 75 Deg. (F) and may run at 40 Psi when the ambient temperature is at 90 Deg. (F). (it can be even higher if ambient temperatures are higher than 90 Degrees.)

To properly diagnose a malfunctioning A/C system, the HIGH side pressure must also be checked. Restrictions in the system can make the low side pressure look normal while the high side pressure is high enough to blow hoses apart. (400+ Psi.)

The system could be low on refrigerant. The A/C pressure switch will not let the compressor run if the system pressure is below 28psi or above 455psi to protect the compressor from damage. At low pressures not enough oil (mixed in with the refrigerant) will circulate, and at high pressures, the compressor seals will likely blow. You can get a set of A/C manifold gauges for R-134a and check the system pressures yourself or get a shop to check it and possibly add some R-134a. If the system pressure checks good, then the A/C pressure switch, the compressor clutch, or some relays will need to be checked.

Hi - There may be 2 separate problems. On my 1991 Lincoln Town Car, there is a Thermal Blower Lockout sensor that disables the heater from blowing until the engine temperature reaches a specific temperature. This way if you have vent or floor settings and 90 degrees set and it is winter, you won't get blasted with cold air. When the vehicle warms up, the TBL circuit enables the climate control to blow the hot air. If you use the flr/def instead of the floor or vent control, the TBL should not prevent the heater from working even with a cold engine. On my car, the TBL went bad and always prevented hot air from blowing on the floor and vent settings, even when the engine was hot. Another thing to check is a thermal limitor resistor. It is designed to cut off the blower at very hot temperatures, except if the blower fan is in the hi setting.

As far as the A/C is concerned, can you verify if the clutch is engaging or not? A rapid cycling of the clutch is an indication of low freon. If you are absolutely sure that you are fully charged on freon, I think there are 2 possible causes: (1) Bad pressure switch on the accumulator is indicating low freon that is telling the compressor not to work; (2) a blocked orifice tube.

This problem is usually due to bad compressor. They adjust the temp of A/C by the outside temp.