It is usually possible to charge a system w/o the comp running and get enough charge in to make the LP switch shut and start cycling the comp. There are some ways to help this along. You can also bypass the LP switch to make the comp cycle on early for charging. But from what we know here I would definitely not recommend it.
Based on the r134a pressure/temp chart (see link below) if you have 28 psi with the comp not running at anything like summer ambient temps you system is seriously discharged, meaning a bad leak somewhere.
BTW, low side pressure, high side pressure... it's all the same when the comp isn't running. The whole system is at equilibrium and the pressure should be as indicated in the table linked below. But even if the pressure matches it doesn't mean your system is correctly charged. Like Octaneman said the surest way is to charge the exact recommended weight of r134a into an evacuated system.
http://dodgeram.org/tech/repair/HVAC/R134a.htm