Featured image by Dana60Cummins, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Working on off-road 4×4 vehicles can provide a Zen experience, but imagine the frustration when you work on an older 4×4 truck with a completely familiar mechanical setup, then switch to a newer truck and find a huge change in the electronics and everything else. What happens if you switch back and forth between working on a transfer case from the 1970s and a truck with one from the 1990s or later? If you’re totally new to mechanics and just learning, read on for a short primer on transfer cases.
What Is a Transfer Case
The term transfer case refers to the gear housing in which the gears distribute driving power between vehicle axles, https://reman-transmission.com/transfer-cases. You’ll only find a transfer case on a vehicle featuring more than one driving axle. These vehicles typically include a shifting lever so the driver can disengage the front-wheel drive.
The transfer case comprises one component of a vehicle’s four-wheel-drive system. It houses the gears and the drive chain, working with your transmission to deliver power to each wheel as needed. Check the transfer case when you notice issues with your four-wheel drive operation. If it malfunctions, the four-wheel-drive does, too. Signs that your vehicle has a transfer case issue include:
- Hard to drive in four-wheel-drive mode
- Runs rough
- Grinding noises during gear shifts
- The truck balks when switching to four-wheel drive
To put it poetically, your 4×4 needs its transfer case like flowers need rain.
Classic Transfer Cases
Like all other parts of a truck, transfer case design has evolved. If you’re new to truck repair, you might not yet have developed the vehement loyalty to either gears or chains that many mechanics have. In the early days of 4x4s, all transfer cases featured gears only. These pre-1970 created a racket when driven. You’d hear your buddy driving from a block or so away, no exaggeration at all. Gear-only vehicles made noise. You felt every bump in the road when driving one, but they functioned durably. These are typically hidden inside a cast-iron case.
Example: The NP205 appeared in most medium-duty vehicles pre-1970s. In anything up to a one-ton truck, you’d find this all gear four-wheel-drive system.
1970s Changes in the Transfer Case
In the early 1970s, the chain drive entered the picture in an effort to create a quieter and smoother ride. While these cases achieved the goal of a nicer ride, the early chains tended to stretch, then break. The case itself underwent a transformation, too, going from iron to aluminum in composition. Although lighter, the dainty cases didn’t handle broken heavy parts well.
Example: The NP231 appeared in three makes of trucks and SUVs – Dodge, GM, and Jeep.
Modern Transfer Cases
Modern transfer cases refer to those that came into use during the 1990s and persist today. Today, they’re crafted of aluminum and require the vehicle buyer to add a skid plate if they intend to do any rough and ready driving. They went through morphing, too, though.
1990s Changes in the Transfer Case
Forget the beast mode of cast-iron. Most automakers are stuck with aluminum. General Motors (GM) and Chevrolet (Chevy) played with the notion of a magnesium transfer case. Think of this expensive choice as the luxury model of a T-case. It cost so much that it only made it onto a few models of vehicle – the GMC Sierra 1500 HD, Chevrolet Avalanche 1500, and the Chevrolet Silverado during the period from 1999 to 2002.
Example: The NV246 used a magnesium case, providing the toughness of an iron case, but the lightweight design of aluminum.
A Quick Guide to the Variety in Transfer Cases
Just as a multitude of makes and models of vehicles exist, so do transfer cases. This has more to do with the overall evolution of automotive design than it does with auto manufacturer competition. The design options include:
Part-Time Transfer Case
Mostly used in trucks, the part-time transfer case, or T-case, lets you choose between sending power to both wheels or just the rear set. This design uses fewer components, so you have fewer that could wear out. A part-time T-case can lead to greater power and fuel efficiency. These typically feature a two-speed unit.
Full-Time Transfer Case
Mostly used in sports utility vehicles (SUVs), the full-time T-case consistently sends power to both axles, so both sets of wheels receive power. These transfer cases typically use either an open differential or a limited-slip differential. This choice in T-case results in a more refined ride but uses more parts, which means there’s more opportunity for parts to break or wear out. To achieve the even split of power needed for off-roading, most of these T-cases come with a differential lock button. Some require you to shift into the low range. With most vehicles, you can switch out a full-time for a part-time to increase the vehicle’s fuel efficiency.
Two-Speed Transfer Cases
You’ll find these two-speed transfer cases in crossover SUVs and select pickup trucks. The Honda Ridgeline features a two-speed T-case. These vehicles typically use all-wheel-drive traction control systems. With a two-speed, you can engage a single different gear ratio. With a 1:1, you’d either put the vehicle in 4-Hi or two-wheel drive, but shifting into low range, the ratio changes to 4:1. The ratio refers to the T-case output (spin) revolution for each rotation of the transmission’s output shaft.
Multi-Speed Transfer Cases
You’ll need to put in the work to install this transfer case. You would do this to achieve even further gear reduction. The most common combination consists of a 4×4 stock two-speed T-case with an underdrive unit. Doing this can give you four transfer case speed options.
Replacing Transfer Cases
If you want to rebuild or modify your vehicle to its original state, you can typically find OEM parts from a major auto parts store. If you want to find the exact part to restore your vehicle, hit the junkyard or scrap yard. You’ll need to shop used. That doesn’t mean abuse. With savvy shopping, you can find parts off of a vehicle that had little wear and tear but became undrivable. These typically were involved in an accident that caused damage that led the insurance company to total them. Think in terms of frame damage. There’s typically nothing wrong with their mechanical parts, so you can shop around the puller parts market for the classic transfer case of your dreams.