From Our Latest Issue — AugustAug 2nd, 2013 | By Editorial Staff | Category: From our latest issue
Editor’s Note: This is a lengthier, online version of Ed Johnson’s TT Learning Center article that appears in the Tow Times August issue.
Towing 4-wheel Drive Vehicles
By Edward D. Johnson
I am often asked why an owner’s manual allows towing a vehicle with all wheels on the ground
without limits but restricts towing with one end lifted. It is a logical question with simple answers. When a vehicle is towed with all wheels on the ground, the fluid level in the transmission and transfer case remains fairly level and moving parts are able to dip into the fluid to obtain lubrication and circulate the fluid. Some parts pick up fluid and throw it around and lubricate other moving parts. When one end of the vehicle is lifted, the fluid level shifts and parts are no longer able to reach the fluid. Manufacturers specify speed and distance restrictions based on whether the transmission is level or tilted. As a tow provider for a local transmission shop for over 30 years, I have seen transmissions and transfer cases that have been destroyed by bad towing. Among those are transmissions whose parts got so heated that they welded into one piece. A few years ago, I was asked to examine a Ford T-Bird after a 70-mile front tow and whose entire drive system broke. My examination determined that the transmission locked first and then the driveshaft broke because the transmission would not allow it to turn and the locked driveshaft then snapped the rear differential. The cost of repair was over $6,000.
A lot of misconceptions about towing 4-wheel and all-wheel drive vehicles circulate through the towing industry and acting on the misinformation causes a lot of damage. Just because a 4-wheel drive system has a neutral position does not mean it is towable. On some vehicles, when the transfer case is shifted into neutral, transfer case parts are disconnected from the transmission but the front and rear wheels remain joined through the drive shafts in the transfer case and towing with any wheels on the ground will cause serious damage. In the 1970s, many Chevrolet 4WD pickup trucks were designed to be sling towed with the transfer case and transmission in neutral, but when wheel lift trucks came along those settings would not work. The truck would try to climb out of the wheel lift. It was necessary to shift the transmission into neutral and the transfer case into 2Hi. in order to tow with a wheel lift. Manufacturers called for Jeep CJs built from 1976-1986 to be towed from the rear with the manual hubs locked so that front axle fluid would be circulated during the tow. Towing with the manual hubs unlocked did not circulate fluid and the front axle would overheat.
Several years ago, I was given a Subaru Forester for disposal and it became an excellent guinea pig for experimentation. I towed it with the rear wheels down and the front wheels on a wheel-lift. As I started towing, I felt a slight tugging which continued for about four miles. The tugging suddenly stopped and the truck began to tow real easy. I got out with a remote thermometer and the temperature of the transfer case was well over 200 degrees. After another few miles, the Subaru began to resist towing to the extent that the rear wheels began jumping and finally would not turn at all. Smoke poured out of the vehicle and fluid was scattered all over the underside.
I also had the opportunity to tow a 1963 Mercury Comet. It began resisting the tow almost immediately and soon smoke started pouring out of the transmission. Inside the engine compartment, transmission fluid was pumped all around the engine and transmission. This happened because Comets had internal pumps that would be turned by the driveshaft during towing. Some cars like the Comet depend on engine operation to drive the pump during driving, but when towed, the pumps are turned by the drive shaft and the transmission will try to shift during towing.
All-wheel drive vehicles such as Hondas have an on-demand drive system that operates in front wheel drive most of the time during normal driving conditions. The system only actuates the rear wheels when the computer detects wheel slippage from the front. Once the wheel slippage stops the vehicle shifts out of all-wheel drive into two-wheel mode. While it is possible to tow these Hondas from the front with the ignition key removed (no electrical functions operate), you have to accept the risk that the electrically applied clutches located at the rear differential are not fully engaged and will begin to heat and be burned.
Haldex is the Swedish manufacturer of all-wheel drive systems for a number of foreign car manufacturers. Their excellent product specifically allows all-wheel drive vehicles to be towed with one end of the vehicle on the ground. It has developed several systems that allow vehicles to operate in full-time front wheel drive until traction is needed at the rear wheels. When the Haldex system operates all four wheels, it is capable of determining exactly how much torque is needed at each end of the vehicle. This increases the usefulness of the drive train and allow more precise shifting. On Volvo cars, it even allows for towing newer all-wheel drive vehicles without risk. A key advantage of Haldex is that it increases fuel mileage and driver control while allowing for a simpler operating system. In the future, we can expect to see more all-wheel drive vehicles that can be towed without problem by wheel-lift trucks.
The Rule: Never tow any vehicle on its drive wheels unless the manufacturer’s manual gives you the specifications.