Servicing 4WD and AWD Systems
4WD and AWD drivetrains have become increasingly popular and vehicle populations continue to rise that feature traction-controlled all-wheel-drive technology. Familiarity with and the ability for your shop to service these vehicles is a major factor in terms of your shop’s capabilities and bottom line.
The popularity of four wheel drive (4WD) and all-wheel drive (AWD) vehicles continues to increase. Although 4WD has been around for many years it has become quite refined with fully automatic shift on the fly that doesn’t require much more than the flick of a switch to engage and fully automatic locking hubs.
And today’s AWD has become very popular with consumers. Thirty-five years ago when AWD technology was in its infancy on the AMC’s Eagle and the Audi Quattro, it was primitive; but the technology has caught up and today AWD vehicles are common. In 2012 Ford reported that AWD vehicle sales had increased more than 72% from 2009, and the popularity was growing in areas of the country that traditionally don’t need AWD (they don’t get a lot of snow).
Miami and Phoenix have seen a surge in AWD-equipped vehicles and most techs don’t even realize that they are working on one until the vehicle goes up in the air and you can see the extra parts.
There are significant differences in the AWD/4WD setups used on today’s vehicles, but many of the parts and pieces overlap and the terms are often used synonymously.
The classic definition of a 4WD vehicle allows constant tractive force (torque) to be applied to all four wheels continuously, using a transfer case and a front and rear differential. This system was originally designed primarily for enhanced off-road use. The typical AWD vehicle will incorporate a transfer case, a front and rear differential, but also some form of coupling device. This coupling device allows torque to be transferred from the wheels that are slipping to others that aren’t slipping, but it won’t allow permanent tractive force to be applied to all four wheels continuously. This is the fundamental difference between 4WD and AWD.
The popularity of 4WD and AWD is adding service opportunities, but it is also adding issues and customer complaints that weren’t as common on simple front- or rear-wheel-drive cars and trucks. AWD and 4WD systems incorporate extra rotating parts, lubricating fluids, bearings, driveshafts, electronic components and other parts that will wear out and require maintenance over time. Today, complaints of non-functional AWD/4WD, illuminated dash lights alerting of a failure, vibrations and noises are most common. But there are a few issues and pattern failures that stand out.
Honda: The 1996-2012 AWD Honda CR-Vs, HR-Vs and Elements are primarily front-wheel-drive vehicles that incorporate Honda’s Dual Pump system for AWD operation. To achieve automatic AWD operation, these Honda products use a rear differential housing that integrates two hydraulic pumps, a valve body, and a multi-plate hydraulically operated clutch and a special dual pump fluid. The driveshaft from the transmission-mounted transfer case turns one hydraulic pump in the differential housing, and the rear wheels drive the other pump.
Under normal driving conditions (no front wheel slippage) these two pumps turn at the same speed, creating equal hydraulic pressure and no AWD operation. But if the front wheels start to slip and spin faster than the rear wheels, the front pump will increase hydraulic pressure and open a valve in the valve body activating the multi-plate clutch initiating AWD operation. The multi-plate clutch allows a specific amount of torque to be delivered to the rear wheels in an attempt to resynchronize the not only front and rear wheel speeds but the pumps’ hydraulic pressure. The higher the speed variance between the front and rear wheels, the greater the amount of torque that is transferred to the rear wheels because of the hydraulic pressure difference. As soon as the wheel speed is synchronized or if the brakes are applied, the clutch disengages and the vehicle will resume 2WD operation.
The operation of this unit is simple and self-contained but it has been prone to fluid failure due to lack of maintenance or moisture intrusion. A common customer complaint of a groaning, howling, whistling or clunking noise on turns can many times be repaired by simply changing the dual pump fluid. In some cases the rear differential may have to be serviced a couple of times in order to get the fluid to flow through and clean out the old fluid from the multi-plate clutch pack. There are specific TSBs to address these issues and they contain a burnishing procedure to force the dual pump fluid into the clutches, but many times driving in a figure eight for a few minutes and then draining and refilling the differential with fresh dual pump fluid is enough to cure the issues. In extreme wear cases that can’t be rectified by a fluid exchange and burnishing procedures, the clutch plates may have to be replaced.
Cadillac SRX/Buick LaCrosse AWD: These 2010-2012 vehicles feature a transmission-mounted transfer case and a driveshaft to transfer power to a limited slip rear differential that is fitted with an electro-hydraulic multi-disc differential clutch assembly that GM calls the torque transfer device (TTD). Torque transfer to the rear wheels through the TTD is controlled by the rear differential clutch control module (CCM) that is bolted onto the differential clutch housing. This TTD unit has its own oil supply, separate from the rear differential: the electronic limited slip differential (eLSD) clutch fluid. The eLSD clutch fluid is used to apply the multi-disc clutch if wheel slip is detected and the rear wheels are to be driven even on dry pavement under heavy acceleration.
