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Old 08-20-2014, 03:39 PM   #15
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Falcon190,
Everything you said is correct, except on the Freightliner chassis (since around 2007) when the compression brake (jake break) is on, it also turns on your brake lights. Their theory is you are "slowing down your RV" and therefore it is is same as pressing the brakes. So when you see the light on the dash indicating your brake jake is activated; it also means your brake lights are on.
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Old 08-20-2014, 03:40 PM   #16
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Quote:
Originally Posted by DieselTech39 View Post
The answer is no. Exhaust brakes and Jake brakes are equipped on diesel engines none that I know of for a gas engine.
Compression brakes, exhaust brakes and transmission retarders are needed on diesel-powered trucks and RVs because diesel engines do not exhibit engine braking the same way gas engines do. In a gas engine, speed is primarily managed by controlling air flow. The combustion fuel has always been a function of the air flow (that's what makes a carburetor work). Hence, when the throttle is closed the air flow is restricted and engine braking occurs.

With diesel engines, air flow is not controlled, fuel is the principal controller of engine speed. If fuel flow is stopped the engine will provide some engine braking but the fact that the air flow is not restricted causes this to be smaller than would occur with an equivalently-sized gas engine. Since diesels are often used on heavy vehicles, this lack of engine braking can result in more use of the service brakes especially in downhill situations.

Therefore, the compression brake was developed to provide additional braking by essentially using some of the engine's own power to assist in stopping the vehicle. The compression brake was invented and patented in the early '60's by Clessie Cummins, the founder of the Cummins Engine Co, but it was co-developed and marketed by Jacobs, hence the name "Jake Brake".

For smaller diesel engines the exhaust brake plays much the same function on a diesel's exhaust gas stream as a gas engine's throttle plate would on the intake air stream. The resulting constriction of the air flow provides additional engine braking.
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Old 08-20-2014, 04:59 PM   #17
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Quote:
Originally Posted by dengraham View Post
Falcon190,
Everything you said is correct, except on the Freightliner chassis (since around 2007) when the compression brake (jake break) is on, it also turns on your brake lights. Their theory is you are "slowing down your RV" and therefore it is is same as pressing the brakes. So when you see the light on the dash indicating your brake jake is activated; it also means your brake lights are on.
I'm on a Spartan chassis and I don't think the Jake turns on the brake lights, but I've never actually looked! Next time my DW is following me, I'll have her look.
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Old 08-20-2014, 05:22 PM   #18
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My 2000 freightliner turns the brakelites on with the exhaust brake.Found this trying to use cruise control with exhaust brake.
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Old 08-20-2014, 07:05 PM   #19
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Excellent explanation by docj!
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Old 08-20-2014, 07:09 PM   #20
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Glad someone has finally agreed that some MHs do the logical thing and bring on the brake lights when the auxiliary brake is in use. Trouble is - as one poster said, that means the cruise control drops out on undulating roads and has to be re-engaged. That is why I didn't bother using the auxiliary brake on flattish roads.
I've since mounted a floor switch so I can more easily engage the exhaust brake when it serves a useful purpose and then quickly switch it off again.
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Old 08-20-2014, 07:38 PM   #21
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Mine is labeled "retarder" assigns 2nd gear and works it's way down to 2. Has no affect one way or the other on my cruise control. I am fairly new to the whole diesel and the retarder but I use it almost all the time except on the open highway. I especially like it around town. I rarely us the brakes. I like the idea of a foot switch....please elaborate Tony Lee. I have always thought there should be a flap type switch on the back side of the steering wheel, much the way they make shifters now.


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Old 08-20-2014, 11:23 PM   #22
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(Snip)Later, on the same trip, I applied this new wisdom on the up to 11% downgrades on I-5 in southern Oregon.

One correction. The grades on I-5 in southern Oregon are not 11%. Interstate highway design criteria limits grades to 6%. Trucks trying to negotiate 11% grades weighing 80,000 to 100,000 lbs would probably be unable to climb an 11% grade at speeds greater than 10 MPH or so. In that case, there would be lots more rear Enders due to the speed differential.
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Old 08-21-2014, 12:58 PM   #23
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That's great information
Thank you
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Old 08-21-2014, 01:54 PM   #24
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Quote:
Originally Posted by Falcon190 View Post
I'm on a Spartan chassis and I don't think the Jake turns on the brake lights, but I've never actually looked! Next time my DW is following me, I'll have her look.
It does...unless you have an ECM flash to change that.
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Old 08-21-2014, 01:59 PM   #25
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Quote:
Originally Posted by Tony Lee View Post
Glad someone has finally agreed that some MHs do the logical thing and bring on the brake lights when the auxiliary brake is in use. Trouble is - as one poster said, that means the cruise control drops out on undulating roads and has to be re-engaged. That is why I didn't bother using the auxiliary brake on flattish roads.
I've since mounted a floor switch so I can more easily engage the exhaust brake when it serves a useful purpose and then quickly switch it off again.
I'm not sure I follow what you are saying regarding cruise control. If I active my EB and then set my cruise control the EB kicks in automatically at speeds about 3 MPH above the cruise control setting. For "undulating roads" such as rolling hills without extreme grades that works fine.
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Old 08-21-2014, 05:56 PM   #26
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I'm not sure I follow what you are saying regarding cruise control. If I active my EB and then set my cruise control the EB kicks in automatically at speeds about 3 MPH above the cruise control setting. For "undulating roads" such as rolling hills without extreme grades that works fine.
There are several different ways exhaust and compression brakes can be set up. On my MH the Jake can't be used with the cruise control, but changing the setup is something I could have my CAT dealer easily do. However, I'm used to the way it is and don't see this change as something I need to do.
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Old 08-21-2014, 07:39 PM   #27
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Different chassis builders implement their brake and cruise control interfaces differently. When mine overspeeds slightly from the cruise setting, the auxiliary brake comes in, the brake lights go on and the cruise control drops out.
All quite logical but would be a bit annoying to those who buy their MH with the exhaust brake switch in the on position and never figure out how to switch it off.

