Identifying drivetrain components part 2: rear wheel drive

When an automobile is involved in a collision, it is important to not overlook damage to drivetrain components that are attributed to the collision. Not only is it in the best interest of the vehicle owner to get their vehicle made whole, a collision shop opens up a whole new revenue source when they perform repairs beyond the sheet metal.

This month we will cover typical RWD components and what to look for when assessing a vehicle post collision. RWD has been around since the early days of combustion engine powered automobiles.

RWD systems can be found on cars, vans, mini vans, SUVs and trucks.

Automatic Transmission

When inspecting the exterior case of a transmission, pay special attention to any impacts and look for cracks. Automatic transmissions typically utilize aluminum housings and aluminum tends to crack and/or break when impacted. One of the reasons engineers use aluminum is because it is quite ridged and will retain its form under normal use. Unfortunately, this rigidness becomes a detriment during a collision. While this is bad for the crashed car, it is good for those of us doing the inspecting as generally aluminum damage stands out quite well.

(Figure 1) RWD transmission of a 2003 Ford F350 Automatic

Manual Transmission

RWD manual transmissions will come in two case types, aluminum and cast iron. Both are quite rugged, but both can be damaged in a collision.

Bell housing

The bell housing is the section that bolts to the engine. This is a common spot for fracture as the material can be pretty thin in spots. Any signs of cracking or fractures should be a flag that the transmission will need to be removed and inspected more carefully. Either the entire transmission will need to be replaced or, for the more technically savvy, the damaged housing can be replaced. When inspecting the bell housing, do not forget to look at the engine too. Many cars utilize an aluminum block engine and if a transmission is damaged often the engine will suffer damage as well.

Transmission case

Cracks in the automatic transmission case often result in leaks because you have an internal pump pressurizing passages inside the case. So, if you have a crashed car spewing out transmission fluid that is an indicator the main housing of the transaxle may been fractured. Usually just cleaning off the fluids and having a helper start the car and cycle through the gears is sufficient to nail down the source.

Manual transmission cases generally won’t spew fluid running so you may have to drive and re-inspect for leaks.

Transmission coolers

Like case sections, damaged transmission coolers tend to leak badly when the engine is running. Keep in mind transmission coolers are exclusive to automatic type transmissions. Generally, if equipped with a cooler it will be located in front of the radiator, integrated into the radiator or both.

Tail shaft

The tail shaft of a RWD transmission is where the drive shaft connects to the transmission output shaft. You will generally find two types of connections: slip yoke or a fixed yoke (fixed yoke is more common on independent suspension RWD and 4x4 drive trains). To tell the two apart, look for bolts that hold the drive shaft to a flange on the output shaft. This would be a fixed yoke. A slip yoke is generally bolted to the differential pinion flange and then instead of being bolted to the output shaft it free floats on it.

(Figure 2) Typical fixed yoke transmissions will incorporate a slip section on the driveshaft to account for suspension travel and drive train movement. (Figure 3) The more common type of output is a slip yoke. This one is on a 2002 Chevrolet 2500 Silverado. The slip portion in the tail housing serves the same purpose as the slip section on a fixed yoke driveshaft.

Driveshaft
The driveshaft connects the output shaft to the rear differential. Constructed of aluminum or steel, if impacted it is no longer fit for service. Some drive shafts can be sent out and repaired, including getting them rebalanced. Most driveshafts have a means to extend and retract either by utilizing a slip yoke on the transmission output shaft or a slip yoke built into the shaft itself. This allows for suspension compression and extension.

During significant rear end collisions, inspect the slip yoke section of the shaft. If the slip portion has bottomed out, the transmission output shaft should be measured with a dial indicator. If bent, the output shaft should also be replaced.

Differential

The differential is used to convert the power coming from the transmission and split it to both drive tires. For RWD vehicles you will find 2 types of rear differentials, solid axle units that travel up and down as terrain changes and independent suspension based differentials that keep the differential fixed in one place and allow the axle shafts to move with suspension movement.

(Figure 4) Typical solid axle configuration from a 2004 Cadillac Escalade at Pete's Garage Inc. (Figure 5) Typical independent suspension diferential. This one is compliments of Jim Testa at JD Automotive in Dover, NJ. The car is a 2015 Challenger Scat Pack Shaker.

Like a manual transmission, the differential is something that may need to be driven and double checked post repair. On top of physical damage externally, differentials can be broken internally during a collision so when performing quality control check post repair, pay attention to any odd noises or vibrations. A 4 wheel alignment is a good way to assess if a solid axle is bent.

Axle shafts

Some independent rear suspension RWD vehicles have 2 Constant Velocity (C.V.) axle shafts that deliver power from the differential of an independent suspension system to the wheels. These are very common components broken during a collision. They are comprised of 3 major components: shaft, joint (2) and boot (also 2). Shafts and joints can break and boots can tear during a collision. Any damage to these component should be remedied by replacing the respective assemblies.

Electrical

You won’t find a modern car that doesn’t have some sort of wire or sensor going to it. Many automatic transmissions even have Electronic Control Modules (ECMs) and/or solenoid packs mounted internally or externally to the case. Damage to ECMs, solenoid pack or sensors warrant replacement. Damaged wire harnesses should be assessed for either reconstructing or replacing depending on your shops skill level. Do not reuse cracked or smashed electrical connectors and remember that ECM and solenoid pack replacement may require special manufacturer specific scan tool procedures or programming to get them operational.

RWD exceptions

Some RWD configurations are built as a mid or rear engine configuration. These powertrains most closely resemble Front Wheel Drive (FWD) systems and the guide for FWD vehicles should be utilized in those cases.

Fluids

Often overlooked, it is important as a service professional to make sure you are putting the correct Automatic Transmission Fluid (ATF), (gear oil or other OEM specified fluid) in when you are topping up after a repair. Red is not an indication of transmission fluid type and failure to use the OEM spec fluid could cause shifting concerns or, even worse, a premature failure. Universal transmission fluids often do not meet the OEM spec. Use your service information to ensure you are ordering the correct ATF. Likewise using the wrong oil in a rear axle could lead to bearing or internal differential failure.

Taking the time to identify powertrain concerns while doing your post collision damage assessment adds a higher level of service for your customer and will bring a lot of money to your bottom line in broken components. Failing to take the time to verify the condition of power-train components is doing your clients a huge disservice!