The CCM controls an electric differential clutch pump that pressurizes the eLSD fluid; the CCM monitors the eLSD pump’s current draw to calculate the pressure that is created. The CCM wants to maintain an operating pressure of 392-464 psi and the pump may be active anytime the engine is running to maintain this pressure (more on this in a moment).When the vehicle detects wheel slip, the CCM is notified through the vehicle’s high speed communication bus. Using the rear differential clutch solenoid valve, it will begin to regulate hydraulic pressure to the clutch pack controlling the amount of torque transferred to the rear wheels. The amount of torque transferred is determined by various sensors and modules on the vehicle but controlled by the CCM.
A number of concerns involving the TTD system have materialized, but the most common customer complaint is a service AWD message of some description on the driver’s information center (DIC). Internal part failures of clutch pistons and pumps have been noted but so have housing, pipe and fitting leaks that can easily cause trouble codes and concerns. Low fluid level in the TTD is a common concern. Another common issue is moisture intrusion into the CCM that is attached to the side of the TTD unit. If the CCM unit is removed for any reason, it must be reinstalled properly. Proper installation is imperative or moisture intrusion will be a problem.
There have been a number of revised calibrations and reflashes released for concerns such as booming noises on the highway and rough-feeling turns, but improper eLSD clutch fluid level is a frequent issue and it won’t be fixed by a reflash. Because of the high pressures involved, a minimum of 10 minutes should pass before any work is done on the TTD to allow the pressure to dissipate.
The TTD is filled properly when the eLSD fluid level is at the bottom of the TTD clutch cover plug and the pump cavity is full. A scan tool is used to determine if the pump cavity is full, monitoring the current draw of the TTD pump rear limited slip differential clutch pump motor current parameter.
A full pump cavity will show a current draw of 2.5-3.5A while the pump is running and a cycling current of 0 to 2.5-3.5A as the pump turns on and off. Both of these conditions have to be met or the level may not be correct (remember, the engine has to be running).
If the pump motor current is steadily below 2.5A, then the pump cavity is low on fluid. If the eLSD fluid level is low or service on the unit has been performed, the level will have to be topped up. The filling and top-up procedure must be adhered to carefully and followed specifically. It involves a scan tool, engine running, WOT engine cycles, multiple level checks and a six mile road-test to ensure the level is correct, and to prevent trouble codes and dash warning messages. Because of the internal design of the TTD device (it has a separate compartment that has to be filled by this procedure before the level will be accurate), care must be taken to follow the step-by step directions carefully or the level in the TTD won’t be correct.
Ford: The 4WD system that Ford has used for many years involves an integrated wheel end (IWE) assembly. This vacuum-controlled system has caused more than its share of confusion and concerns with technicians and owners since its inception. The system is intended to allow the front hubs to freewheel using the IWE assembly. The IWE allows the hubs to separate from the driveshaft at the wheel end, resulting in less wear from back-driving all of the 4WD parts and pieces, less drag and slightly improved fuel economy.
The operation of the IWE is fairly simple. When the operator requests 2WD operation, vacuum is applied to the IWE actuator, retracting a clutch ring from the hub coupler and disengaging the hub from the drive axle. When 4WD operation is requested, the vacuum is removed from the IWE actuator and the clutch ring is re-engaged into the hub coupler by spring pressure joining the hub to the axle, now driving the wheel.
Proper vacuum throughout the system is imperative for proper operation. A vacuum leak, low vacuum or issues with check valves or solenoids is going to cause customer and driveability issues, especially if the operator is requesting 2WD operation.
Common complaints involve torque steer, ratcheting, grinding sounds or a whining noise at highway speeds when the vehicle is in 2WD mode. Low or missing vacuum is going to allow the clutch to engage or partially engage in the coupler. Disconnected vacuum lines at the IWE, cracked, melted or rodent chewed hard plastic vacuum lines, solenoid failures and check valves are the most common culprits. But so are corrosion, water intrusion and physical damage on the actual IWE components, especially in the Rust Belt areas where road salt and other chemicals are used to clear the roads in the winter.
Diagnosing the IWE system can be done with simple hand tools: a vacuum pump and a vacuum gauge. When the selector is set at 2WD there should be at least 5in/Hg vacuum at the IWE assembly. The best practice is to tee into the vacuum fitting between the IWE hose end and the IWE itself. If there isn’t 5in /Hg, then you have to start working backwards in the system and find the leak.
