I am replacing my old Honeywell thermostat but am not sure of the correct part number of the old unit. The following numbers are printed on the old unit - T8085D1008 or 890121The replacement is a Honeywell, part# RTH7400D1008 Programmable Thermostat and I'm badly in need of some guidance regarding the wiring. My A/C system is an apartment combination heating and cooling unit.

Thanks!!

Old thermostat wiring.

http://customer.honeywell.com/techlit/pdf/69-0000s/69-0258.pdf

New thermostat wiring

http://customer.honeywell.com/techlit/pdf/69-0000s/69-1726.pdf

It is all here for you from these two docs.