BMW 23001220371 Exchange 5 Speed (Overdrive) Gearbox

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The automatic transmission allows engine torque and power to be transmitted to the rear wheels within a narrow range of engine operating speeds. It uses a light fluid as the medium for the transmission of power and also works in the operation of various hydraulic control circuits and as a lubricant. The torque converter replaces the conventional clutch and has three functions: allowing the engine to idle with the vehicle at a standstill, allowing smooth shifting between ranges without closing the throttle, and multiplying engine torque as vehicle speed drops and throttle opening is increased. The torque converter consists of a metal case with three sets of blades: the torus or pump, the turbine, and the stator. Under low speed acceleration, the fluid flows through the torus and turbine, picking up speed and energy, and is redirected by the stator to allow the torus to turn faster than the turbine, resulting in higher engine power output and torque multiplication. A planetary gearbox is used to carry the power output of the turbine to the driveshaft, providing various reduction ratios. Servos and accumulators are used to engage the bands and clutches, with the servos being hydraulic pistons and cylinders and the accumulators cushioning their engagement. The hydraulic control system uses pressure from the main transmission oil pump, channeled through shift valves, to operate the servos. Governor pressure and modulator pressure, affected by vehicle speed and throttle position, respectively, control the operation of the shift valves and delay upshift points to optimize engine power output. An auxiliary circuit is used for downshifting, actuated by various means such as the throttle linkage or a solenoid. The transmission modulator also governs the line pressure to match the torque output of the engine for proper actuation of the clutches and bands.

Internal combustion engines produce torque within a specific speed range. Overhead valve pushrod engines typically achieve peak torque around 2500 rpm, often diminishing by 4500 rpm. Overhead camshaft engines generally have a higher, but narrower, torque peak. Manual transmissions and clutches adjust the relationship between engine RPM and wheel speed to ensure adequate power. The clutch gradually applies engine torque to the transmission input shaft due to mechanical slippage, allowing smooth starts from a stop. The transmission modifies the rotational speed ratio between the engine and the wheels using gears, with 4-speed or 5-speed versions being common. Lower gears ensure full engine power reaches the wheels during low-speed acceleration. The clutch driveplate, splined to the transmission input shaft, is coated on both sides with a material similar to brake lining that allows smooth slippage. The clutch cover, attached to the engine flywheel, uses a diaphragm spring to engage the clutch and houses the pressure plate. When the clutch pedal is let go, the driven disc is pressed between the pressure plate and flywheel, making the disc rotate at the engine crankshaft's speed. The transmission features a main shaft that extends from the clutch to the driveshaft, segmented to allow different rotational speeds for its front and rear. Power in lower gears and reverse is transmitted via a countershaft. This countershaft's gears mesh with the mainshaft gears, transferring power between them. Several mainshaft gears either rotate freely or lock to the shaft. Shifting gears releases one and engages another, facilitated by internal dog clutches sliding between the gear's center and the shaft. Forward gears typically use synchronizers to align gear and shaft speeds before the dog clutches engage.

Inspect the area surrounding the transmission and remove the starter motor.

The required tools include a support for the transmission, BMW Tool No.24 O 120 and 00 2 020 or equivalent, and a tool for tightening the driveshaft locking ring, BMW Tool No. 26 1 040 or equivalent.