Part numbers are substantially different between the two gearboxes. A steering column-mounted lever engaged the overdrive when an 1800 was cruising in fourth gear. Both transmissions have an overdrive unit that requires fluid and filter checks. Replacement of the overdrive solenoid is sometimes needed. A rubber transmission mount located on the rear bottom of the transmission and attached to a frame crossmember is a candidate for replacement on high-mileage 1800s.
Separate parts are available for rebuilding purposes.
All automatics had a floor-mounted shift lever. Unlike the hydraulic assembly in P120, the 1800s have a mechanically operated clutch assembly. A cable and release fork may also be needed in addition to the usual pressure plate, disc and release bearing. As a general rule for all pre-1975 Volvos, when more power was obtained from an engine, the driveshaft and U joints were made stronger. The rear axle assembly contains the differential and axles.