What Are Jake Brakes and How Do You Use Them?

Jake brakes are an important part of a semi-truck that can save its owner a significant amount of money over the life of the truck due by reducing the amount of service needed. In addition, these brakes also provide an additional layer of safety to the driver of the truck.

In the article below, I will cover how a jake brake works as well as the pros and cons of using it.

For now, to answer the basic question of “What is a Jake Brake?” It’s an additional braking system that allows a driver to slow their trucks without having to apply their air brakes which saves wear and tear on the brake lines and pads.

Inventor and History

The Jake Brake was originally known as the Jacob brake which was named after the company that began producing it in 1961. Ultimately, the original patent for the jake brake was submitted and awarded to Cummins Diesel Company in 1962.

The person credited with inventing the jake brake was Clessie Cummins, who is famous for founding the Cummins Diesel Company in 1919.

Up until Mr. Cummins founded his company, the diesel engine was not a popular choice for engines except for farmers and the military. His company struggled in the earlier years and was at risk of folding several times due lack of profits.

It wasn’t until the start of World War II that Cummins Diesel Company began to experience consistent sales due to the military purchasing engines as quickly as they could be produced for the war efforts in Europe. (Source)

How Engines Run

All engines gain their energy by creating small, controlled explosions within the engine block. Whether it’s a small engine on a lawn mower or a large aircraft engine; they need an explosion to occur to gain the energy that will be used to move the vehicle.

Modern technology has progressed so that computers and sensors are used to ensure the engine is using the most efficient mixtures of air and fuel to create these explosions. If the engine isn’t getting the proper ratio of these three elements it won’t run efficiently. If any one of these is missing, your engine will not run.

Before covering the way that jake brakes work, I think it’s important to touch on how gasoline engines work. This helps compare how the two engines work and will allow better understanding of how the jake brake works.

Gasoline Engines

If you are driving a gasoline car or truck, you open or close the throttle by pressing down or letting up on the gas pedal. When you press down on the gas pedal, not only are you sending additional fuel to the engine, but you are also opening the air intake to receive more air into the engine.

Computers and sensors in the engine are constantly calculating the fuel and air intake amounts to find the correct mixture.

The opposite actions occur when you let off the gas pedal. Odds are, you’ve been driving a gasoline car before and needed to slow down a bit but didn’t need to engage your brake. Therefore, you simply let off the gas pedal and your vehicle began slowing on its own. Part of the slowdown is due to friction between your tires and the road, but most of your slowing is due to the throttle being loosened which significantly reduces the amount of air going thru the intake.

This reduction in air causes less pressure to build in the engine, which results in less energy, thus allowing your RPM’s and speed to slowly decrease.

The key point to remember is that the throttle cable (gas pedal) reduces the amount of air going into the end when the pedal is let up on.

Diesel Engines

Throttle cables do not control the amount of air that enters a diesel engine. As a result, when a truck driver lets his foot off the gas pedal, the air going into the engine is not cut off. Pressure and combustion continue to build until the amount of fuel being delivered to the engine is reduced. It’s estimated that the pressure build-up in a diesel engine is about double that of a gasoline vehicle. This additional pressure is what gives the trucks so much extra horsepower to carry their loads.

Prior to the invention of the jake brakes, the only way a driver could slow down is by using his air brakes. Using your air brakes constantly with the amount of weight a semi carries will result in the driver having to constantly replace their brake pads.

This is the reason the engine brake was invented. When the driver engages the engine brake switch in the cab of the truck, it triggers openings on the engine which releases the pressure that has built up in the engine. Once that pressure is released, the engine loses it’s power and the truck begins to slow down.

This sudden release of all the built up pressure in the engine is the reason why engaging the jakes are so loud.

Where Is The Jake Brake Located?

As we covered in the section above, jake brakes are used as an engine braking system, therefore they are located within the motor of the truck. In order to engage the brake, there is a switch or handle that must be engaged or turned on inside of the cab of the truck.

Are Jake Brakes Bad For The Engine?

This is one of the biggest myths in trucking. The answer is NO. Using an engine brake has zero impact on the life of your truck. The jakes are simply releasing compression in the engine as a way to reduce the energy being sent to the crankshaft. There is nothing in this process that is harming the engine. I’ve never spoken to a truck who have blamed their engine troubles on their jakes.

What Do ‘No Jake Brake’ Signs Mean?

