I have an old 25yrs + ac unit ...the blower motor seems to work properly when the thermostat kicks in, but often, the AC compressor outside does not. I have replaced the thermostat 2x with brand new ones, and it seems to work for awhile, and then once temperature has been reached, although the blower motor will kick in again, often the AC unit does not. It will kick in if I jumper the RY wires however, and run cold until I un-jumper them.
Any idea what might cause this intermittent problem? Is there some other solenoid or switch or something in line that is preventing it from starting by the thermostat alone (without jumping the wires)?

(quote) if I jumper the RY wires,and run cold until I un-jumper them

explain what are you jumper this ry to/with?
a lot of ,,mos thermostats require a jumper from RY to RW,,as per the instructions

what brand and model number of tstat are you using?

Thanks. Maybe jumper isnt the right word. what I mean is I take the R and Y wires off the tstat terminals and twist them together.

Using a Globe Energysaver 59010 tstat

Should also mention, only 4 wires...R, W, G, Bl (Y)

I can`t seem to find anything on Globe Energysaver 59010 tstat
by tieing the R and Y wires together you are bypassig the tstat,,,when you do this does the indoor airhandler fan run?
next time with the tstat wired like it should be and the inside blower kicks on when calling for ac,,go outside and see if the contacter in the ac unit has kicked in (the contactor is behind the cover and the main hot wires + the smaller tstat wires are connected to the contacter)from the furnace

most t stats have a 5 minute time delay in them if they are energy savers. this means once it calls for cooling it may take up to five minutes for the ac to kick on. bypassing the tstat will make them come on immediatly.

also you could have a low voltage problem somewhere between t stat, furnace, and ac. might want to have it looked at.