But if you don’t find any vacuum control, hose or fitting leaks you have to look at the IWE unit itself. The IWE unit has to be able to hold a good vacuum (20in/Hg) for at least five minutes. If I find the IWE won’t hold vacuum, I often use a smoke machine to verify leaks and cracks caused by corrosion and determine what parts need to be replaced. Don’t forget that this system still has a transfer case and a shift motor, but the takeaway here is that 2WD requires constant high vacuum, and 4WD no vacuum.
Propeller shaft vibration issues: All the extra rotating parts of the AWD/4WD system will form natural resonance that the engineers will work very hard to eliminate. They don’t want the customer complaining about driveline vibrations.
But as parts start to wear and mileage accumulates, vibrations and noises can become a common complaint. Most AWD/4WD vehicles will have a hanger bearing to support the propeller shaft that transfers the power from the transfer case to the rear differential. These hanger bearings are frequently rubber insulated and when first installed are round. Over time the rubber will become distorted and start to sag and become egg shaped, allowing the shaft to droop and create an imbalance.
A good visual inspection may not show this wear until the propeller shaft is grabbed and pushed up or sideways to reveal the wear in this bushing. They don’t tear like they older models did previously.
U-joints and driveshaft CV joints are also susceptible to wear and seizing up. It is now common to have to replace entire propeller shaft assemblies, as repair parts aren’t available. The manufacturers are very concerned about propeller shaft balance and the vibrations they can cause to the point that they recommend a driveshaft be scrapped if it’s dropped.
Electronics: The use of electronics has become the preferred method of actuation and control. Honda used the non-electronic AWD setup with great success for many years, but even they have succumbed to electronic rear differential control. Most manufacturers are now using electronics to control transfer cases, differentials, multi-disc clutch packs (electrically or electro-hydraulic) and viscous clutch operation. The ability to precisely regulate torque flow for constant traction on all four wheels is much easier accomplished with the total integration of electronics and electronic components that are linked to various other sensors to control traction.
Because of this extensive use of electronics, the use of a scan tool is now going to be imperative. Scan tools will be needed for retrieving codes, bidirectional testing, reading data, proper level checking and monitoring system operation. Another concern created by the heavy use of electronics is the frequent updates or reflashes released to correct operational issues, customer complaints and even noises created by the system’s operation.
Ford has an update for 2011-2014 AWD Explorer models for ghost terrain management warnings on the dash that will not store any codes. The anti-lock brake module has an available update to cure this situation. Ford also has an issue with some 2013 Flex, Explorer, Taurus, MKS, MKX and MKT AWD/4WD vehicles. They may not give you any diagnostic PIDs until the PCM is updated. But troubles codes don’t always indicate an electronic or control issue either and care has to be taken to ensure all the basics are covered as with any diagnostic.
A number of GM models equipped with AWD (2007-2010 Enclaves, Equinox, Acadia, Torrent, Terrain, Vue, and Traverse) can set DTC C0550 or C056D codes in the CCM and may illuminate the Service All Wheel Drive message and render the AWD inoperative (2WD operation works fine). In many cases, other issues such as a no-start, slow cranking and or a dead battery situation that required boosting will have occurred around the same time that the message and codes set and that issue has to be dealt with first. Typically these codes will set if the battery voltage dips below 9 volts, so the charging and starting system has to be analyzed and verified first. After verifying or repairing any concerns, allow the battery voltage to drop below 9 volts and clear the codes and leave the ignition switch off for two minutes.
Then cycle the key on and if after 40 seconds no codes reappear in the CCM the issue was a voltage concern. If the codes reappear, then you will have to follow the proper diagnostic information.
The popularity of AWD and 4WD vehicles will only enhance our services in the coming years. There are common issues and pattern failures that stand out and service items and techniques that have to be adhered to for successful repairs, but following proper maintenance and diagnostic procedures are still going to be a technician’s best friend.
All those extra rotating parts, pieces and fluids will need to be serviced at some time. Although many fluids are advertised as “lifetime,” the service information will tell you that many conditions such as towing and temperature extremes exclude the fluid from the lifetime category, and regular maintenance will be required. The proper fluid in transfer cases, rear ends and torque transfer units has never been more important.
Considering the potential use of universal fluids that may not meet the manufacturers’ specifications, and with AWD/4WD now being part of the vehicle’s stability control, the proper fluid must be used to ensure the customer’s safety. ●
Jeff Taylor boasts a 31-year career in the automotive industry with Eccles Auto Service in Dundas, Ontario, as a fully licensed professional lead technician. While continuing to be “on the bench” every day, Jeff is also heavily involved in government focus groups, serves as an accomplished technical writer and has competed in international diagnostic competitions as well as providing his expertise as an automotive technical instructor for a major aftermarket parts retailer.
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