817383A 1 Mercury TOOL KIT (55)

A compression brake is used to slow a vehicle, not to stop a vehicle. A compression brake is not a substitute for the service braking system. The vehicles service brakes must be used to bring the vehicle to a complete stop. Proper use of the compression brake will keep the service brakes cool. This will keep the service brakes ready to provide maximum stopping power.
Driver Controls
It is important to be familiar with the Jake Brake controls in your vehicle. The Jake Brake controls will vary slightly depending on the Jake Brake configuration. The Jake Brake controls will vary depending on the design of the vehicle cab. However, basic operator controls will be similar for all models. The various types of dash switches that may be used in the cab of your vehicle are shown in the illustrations below.Note: Switches that are supplied by Jacobs Vehicle Systems may be different from the switches that are installed in your vehicle. The physical appearance of the dash switches may vary but the function of the dash switches should not change.The following information is for a typical in-line six cylinder engine.
Illustration 1 g00731591
"LOW/MED/HIGH" Switch
The "LOW" setting on the dash switch will activate two cylinders, which will provide approximately one-third of the braking horsepower. The "MED" setting on the dash switch will activate four cylinders, which will provide approximately two-thirds of the braking horsepower. The "HIGH" setting on the dash switch will activate all of the cylinders, which will provide full braking horsepower.Note: A foot-operated switch may be installed in the cab of the vehicle. The foot-operated switch will control the on/off functions of the compression brake. Some vehicle manufacturers may offer a switch that is mounted on the gear selector for the Jake Brake controls.Engine Controls
All Jake Brake applications have two additional controls. One control is activated by the position of the clutch pedal and the other control is activated by the position of the accelerator pedal. The two additional controls can provide fully automatic operation of the Jake Brake.Operation of the Jacobs Engine Brake (Jake Brake®)
The Jake Brake depends on the free flow of engine oil for operation. Make sure that the engine is at full operating temperature before switching on the Jake Brake. Normally the Jake Brake is left in the ON position whenever you are driving. The exception is when roads are slippery due to bad weather conditions. Refer to the Slippery Pavement topic in this Manual for specific operating instructions.
Illustration 2 g00731601The operation of the Jake Brake is fully automatic once the brake is turned on. When your foot is off of the clutch and you remove your foot completely from the accelerator pedal, the Jake Brake is automatically activated. There are some systems that will activate the Jake Brake only once the brake pedal is depressed.
Illustration 3 g00731603When you apply pressure to the accelerator pedal, the Jake Brake is deactivated.
Illustration 4 g00731767During shifting, the Jake Brake is automatically deactivated when the clutch pedal is depressed.
If the Jake Brake is on, shifting without using the clutch or double-clutching (to use the compression brake to reduce engine rpm--also known as "Jake Shifting") is strongly discouraged. Serious powertrain damage can result.
Illustration 5 g00731909Note: The Jake Brake will remain activated after the brake pedal has been depressed. This function gives the combined power of both the compression brake and the service brakes to the drive wheels.The ECM on vehicles that are equipped with ABS (Anti-Lock Brake System) has the ability to deactivate the Jake Brake if wheel slippage is detected. The Jake Brake will be automatically reactivated when wheel slippage is no longer detected.The ECM will deactivate the Jake Brake when engine speed falls below approximately 1000 rpm or when the vehicle slows down to a preset speed. These parameters may vary depending on the vehicle and the engine configuration. This prevents stalling the engine.
Be sure to turn the Jake Brake dash switch to the OFF position when the engine is turned off. This will prevent the switch from being in the ON position at engine cold start.
Automatic Transmissions
Operating the Jake Brake in a vehicle that is equipped with an automatic transmission is similar to operating the Jake Brake in a vehicle that is equipped with a manual transmission. The Jake Brake is activated when you move your foot off the accelerator pedal. The Jake Brake is deactivated when you apply pressure to the accelerator pedal. The ECM will deactivate the Jake Brake when the engine speed falls below approximately 1000 rpm or when the torque converter shifts out of lockup. The torque converter will usually shift out of lockup between 10 and 25 mph. This is dependent on the type of transmission.Note: With Autoshift transmissions, the Jake Brake may actuate in order to help the transmission upshift. This is done automatically through the transmission control module. This can happen even if the Jake Brake dash switch is in the OFF position.