Driveline Characteristics for All-Wheel Drive (AWD) and Four-Wheel Drive (4WD) Systems

This bulletin is being revised to update the model years. Please discard Corporate Bulletin Number 01-04-18-001E (Section 04 -- Driveline/Axle).

The purpose of this bulletin is to help explain the operating characteristics of 4WD/AWD systems.

For specific operating instructions for individual transfer cases, please refer to the Owner Manual or Service Information.

AWD vs. 4WD

The very basic difference between AWD and 4WD is the intended usage of the systems.

AWD is usually intended for on-road use in inclement weather conditions, while operating smoothly on dry pavement by allowing for a difference in speed between the front and rear axles while turning. These systems are not selectable and do not have low range gearing for the transfer case. These systems can be found in cars or trucks.

4WD is primarily found in trucks and can be broken down into Part-Time, Full-Time, and Automatic Transfer Cases. These selectable systems have modes intended for on-road use and other modes intended for off-road or low traction situation usage. All current GM 4WD systems have a low range available in the transfer case.

Disclaimer: Even though a certain amount of noise or feel can be expected, GM may offer service procedures or components, or change vehicle design, which may reduce perceived noise levels in the interest of customer satisfaction.

AWD

There are two different categories of AWD systems. The first category is full-time AWD. This type of transfer case delivers torque to the front and rear axles at all times. This ratio can vary depending on the system, but is usually about a 30/70% front to rear split but also can vary depending on traction conditions, up to 100% front or rear. This type of transfer case can have a viscous coupling for low traction conditions along with a planetary gear set to allow for difference in speeds between the front and rear axle, or an open type of planetary gear set differential, which uses brake based traction control for low traction conditions. An example of a vehicle with an open differential/traction control type of transfer case is a 2003 Escalade. RPO codes for these types of transfer cases are NP3 (NVG 149, BW 4473 viscous clutch) and NR3 (BW 4476, 4481, 4485, open differential) or NR4/NR6 (BW 4493, 4494 open differential).

The second category is an on-demand AWD. This type of AWD basically delivers torque only to a primary driving axle unless reduced traction is experienced. At that point, the system electronically or mechanically will apply torque to the other axle. Depending on the type of system, this can provide up to 100% of the torque to the axle with traction. These transfer cases use an electronically actuated clutch pack, a hydraulically actuated clutch pack, or a viscous coupling to allow for a difference in speed between the front and rear axles. An example of an electronically controlled version of the On-Demand AWD is in the Smart Trak system in the 2003 Bravada. An example of a hydraulically operated On-Demand AWD is the Versa Trak system in the Aztek, while a viscous coupling is used in the Vibe. The RPO code for the Bravada is NP4 (NVG126). For some vehicle lines, there is not an RPO code. The only way to tell is by the Line Chassis VIN code, B for the Rendezvous/Aztek, V for the Venture/Montana or by SM for the Vibe.

PART-TIME 4WD

Part time 4WD refers to vehicles equipped with a transfer case to split power between the front and rear axles of the vehicle. This traditionally is a 2-speed selectable transfer case that can be shifted into 2HI, 4HI, 4LO and usually a Neutral position. The 4WD modes of Part time systems do not allow for a difference in speed between the front and rear axles while turning. This system effectively locks the front and rear propeller shafts together. When turning, the tires must allow for the different turning radius of the front and rear axles, which is why this is intended for low traction or off-road use. These systems have low range gearing for the transfer case. An example of a vehicle with this style of transfer case would be a Silverado with a manual shift transfer case (a shift lever on the floor) (RPO NP2) or a Colorado with a push button transfer case with a 2HI, 4HI, 4LO and Neutral position (RPO NP1). The RPO codes for this style of transfer case are NP1 (NVG 233, 243, 263, and T-150 push button) or NP2 (NVG 231, 241,261, BW 4401, 4470 shift lever).

FULL-TIME 4WD

A second version of a 4WD transfer case is a full-time 4WD transfer case. This style of transfer case has an open center differential to allow for different speeds between the front and rear axles and operates similar to an AWD system. This transfer case can be locked to operate like a Part-Time 4WD transfer case (no difference between front and rear prop-shaft speeds) and/or uses a traction control system to assist in low traction situations. These transfer cases also have a selectable low range. An example of this type of 4WD is the H2. RPO code is NR4.

The NR4 transfer case (available in non-luxury utilities) has no switch selection to lock into a part-time 4WD mode as described above. There are only 3 selections on the controls: Disable Stabilitrak, AWD and 4LO. The Owner's Manual describes this system as an AWD system.

AUTOMATIC TRANSFER CASES

The last category is a combination of 4WD and On Demand AWD. These transfer cases have a 2HI, Auto-4WD, 4HI, 4LO and Neutral position and would fall in the general 4WD category. This transfer case has the operating characteristics of both an On Demand AWD and a Part-Time 4WD system depending on the mode selected. This transfer case uses a clutch pack to allow for a difference in speed between the front and rear axles in the Auto-4WD mode. In the 4HI or 4LO modes, there is no allowance for the difference in speed between the front and rear axles. An example of a vehicle with this would be a Sierra with a push-button transfer case with a 2HI, Auto-4WD, 4HI, 4LO, and Neutral positions. The RPO code for these transfer cases is NP8 (NVG 226, 236, 246, 246 EAU).

DIAGNOSTIC AIDS

Always follow Strategy Based diagnostics and service information in Service Information (SI). SI is constantly updated with new and more thorough information.

