Hey guy, I have a 1997 F350 Diesel Automatic w/OD and I believe I have a "J-brake"/Exhaust brake. I took took off my exhaust from what I thought was part of the turbo and saw inside there was a lever that could be opened and closed (but right now its open at half way). I can not turn it by hand and I felt underneath and took some pics of what looks like a lever that would turn the valve open and closed. Does the OD off button turn on and off the brake? There was a switch in the cab when i first bought the truck but no wires hooked up to it. There also is a random plug in the cab thats not hooked to anything. Once I figure out how to put pictures up it'd help my explanation. Any advice would be very helpful. Thanks.

 

First of all the device you are referring to is called a jacobs brake or jake brake. It is used on larger diesels to slow the vehicle with engine compression rather than relying on the braking system. The systems commonly sold for light duty diesels are called "exhaust brakes" and operate a flap in the exhaust system to add backpressure to slow the engine down. These typically are far less effective than jake brakes but are much easier to retrofit onto engines than jake brakes, since those typically require modifications to the valvetrain, heads, cam, etc.

Now, the flap that you found on your truck has nothing to do with either system. That device is called the exhaust backpressure valve, or EBPV. These are operated by oil pressure and commanded by the PCM to speed up the warm-up process of the engine by placing extra load on the engine. There are ways to wire up these valves to act similarly to an exhaust brake, but in stock form, that is not this valve's function.

 

Just expanding on what boggerted said. As I understand it, a Jake brake works by opening the exhaust valves at the top of the compression stroke. This releases the pressure so it doesn't push the piston back down. Obviously the piston had to work to compress it in the first place, so this is a VERY effective way to compression brake. It also makes a lot of noise as the high pressure compressed air is suddenly released. That's why it's banned in many communities (you'll see signs in places saying something like "no engine braking" or "no trepanning").

By the way according to wikipedia, "trepanning" or "trephination" is "a surgical intervention in which a hole is drilled or scraped into the human skull." It's also used as a generic term to describe a Jake brake (which as boggerted said is short for Jacobs, which is a brand name). I guess it comes from putting a hole in a head.

I was thinking that exhaust brakes were basically throttle valves in the exhaust, but I have no experience there, so I could be wrong. as boggerted said, exhaust brakes are commonly available for light truck diesels and they are a pretty good way to get engine braking out of a diesel. Not as good as a Jake brake, but a lot cheaper and quieter.

 

Many engines are not equipped to support jake brakes since it involves significant internal mods to work. Exhaust brakes, on the other hand, do not require the same level of modification.

 

Like boggerted and Nothing Special said, The "Jake" or engine brake is integral to the engine while an exhaust brake works by creating back pressure and not letting the engine exhaust efficiently. I read on the Jacobs Vehicle Systems site that an engine or Jake brake won't work due to the possibility of floating valves.

Jacobs Vehicle Systems | FAQs

Went back and looked and its the 4th FAQ on the list.