How to Use a Jake Brake

Jake brakes control truck speeds on downgrades.

The Jacobs engine brake, more commonly known as a Jake brake, is a device that retards the valve timing on a diesel engine in order to use the engine to slow the vehicle. Using a Jake brake to slow a truck down gives the driver better control over the vehicle, while saving wear and tear on the service brakes and preventing the brakes from overheating on steep downgrades.

Things You'll Need

• None

Instructions

1. Start the diesel engine and allow it to reach normal operating temperature. The constant flow of oil at operating temperature is necessary for the operation of the Jake brake.

2. Turn the Jake brake "On/Off" switch, normally located on the truck's dashboard, to the "On" position.

3. Set the "High/Low" switch, usually located next to the "On/Off" switch, to the desired position. Setting the switch to "Low" activates the Jake brake on three of the six cylinders of the diesel engine. Use "Low" for light loads on flat ground or slight grades. Setting the switch to "High" activates the Jake brake on all six cylinders. Use "High" for heavier loads, descending a steep grade or if slowing the vehicle still requires the service brakes.

4. Remove the right foot from the accelerator pedal. This will activate the Jake brake. To deactivate the Jake brake, depress the accelerator pedal.

Tips & Warnings

• Some Jake brake models have a "Low/Medium/High" switch. In this case, "Low" activates the Jake brake on two cylinders, "Medium" on four cylinders and "High" on all six.

• Test the Jake brake before beginning a steep descent before proceeding down the hill.

• Some truck manufacturers position the Jake brake switches on the gearshift.

• Some cruise-control systems work hand in hand with a Jake brake. While in cruise control, the Jake brake will activate when the vehicle exceeds the cruise-control set speed. The Jake brake will disengage when the vehicle speed drops 1/2 mile per hour below the set cruise-control speed.

• Exercise caution when operating the Jake brake on wet or slippery pavement. If the wheels lock up or the truck cannot be controlled while using the Jake brake, turn it off and use the service brakes to control vehicle speed.