Help understanding Jake (exhaust) brakes

[Ray,IN]

FIRE UP is exactly right!
There is a caveat when descending a grade and using your exhaust brake. You must keep your speed under control!!!! If the TCM and ECM determines engine RPM's are exceeding the programmed limit, THE ALLISON TRANSMISSION WILL UPSHIFT to protect the engine. There is nothing you can do to prevent that. This means you only have your service brakes to bring your MH speed under control until your speed is reduced enough for the Allison to downshift again, and begin controlling speed again.


reference: Mountain Driving Tips from Workhorse | MotorHome Magazine

[RKins]

Originally Posted by lorim

It is a Jacob brand, but its an exhaust brake.. I know this for a fact, I have the paperwork. This is a 1999, 5.9 Turbo Diesel on a Freightliner with an Allison Trans, nothing auto downshifts when going down any hill and there certainly isn't any tow/haul mode either. If I want a lower gear I have to manually gear down or use the breaks to force a downshift which is not what I want to be doing on a long hill. I have an on/off switch on the dash and for the longest time I wasn't even sure which way was on or off. I came down a 5% grade from the tire shop this afternoon. This hill is short, maybe a little over a mile, but steep enough that it has a pull off for big trucks halfway down and a runaway sand-pile. Exhaust brake on, manually pulled down into 2nd gear and the exhaust brakes never engaged. I was at about 35 mph when I got to the bottom using the lower gear and stabbing the breaks as needed (there's a light at the bottom of the hill) The exhaust brakes never engaged, my suspicion is because I wasn't letting the MH speed or the RPMs get very high knowing I have to stop.

Have you found a switch labelled exhaust brake on the dash? Sounds like the system is off. When on, the tranny should start down shifting and the exhaust brake should activate as soon as you take your foot of the fuel pedal.

[Gary RVRoamer]

Lot's of "shudda" here but it's hard to be really sure which is correct. On a 1999 Freightliner, the Jacobs exhaust brake is an optional add-on, and there are a couple ways it could be programmed to work (or not). Given that there is an On/Off switch on the dash, we can probably rule out the floor pedal type. Don't know what Allison tranny model it is, though, and they have some differences in how they handle exhaust brakes. Generally, though, the engine brake is triggered by the accelerator pedal returning to zero (idle) when the EB switch is ON. The tranny is programmed to select a lower target gear (typically 4th or 2nd) as soon as the engine brake is triggered. However, the transmission will not actually downshift to the target gear until it can do so without over-reving the engine. If it cannot downshift at the current speed, use the service brakes to bring speed down enough for a auto-downshift. Note that if the revs are too high, manually shifting the selector won't help either - the tranny will still refuse to shift.


When you head down the hill with the EB switch ON, what gear number shows in the Allison gear selector window? Some show just one gear number - that would be the Target gear. Others show two numbers and those will be the Target and the current Actual gear, so you can see how the downshift is progressing.

[ASU777]

Here is a picture of a Jacobs Exhaust Brake from 2002 that was an add-on by Spartan. [Edit: Spartan calls it a Jacobs Engine Exhaust Brake in my brochure, FWIW] Like Gary said, it is connected via a coupler (with Jacobs logo) downstream of the turbo.

I don't have a picture of the damper inside the coupler but it's controlled by the piston you see in the picture. The damper looks like what you'd see in a multi-zone ducted AC system.

Mine is controlled by an on/off switch and tied to an Aux Braking light in the dash. I don't recall if it only lights up when the damper actually engages or when the switch is on. My knowledge is limited as I only got a chance to use it for a few seconds and had to turn it off due to a problem.

[Ray,IN]

Originally Posted by RetiredinFL

Here is a picture of a Jacobs Exhaust Brake from 2002 that was an add-on by Spartan. [Edit: Spartan calls it a Jacobs Engine Exhaust Brake in my brochure, FWIW] Like Gary said, it is connected via a coupler (with Jacobs logo) downstream of the turbo.

I don't have a picture of the damper inside the coupler but it's controlled by the piston you see in the picture. The damper looks like what you'd see in a multi-zone ducted AC system.

Mine is controlled by an on/off switch and tied to an Aux Braking light in the dash. I don't recall if it only lights up when the damper actually engages or when the switch is on. My knowledge is limited as I only got a chance to use it for a few seconds and had to turn it off due to a problem.

Attachment 258607

Attachment 258608

Switch on, light is on. If I remove my foot from accelerator, my shoulder harness/seat belt keeps me from leaving the seat when I forget to turn off the switch.

[RKins]

LOL
Mine doesn't seem that powerful but I can tell you I can tell the difference when the "system" is working and when it isn't.

[jcussen]

Think most will find that exhaust braking performance will depend on your exhaust valve springs. On my 3208 could not install effective exhaust brake with standard exhaust valve springs because they would float with high back pressure. Heavier springs solved the problem. Exhaust brake will still work with standard springs, but flapper on exhaust will not close as far to prevent valve float and not provide as effective braking.
https://www.cumminsforum.com/forum/8...e-springs.html

[Gary.Jones]

Roadkyll

The answer to your questions is that some owners leave the Jake on all the time, and some use it only when going down big and long and steep hills. Now, one of the engineers at Cummins/Spartan used to always say in his lectures (he's retired now.... "Big Mike") that in his opinion, motorcoach owners often use their jake brakes way way way too much. His opinion was that service brakes are on the coach for most routine stopping and they do their job very well. He advocated that the jake/engine brakes should be used primarily when your situation is overstressing the service air brakes.... e.g. when going down very long steep hills.

