The Mechanic's Corner: Always confirm vehicle's drive status
October 13, 2009
BAGHDAD, Iraq - Driving a vehicle in four-wheel drive when it should have been in two-wheel drive is not the best of circumstances.
As soon as the driver finds out of the mistake, they're left with an awkward feeling that is compounded when they realize the vehicle was driven like that for a long period of time. Usually, the first thoughts are concern for the vehicle and finding out whether damage was done.
If there is damage and problems exist, the vehicle will show certain tendencies.
"MRAPs [Mine-Resistant Ambush-Protected vehicles] come in and they're reporting that they couldn't get over 30 miles per hour, they were having problems stopping, or there's trouble driving on paved roads and the vehicle didn't respond correctly," said Spc. Charles Marshall, a light-wheel mechanic, from Wheeling, W.Va., assigned to quality control, Headquarters Support Company, Division Special Troops Battalion, 1st Cavalry Division. "They had engaged the transfer case and turned the four-wheel drive on and didn't realize it."
The problem with Soldiers not using the proper drive status for the proper terrain mostly effects the transmission and the transfer case, which changes the range in the gears. It begins with the vehicle losing power from the transmission to the wheels, but also causes the back of the transmission to become stripped of its grooves, Charles said. There was one incident in which a transfer case completely fell out because all of its bolts snapped off.
"Excessive torque from the transfer case to the front axle causes unnecessary wear and tear," said Sgt. Michael Clark, quality control and assurance shop foreman, from Arlington, Texas,. "It's [four-wheel drive] only made for when you're in rough terrain or in a rut."
According to Charles, transfer case problems come from neglect.
"They are not checking their settings before they start to drive," emphasized Charles.
It is a problem easily corrected.
"For an MRAP, it's simply flipping a switch, for a humvee, it's by shifting a lever."
The switches for the gears are delicate though and can be activated without the driver's knowledge. Soldiers need to be careful of accidentally tapping the gear switch, said Sgt. Christopher Kendall, a light-wheeled mechanic, from York, Pa., assigned to quality control. They carry so much gear on their person and in the vehicle, a switch is pressed and no one is aware of it..
The same principle applies to humvees. Mechanics warn to be careful when disengaging the brakes or shifting the humvee from park to drive because the hand is close to the transfer case controls and can accidentally put the vehicle into four-wheel drive.
In a humvee, High-lock and Low-lock are the four-wheel drives, High is the regular two-wheel drive, said Sgt. Abraham Kay, motor pool maintenance squad leader, from Coleman, Texas. "Low-lock is for deeper sand and mud, and drives half the speed of High-lock."
For MRAP vehicles where four-wheel drive is needed, use Low-three for speeds of 20-35 miles per hour, Low-two for 10-25 mph and Low-one for speed 0-15 mph, said Charles.
Soldiers should remember that lower gear ratio gives more torque, meaning the wheels don't spin as fast, which allows vehicles to cross rougher terrain more easily.
Due to the serious nature of the problems associated from irregular use of the transfer case, mechanics stress the importance for Soldiers to discuss terrain features that will be encountered during pre-mission briefs.
It is also recommended that all personnel operating a humvee or MRAP vehicle have their operator's manual easily accessible to look at the sections on operating under usual conditions and operating under unusual conditions.
Preventing extended periods of unnecessary driving time in four-wheel drive could mean the difference between being out in sector dead lined and completing a successful mission.