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 all-wheel drive (AWD)/four-wheel drive (4WD) components and what to look for when assessing a vehicle post collision. Though there are practical differences between AWD and Four Wheel Drive (4WD), you will find that for collision purposes they are fairly interchangeable. The important difference to note between the two types is that AWD is designed to run at highway speeds on clean and clear roads and 4WD is designed for high slip and off road use and typically has a Two Wheel drive (2WD) or AWD mode that can be selected for highway use. Depending on configuration, when in 2WD mode either the front or rear axle could be the drive axle.
AWD/4WD systems can be found on cars, vans, mini vans, SUVs and trucks and will borrow components from either FWD or RWD powertrain plus integrate a few things unique to AWD/4WD.
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.
Manual transmission
4WD 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. A crashed car spewing out transmission fluid could an indicator the main housing of the transaxle may be 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.
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.
Transfer Case
AWD/4WD systems utilize a transfer case. This splits the power from the transmission and sends it to the front and rear differential. These can use a cast iron case, but aluminum is much more common and all the same rules for the transmission case inspection apply.
Available in single speed typically found in AWD only and multiple speed versions that allow for true 4WD and a gear reducing low range.
Tail shaft
The tail shaft of a transfer case is where the drive shaft connects to the transfer case output shaft. You will generally find two types of connections: slip yoke or a fixed yoke. To identify the two, 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 either bolted to the output shaft or free floats on it.
Driveshaft
The driveshaft connects the output shaft of the transfer case to the rear differential and in some configurations to the front differential. Constructed of aluminum or steel, if it is impacted, it no longer is fit for service. Some drive shafts can be sent out and repaired, including getting them rebalanced. Most drive shafts 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 collisions that push the differential into the driveshaft, 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 AWD/4WD vehicles, you will find 2 types of rear differentials and three types of front differential.
- Solid axle units that travel up and down as terrain changes;
- Independent suspension based differentials that keep the differential fixed in one place and allow the axle shafts to move with suspension movement;
- Some AWD vehicles utilize a FWD type transaxle that houses the front differential in the same fashion as a typical FWD transmission.
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. When performing quality control checks post repair, make sure to pay attention to any odd noises or vibrations.
In the case of solid front axles, you will find some steering components affixed to the housing. Inspect carefully for damage. A four wheel alignment is a good way to assess if a solid axle (front or rear) is bent.
C/V Axle shafts
Some AWD/4WD vehicles have 2 or 4 Constant Velocity (C.V.) axle shafts that deliver power from the differential to the wheels. These are a very common component broken during a collision. They are comprised of 3 major components: shaft, joint (2) and boot (also 2). Shafts and joints break and boots tear. Any damage to these components 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 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 shop's 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 and/or programming to get them operational.
Fluids
Often overlooked, it is important as a service professional to make sure you are putting the correct Automatic Transmission Fluid (ATF) 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.
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!
