If you mean changing from R-12 freon to R-134, you don't. R-134 uses a different kind of oil and if you put that oil into your present A/C system it will eat through the seals, destroying the pump and everything. Sorry. I really dont think that is the case. Retrofit kits are available to convert to r134 and new O rings are part of it. Compressor oil is compressor oil, just because the refrigerant is different does not mean the oil is. The new refrigerant actually has poorer cooling abilities than r12 but is better for the environment.
If it hasn't been converted to R-134 if will have to be
Would depend on ambient temperature and freon level in system
There is a leak somewhere in the air conditioning system. Mechanic shops have a special dye they can add to the system to find the leak.
It can be done for some models. Is there already AC on the '90 civic? If not it will require some effort but it is definitely not impossible.
No, they're different chemicals with different properties. A car with an air conditioning system designed for R-12 needs a retrofit in order to be able to use R-134a.
It is easier to build accessibility into the design rather than having to retrofit a solution. He tried to retrofit a fuel injection system into his old Mercedes. The retrofit using new lenses effectively repaired the Hubble Telescope.
You need to convert the AC system to R-134a type freon first (retrofit kit). Then you charge it on the low pressure side.
Because a complete retrofit is a very expensive process, and someone driving a car old enough, for example, to have an R-12 system may not necessarily wish to spend that kind of money on such an old vehicle. Additionally, most customers would come in under the impression that a "magic bullet" exists which would allow them to to cheaply retrofit the system, when in fact there is no such thing.
If it needs a charge you probably have a leak in the system. It would be in your best interest to have the professionals service your ac system.
To properly charge the air conditioner you need gauges and a refrigeraant recovery system.
Probably low on coolant. Re-charge the system then check again.
If the system has a full charge then the thermister is most likely the cause for it not working.