Lucius,
There realy is nothing you can do. Once the transmition (internal gears / turbine) are compromised with debris, your only option is to replace. Although, if it is not from debris (sand,pvc shavings, etc) you can turn the heads upside down and pour a mild dissolvent (bleach & water) in the inlet and let it drain out naturally. This will alow the dissolvent to slowly work its way down through the gears and clean any deposits in its path. You will probably have no luck with this, but, you could try it.
Josh
Irrigation /Landscape Lighting / Pump and Well Specialist