Jack Stand
A safety device that is placed under a car to guard against jack failure. It is usually made of steel and its height is adjustable

Jack knife
The action of a vehicle (tractor) with a trailer in which the trailer and the vehicle form a "V" instead of normally being pulled in a straight line. Usually this is the result of a skid in which the trailer swings around dangerously and tries to overtake the cab.

Jacking point
A strengthened place on the underbody to put the jack. Small cars have one point on each side but larger cars may have two

Jake brake
Usually seen in trucks, jake brake shuts off the exhaust valves so that in the exhaust stroke, the burned gasses cannot escape through the exhaust valves. Instead they press against the head of the piston and causes the piston to slow down. When the intake valve opens, some of the exhaust escapes out the intake valve and gives a distinctive loud rapping noise. Jake brakes are used in large truck engines to assist in slowing the vehicle.

Jig
device used in the accurate machining of good in process by holding the goods firmly and guiding tools exactly to position.

Joule
After James Prescott Joule, English (1818-1889). The joule is a unit of energy.

Jumper cables
Cables used to start a vehicle by sending electrical current from the battery of one vehicle (charged) to another. This way the disabled vehicle can be started and start generating its own power.