The M46 transmission has a separate Laycock de Normanville overdrive unit at the rear. Its engagement is based on hydraulic pressure maintained by small pistons. When the O-rings on these pistons harden and begin to leak over time, the pressure is insufficient to keep the overdrive engaged.

Overdrive only engages at high speeds (when oil pressure rises sufficiently). The tachometer "wanders" or needle jumps when overdrive is on. Overdrive disengages when you ease off the throttle or when transmission oil warms up properly.

O-ring sizes can vary slightly depending on model year (J or P type overdrive), but the most common size is: Size: 32/24 * 3,2 mm (or equivalent imperial size). From hydraulics supplier: E.g., Ramiko Oy / Aseko model MT54032B. Recommendation: Take the old piston with you when obtaining seals, so you get the correct profile and tightness.

Preparation and Disassembly Lift car on stands or drive over pit. Drain transmission oil. Important (700/900 series): Remove distributor cap from rear, so it doesn't hit the fan shield when lowering transmission. Support transmission and disconnect one end of transmission support bar. Lower box slightly to gain access to overdrive nuts. Mark cartridge and flange positions with marker (for balancing purposes). Remove cartridge from transmission rear flange and swing it aside. Disconnect solenoid electrical wire. 2. Overdrive Unit Removal Open overdrive mounting nuts (8 units, 11 mm wrench). Pull overdrive unit carefully backward until it detaches from main transmission. Warning: Ensure output shaft doesn't fall on the floor during removal. 3. O-Ring Replacement Open flanges on overdrive side (or top, depending on model), where pressure pistons are located. Pull out pistons. Remove old, hardened O-rings. Clean cylinders and pistons thoroughly with brake cleaner. Install new O-rings. Lubricate them with clean transmission oil before reinsertion, so they don't cut or get damaged. 4. Assembly Push pistons back and secure flanges. Reinstall overdrive unit at transmission rear using new gasket (or gasket compound). Install cartridge according to marks, raise transmission support bar, and reinstall distributor cap. Fill oil: Use quality Type F/G (or newer Dexron) ATF oil or manufacturer-recommended manual transmission oil.

If O-ring replacement doesn't help, check: Solenoid: Does it click when you turn on power and press the button? If not, the fault is electrical or the solenoid is stuck. Oil strainer: At the bottom of the overdrive (under a large brass plug cap) is a fine filter. Clean it during every oil change, as debris in the strainer prevents pressure buildup.