One of the first steps that can be used in determining if a noise is a characteristic is to compare it to a similar vehicle. The comparison vehicle should be as close to the original vehicle as possible, including mileage, GVW, RPO, chassis style (extended cab, short-bed...).

Attempts to correct characteristic types of noise and/or feel should not be performed. In most situations, these conditions will not be changed. Any attempts to correct these conditions may reduce customer confidence and inconvenience the customer while their vehicle is out of service.

4WD

While operating 4WD vehicles in the 2HI mode, they should perform similarly to an equivalent 2WD version of the vehicle. However there are still additional components on the vehicles and there may still be some slight differences in characteristics. Some Part-Time 4WD transfer cases may make a slight gear rattle type of noise when operated in 2HI at low engine speeds, similar to a manual transmission gear rattle type of noise. This can originate in the synchronizer assembly from engine harmonics. This noise will usually be reduced or eliminated while driving in a 4WD mode because the synchronizer assembly clearances will be taken up once engaged in 4WD. Operating the engine at a slightly higher rpm should reduce this noise.

While a vehicle is traveling down the road, the tires rotate a certain number of times per mile depending on the true tire radius. If all the tires do not have the exact same true radius (due to load, tire pressure, wear, build variances...), they will turn at slightly different rates. Also, unless the vehicle is traveling in a perfectly straight line, the front and rear axles are traveling in a slightly different arc, which means the front and rear axles are traveling at slightly different average speeds.

These systems either do not have a center differential or it has been bypassed (when used in 4HI or 4LO in Part-Time or Automatic systems, or 4HI Lock or 4LO Lock on Full-time 4WD), so the front and the rear propeller shafts will turn at the same speed, which leads to the front, and rear axles rotating at the same average speed. Using the 4WD modes (4HI, 4LO, 4HI Lock, 4LO Lock) will usually increase noise in the axles, transfer case and the rest of the driveline and is usually greater at higher speeds and will usually increase the more the vehicle is turned. As the vehicle turns, the front and rear axles follow a different arc. When this occurs, the only place to compensate for this binding is between the contact patch of the tires and the ground. This can feel like the vehicle is vibrating, crow hopping or grabbing. Even if the vehicle is driven in a straight line, there are slight differences in tire circumference that will cause some driveline binding. If a vehicle had the exact same size tires and was driven in a perfectly straight line, the fact that more parts are moving would mean that there would be more noise and possibly some feel of the system operating.

Use of 4HI or 4LO and 4HI Lock or 4LO Lock on Full-time 4WD is intended for use on a low traction surface such as snow, ice, mud or sand. On a low traction surface, the differences in front and rear axle speeds will not have as much effect on binding because of the lower traction levels between the surface and the contact patch of the tires. On a high traction surface, the higher traction levels will create more binding and noise in the driveline.

As a result, a small amount of noise or feel of the system operating can be expected when using 4HI or 4LO in Part-Time or Automatic systems, or 4HI Lock or 4LO Lock on Full-time 4WD. The noise and feel can vary depending on the transfer case type, GVW ratings (generally the higher the GVW the more noise it will make), vehicle build variations, gear ratios in the axles, axle type, tires, and importantly driving conditions.

While the transfer case is in Neutral, with the engine running, some noise can be expected.

When transfer case is shifted to 4LO (or any time through or from neutral, for example from 2HI to 4LO or back) with the engine running, or with the vehicle moving, some grinding noise can be expected. In some vehicles you may experience a slight bump as the shift to 4LO is completed. This occurs because the 4LO range is not synchronized and there may be a difference in transfer case input shaft and output shaft speeds, especially if the vehicle is moving or equipped with an automatic transmission (residual torque from the transmission may be driving the transfer case input shaft). To minimize this noise the shift may be completed at a stop and in an engine off, key on mode. However, there may be an increased chance of having a blocked shift (some models must have the engine restarted to complete the shift). Noise and bump levels will vary between vehicles and even between shifts on the same vehicle. As a reminder, if attempting to determine if noise or bump experienced in one vehicle is a characteristic, be sure to compare to similar make vehicles. For example the shift to 4LO in a Colorado or Canyon will usually have more noise and/or bump feel than in a Silverado or Sierra. When driving in 4LO, the extra gear reduction will make additional noise compared to driving in 4HI. The automatic transmission shift characteristics will also feel different while operating in 4LO.

AWD

AWD systems are intended for use in high and low traction situations without operator input. These systems will generally be quieter on high traction surfaces than a similar 4WD used in 4HI or 4LO. However, these systems will generally make more noise than a similar 2WD vehicle simply because there are more parts rotating and more gear sets interfacing. Once again, these systems may make more noise in turns, and in some situations you may be able to feel the system operating, even on high traction surfaces. Additionally, you may feel transfer case operation in on-demand AWD systems. These systems generally react to a speed difference between the front and rear axle and it may be possible to detect this by hearing a noise, or actually feeling the engagement of the system.

Automatic Transfer Cases

Automatic transfer cases will have the characteristics of both the Part-Time 4WD and the AWD systems depending on the mode selected. If the transfer case is operated in 4HI or 4LO, it will behave as a Part-Time 4WD system. If it is operated in the Auto 4WD mode, it will behave like an on-demand AWD system.

Driveline noises in 4WD or AWD vehicles caused by loading may be mistaken for transfer case or front axle noise because they can sound very similar. Four-wheel drive noises caused by loading may exhibit the following conditions:

TIRES

Tire rolling rates can be a major factor in operational characteristics. Consider the following important items: