GM used part-time, gear drive, cast iron transfer cases for all 4×4 models from 1958 to 1972. The Dana Model 24 equipped the first factory production GM 4×4 trucks in 1958 and 1959. Rockwell supplied the 4-shaft, T221 unit from 1960 to 1969. The T221 was reasonably strong, but it was often difficult to seal against oil leaks. The T221 also had some problems with inadequate lubrication of the ball bearing at the back of the input shaft plus some other issues related to it’s inefficient, yet costly design. Early Blazer and Jimmy SUV’s built in 1969 and 1970 had the small Dana 20.
The Dana 20 was arguably the best transfer cases installed in 1 Ton and smaller trucks, first appeared in 1/2 and 3/4 ton pickups starting in late 1969. For 1973, GM introduced the heavy, cast iron, NP203 chain-drive, full-time transfer case and used it on most automatic transmission equipped trucks through the 1979 model year.
Despite it’s extreme weight and rugged front range section, the 3-section NP203 was overall a fairly weak unit due in part to it’s chain drive and inner-axle differential. Many fuel-sucking 203′s were converted to part time by a variety of conversion kits that modified the differential and disconnected the chain and front output shaft from the rear output shaft. Most conversion kits were poorly engineered and none solved the lubrication issues resulting from the conversion because the NP203 depended upon the chain to carry oil up to the upper shafts.
Very few converted 203′s lasted more than a year or two before suffering expensive lubrication related failures. Interestingly, the NP203 front range section is now in demand as a doubler box for providing double low range capabilities on custom offroad rigs. The rugged and reliable, part-time, NP205 with a fixed rear output shaft was also used from 1973-1979, primarily on vehicles with the SM465 4 speed manual transmission.
The NP205 was the sole transfer case choice for all 1980 GM 4×4′s and all 1981-1991 K/V 1 ton 4×4′s with the square body style. GM used an ill-starred, fully synchronized version of the NP205 on trucks with automatic hubs beginning in 1982. Most 1980 to 1991 GM NP205 Transfer Cases use a Slip Yoke type rear output. The 1991 V3500 was the last GM production model equipped with a NP205.
Starting in 1981 and used through 1988, GM full-size trucks of up to 3/4 ton rating came with the lightly built, synchronized, aluminum housing, NP208 chain drive, part-time transfer case. The 208 used a planetary gear set for low range, which has been used on all subsequent 2 speed transfer cases used by GM.
Manual Shift NP241 transfer cases debuted in 1988, while the electric shift NP243 version of the same case hit the market in 1991. GM has also used a plethora of other cases in various full size models from 1988 on including the Borg Warner 1370/1372 (1988-2000), Borg Warner 4401/4470 (1989-2000), Borg Warner 4481/4483 (intro 2003), New Venture 261 Manual Shift (1999-2007), New Venture 263 Electric Shift (1999-2007), New Process 243 Electric Shift (1994-2007), New Venture 246 (1998-2007), New Venture 4481/4482 Single Speed (intro 2003), and starting in 2007 cases from Magna Corporation (Magna now owns the New Process Plant in Syracuse, NY which produced New Process and New Venture Transfer Cases.). GM 4500 and 5500 Medium Duty trucks use a remote mount, electric shift, NP273.
GM introduced the mid-size S10/15 series 4×4′s in 1983 which initially used a NP207 chain drive case. The NP207 was used until 1988. Subsequent mid-size 4×4′s used mostly New Process/New Venture units including the 136 Single Speed plus the following 2 speed models: 231 Manual Shift, 233 Electric Shift, and 246. Borg Warner 4472 cases equip some 1995-1997 4×4′s.
Enough technical stuff. It is 98% accurate. You get the idea here, I have been a transmission repair person for over 30 years. It is obvious that I have worked on many of the above transfer cases or I would not have such a detailed memory of them.
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