That is my policy. There are essentially only 2 times I have the jake turned on (my coach is a 605 hp Cummins X-15 which has three levels of jake (low, medium, high)) and those two times are when going down Colorado or major type mountains/hills. Any hill that shows signs that semis should downshift or come to a complete halt at the top of the hill to test brakes and then begin in a low gear are the types of hills that I use the jake. The second time is if I encounter a 360 degree off ramp when I anticipated a gentle turn off ramp of an interstate and I have miscalculated and am still going too fast as I enter the off ramp. Both of those times, the jake is turned on and the level adjusted for what I need. I can descent an 8% 7 mile long grade virtually never touching my service brakes at a nice safe speed, by rocking the jake from off to low to middle to off to low to middle to high to middle, etc. as I descend to maintain, increase, or decrease speed. It is an amazing capability!

Now, I find that if you have it on all the time, your ride is very jerky as the jake kicks itself on and off depending on what you are doing with the diesel foot pedal. You have to learn to "feather" the diesel feed so you are not accelerating but also not decelerating to keep the jake from kicking-in. I don't like the ride of a coach that has the jake on all the time.... most drivers are not excellent enough to keep feathering the diesel feed to coast or coast smoothly. If you leave it off until you need it then you can coast just fine..... that is the way Big Mike says to do it, and the procedure I follow.... off until it is really needed.

Several of the responses that the OP has gotten are excellent and come from very experienced and knowledgeable owners.... if you follow all the advice you have gotten here, you will be in good shape.

Gary

[rodekyll]

Nice answer.

[Dutch Star Don]

This discussion becomes much easier if you refer to one as an exhaust brake (blocking off the exhaust) and one as an engine brake (closing of the valves). Using the word Jake, muddies the water since they offered both an exhaust and engine brake.

EXHAUST BRAKE......These are the least effective of the two. Using them at higher speeds, 60 and above, will rarely offer much braking. You need to experiment with YOUR exhaust brake and see what it does at different SPEEDS and different GEARS.

As an example....I had a 2005 Monaco Diplomat with a 400ISL and exhaust brake. As stated, at higher speeds (60+) it didn't do much unless it was accompanied by some hard foot braking. Once you learn where it works best, you'll get some good performance. I knew on my Diplomat, while towing a truck, the exhaust brake was NOT enough to slow the coach on a 6% grade without using the foot brake regularly. Each time I was able to get the coach to a lower gear, after helping with the foot brake, the exhaust brake became more effective.

On my Diplomat, if I could get the coach going slow enough that it would downshift to 3rd gear, the coach would hold almost any grade at about 45 mph without using the foot brake. You'll soon learn what YOUR speed needs to be on grades you regularly descend. So......play with your exhaust brake and learn it's capabilities. Both engine brakes and exhaust brakes on MOST coaches, will shut off at 15 mph.

TIP.....On the newer coaches, with an engine brake, they interact with the cruise control so as you descend a grade, you can set your cruise and the engine brake will turn on and off to maintain the speed you set the cruise to. On my last two coaches, Cummins set the engine brake to come on approximately 7 mph after I exceeded the cruise control setting on my 2014 Dutch Star and 9 mph on my 2019 Dutch Star. This meant that if I set the cruise to 55 mph, my 2014 would speed up to 62 mph before the brake engaged. On my 2019, it's 64 mph before the brake came on. On both coaches, this was much faster than I wanted to go. There are two solutions, either back the cruise off 7 or 9 mph, or have the parameters changed.

I chose the second on both coaches and just had my 2019 done today and will try it out tomorrow. Cummins, via the Insite computer program, can change when the engine brake comes on in relationship to the cruise setting. I had both coaches reprogrammed to come on at 2 mph above the cruise control setting. This makes it much easier to just set the cruise on a downhill and let the coach do the work. The parameter change only takes a couple of minutes, but you'll typically be charged an hour labor.

[jcussen]

Jacobs is a company that for many years only made a engine compression brake that used engine compression to slow the truck or coach. People shorted that up and it became known as a "Jake brake". Appears "jake brake" is now registered by Jacobs to avoid confusion between it and their exhaust brake. The same company, Jacobs Vehicle Systems, now makes exhaust brakes which they call the "Jacobs exhaust brake" Big difference between "Jake brakes" and "Jacobs exhaust brakes".
https://www.jacobsvehiclesystems.com...ne-brake-works
Most old truckers and owners of big rv's know the difference between "Jake brakes" and any brand of exhaust brakes. But some people use them interchangeably. Have had both, find the "jake brake" much more effective.

[lorim]

This got a lot more discussion than I imagined so I wanted to report back on my trip. First thing, I opted not to tow because I just wasn't comfortable (I've never towed this wasn't the trip to get me acquainted)
I live in PA we have tons of steep, short hills that using the service brakes is not a big deal. I drove a school bus for 10 years and there was no breaking assistance on any of those units.. it was gear down. So I still have this urge to gear down, I don't know if this will ever go away.

I just left the exhaust break switch on the whole trip. On the way up 66/28 I geared down and crept along, this is a very windy and hilly secondary two lane highway, hogback hill has a 9% grade with a few hard turns.. this hill blows to go down in a car. Once I hit 80 I was still a tad rattled and was still manually gearing down on any grade warnings. At times, it was a very slow ride out..
On the way home I decided to really give the exhaust break a chance to do its thing and refrained from manually gearing down. To my surprise on 80, it was holding me back right around 50 MPH and I only felt the urge to stab at the breaks a few times. Now when we got off 80, back on 66/28 things weren't as comfortable and I felt like I was using the daylights out of my service breaks, I doubt I'll ever attempt that again.