11/17/09 - This section currently being re-written and updated with additional images and information! Check back soon!
The transfer case is most fundamental part of the BMW all-wheel drive system. It splits the power from the engine between the front and rear axles. The transfer case is an enclosed (”non-serviceable”) unit attached directly to the transmission output. Driveshafts from the transfer case apply the power to the front and rear differentials.
The planetary-geared transfer case used in all BMW E46 3-Series applications is the model “NV 124″ manufactured by New Venture Gear, INC (the X5 SAV uses the chain driven model NV125). New Venture, once a subsidiary of Chrysler Motor, was purchased by Magna Powertrain in September 2004.
The NV 124 is a full-time, one-speed, four-wheel-drive transfer case. Unlike the chain driven transfer case in the BMW X-series vehicles, the 3-series cases are fully gear driven units. According to BMW, “The reason gears are used is to produce a compact low profile transfer case that could fit in the transmission tunnel of an E46 without excessively limiting the forward travel of the drivers seat.” The gearing provides a permanent 38%F / 62%R torque split (discussed below) between the front and rear drive shafts.
|Model Number||NV 124 (New Venture Gears)|
|Max Torque||3482 ft-lbs (4721 Nm)*|
|Oil Fill Capacity||0.25 qts (see maintenance)|
*BMW has documented the maximum torque to be 221ft-lbs (300Nm). This figure seems extremely conservative based on the advertised NV124 spec.
The following is the basic operation of the transfer case.
“The input to the Planetary Carrier is from the output shaft of the transmission.”
“The Sun Gear of the Planetary assembly is turned by the Planetary Gears, the Sungear then provides torque to the Transfer Gear.”
“The Transfer Gear drives the Output via gear-to-gear contact. The front axle is driven via a flange connected to the output gear.”
“From the Ring Gear , the power is transmitted via the driveshaft to the rear axle differential.”
“The transmission ratio of the planetary gear set provides a fixed torque transfer of 38:62 (front:Rear). The output speeds to the front and rear axle are the same (1:1).”
The transfer box is directly mounted to the Getrag transmission with the transmission output shaft directly driving the transfer case input. The transfer case can be removed by itself, as if often suggested for any drivetrain work (i.e. Bentley Guide). Doing so, however, is not always necessary as it is fully possible to remove the transmission from the vehicle with the transfer case still attached.
The transfer case has two outputs, one each for the front and rear driveshafts. The driveshafts are mated to the case using simple flange and bolts.
Gearing and Torque Split
The transfer case performs much like a common rear (or front) axle differential.
The transfer case, as also goes for the transmission, is filled with “lifetime” fluid and, according to BMW, does not need to be replaced. The logical question is where there is a drain and fill plug on the box, if it doesn’t need to be changed. The BMW training guide specifies:
“The transfer case is filled with a lifetime fill of MTF-LT1 P/N 83-22-9-408-942. The only repairs possible are the replacement of the three oil seals and the hydraulic mount.”
- Oil Volume: 0.25qt (0.24 liter)
- Drain/Fill plug torque: 25 ft-lbs (33Nm)
Fill plugs are shown in the following figure:
As for internal maintenance, the NV 124 is enclosed and there is no opportunity for maintenance or rebuild by the common DIYer. If you are reading this and have attempted such a process, please contact me. I’m sure any information would be useful to someone.
Unlike many claims and rumors, the transfer case does not implement any use of viscous coupling or variable torque control, either passive or active. The torque ratio is 38%-Front / 62%-Rear.
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