The cast aluminum modulator valve sleeve in the GM TH180 and TH180C (3L30) cases are known to wear due to continuous stroking of the steel valve. This valve regulates line pressure into modulator pressure and is also controlled by governor pressure. Excess sleeve wear can result in leakage of line, modulator and governor pressures, leading to a multitude of shift complaints and poor line rise. Sonnax modulator sleeve 41820-08 corrects these issues by restoring hydraulic clearances.
Visual wear is detected on the inner and outer diameters of the sleeve.