When driving on smaller highways and interstates that pass near residential areas, you may notice signs that say “No Jake Brakes Within City Limits” or “Engine Braking Prohibited.” Many cities pass laws banning the use of jake brakes due to the loud noises that the brakes emit.

Anyone who has traveled or lived near a major highway has heard the distinct sound that the jake brakes use when engaged. In this video, you can see and then hear the driver engage the brakes.

[embedyt] https://www.youtube.com/watch?v=qocMoTOVn6Q[/embedyt]

Over the past few years, there have been improvements in the technology used on jake brakes which has significantly reduced the noise levels of them when engaged. The noise levels have been reduced so much, that many truckers are able to use the braking system in towns that prohibit the use of engine brakes.

Can You Install a Jake Brake On a Used Truck?

Yes, it is possible to get a jack brake installed on a truck that didn’t have one originally. If you are buying a used truck that was purchased new from some of the major fleet operators in the United States, odds are those trucks will not have a jake brake. Many times the larger fleets will elect to not have jakes installed on new truck purchases in order to save money.

peterbilt red cabHaving a truck retrofitted with a jake brake isn’t a difficult job, but it’s important to make sure the mechanic doing it knows what he is doing. Total cost of the retrofit will vary significantly based on your truck and where you are located. On average, I would estimate the parts and labor to run anywhere from $3,000 to $5,000 to have the install done.

You may be surprised to learn, that retrofitting a truck with a jake brake will often require the purchase of a new wiring harness, as well as having to get your ECU reprogrammed. Sometimes the wiring harness can be modified depending on the model of your truck.

Getting an ECU re-programmed is not cheap. Though it’s easy to do, the expense comes from the mechanic having to purchase the software license to complete the programming. If you mechanic quotes you $500-$1,000 to reprogram, don’t be surprised. Most often, he’s not the one pocketing the money though.

Snow and Rain

This is a subject that can be a bit controversial. The problem is that many times drivers want to answer this question with an absolute “Yes” or “No” but neither is correct. I’ll attempt to explain the thinking from both sets of drivers.

The one drawback to using jakes in conditions that are not dry is the fact that they work by slowing the trucking using your tractor drive tires. If the driver isn’t careful, the jakes can cause these tires to lock up which can lead to loss of control and even a jackknifed truck.

Due to these reasons, it is highly recommended that new drivers and drivers in a new truck, NOT use jakes in conditions where the roadway is not completely dry. It’s important for drivers to have a solid level of experience both on the road and in their specific truck before using the brakes in less than ideal situations.

The key to using the jakes in wet or slippery conditions is to avoid relying too much on the engine braking. It’s important to keep the jakes in low power and use the air brakes more than you would in normal, dry conditions.

Even if you are an experienced driver with several years of experience, it still may not be worth the risk of using your jakes in slippery conditions. Many companies have policies against using the jakes in wet conditions as well as other things like such as cruise control. Should you be involved in an accident while using the jakes in slippery conditions, it very well could cost you your job.

Do Jake Brakes Decrease Fuel Mileage

This is a myth that has been around for almost as long as the jake brakes themselves. It had to be started by someone who had no clue how the jakes worked. The jakes have nothing to do with the fuel in your truck. In fact, fuel is turned off to the engine when the jakes are engaged.

Additional Related Questions

Why won’t my jake brake work?  As with most everything on the truck, jake brakes will fail on occasion. Fortunately, these failures won’t cause you to be stuck on the side of the road, but they can make driving much more difficult if you are in mountainous terrain. Below are some of the most common issues we’ve seen when it comes to jake brake failures:

  • Solenoid failures – You should be able to locate these under the valve covers to test
  • Electrical/wiring failures – the engine brake is controlled by two wires in the wiring harness which receive operations signals from the ECU. If these wires have suffered any type of damage, you will have issues with your brake. In addition, the wiring between your ECU and the switch in the cab of your truck could cause problems if it has been damaged.
  • Not getting proper oil pressure – When troubleshooting your brakes, it’s helpful to pay attention to your oil pressure levels when you engage and disengage them. Somethings sludge can build up and the oil pressure doesn’t get high enough for them to engage the brakes.
  • Broken Control Valve Springs – Some of the valves in your engine have springs for the jake brake that will brake and cause issues. Expect to spend around $1,000 to have them replaced.


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