For other reasons I can't stand having the rig work its way down to second gear every time I want to lose a couple of mph so it stays off until it is needed.
To make that easier, I first shifter the switch off the dash behind the steering wheel where the ergonomic experts dictated it should be, to the side panel where it is exactly where my hand falls down off the steering wheel. Made it a lot more convenient and more useful.

Next step was to screw a $10 NAPA headlight dipping switch - push on, push off - to the floor on an angle where my left foot normally sits and wire it in parallel to that panel switch. In towns, if right foot needs to go on the brake, then the left foot can switch the auxiliary brake on and then off when brakes aren't needed. Much smoother ride and I still have all the advantages of reduced brake wear and quicker stopping, but only when it is needed. On the open road, no dropping out of the cruise control and no wasted fuel caused by unnecessary slowing down when you really want to coast followed by acceleration again.

Could probably have achieved the same effect by getting the system reprogrammed to the "latch Mode" and maybe changing the default from second (too low on the open road) to fourth. Trouble is anytime I bothered ringing an Allison workshop in my travels, they would always be happy to do it at a price, but not until the middle of next week.
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Old 08-21-2014, 08:57 PM   #28
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Jake Brake

Quote:
Originally Posted by Tony Lee View Post
...For other reasons I can't stand having the rig work its way down to second gear every time I want to lose a couple of mph so it stays off until it is needed...
That can be fixed. Here's what I posted about this in an older thread:

Our coach is built on the Freightliner XC-R chassis. It has the 400 hp Cummins ISL and the Allison 3000 6-speed transmission. This is a very common chassis-engine-transmission combination.

The ISL has a true Jake Brake - a 2-stage compression brake - as opposed to an exhaust brake. However, what I am about to describe ought to work on ANY chassis with an engine brake and the Allison transmission.

When using the Jake Brake, I found the braking action to be very aggressive. The Allison transmission is programmed to shift down to the next lower gear as soon as it can and not over-rev the engine in the next lower gear. It targets 2nd gear and will shift down all the way to 2nd as the speed drops. Those downshifts were dramatic and always caused a heavy braking surge as the downshift took place. That surge is hard on the drive train...and the passengers!

To say it another way, the Jake Brake is programmed to deliver 100% braking effort every time you use it. But there are times when less than 100% braking effort is desirable. Slowing for an interstate off-ramp, driving in stop and go 35 mph in surface street traffic or descending a gradual grade are all situations where the Jake Brake is useful, but not at 100% braking effort.

I took my coach to the local Allison transmission shop and had them reprogram the TCM to stay in 6th gear when the Jake Brake is energized. Now, instead of the computer controlling when the downshifts take place, I control the downshifts via the Allison transmission controller. The job took only 30 minutes and cost about $60. They connect a computer to the diagnostics port and make ONE change in the programming, changing "second" to "sixth" in the Jake Brake program.

What a huge difference!
Now I can actually use the Jake in many situations where I previously could not. Depending on the gear I choose, the braking effort can be gentle or aggressive, as I need. Slowing for an interstate off-ramp is now a joy, using 5th gear in the slow-down. No more slamming into 4th gear at 45 mph! Driving in stop and go traffic at 35 mph in 4th gear provides just the right gentle braking effort needed to speed up and slow down with the traffic. I don't need to use the service brakes until it's time to actually stop.

If you do nothing with the shift controller, the transmission will still downshift to lower gears as you slow down, but it does so at much slower speeds than before. The end result is nice, gentle braking right down to 15 mph where the Jake disengages.

To those of you who claim that doing this will diminish the braking effort available in an emergency stop, you may have a small point. However, an emergency stop is an event lasting only a few seconds. From the time you hit the service brakes to the point where the Jake is providing meaningful braking effort is...a few seconds! By the time the Jake fully engages, the emergency is over.

If you are in the habit of placing your transmission in "D" and forgetting about it, then this mod may not be for you. I don't drive our coach that way. I take a more active role in driving over the road, and often use the transmission controller to choose the correct gear for the particular situation I am in at the moment. The computer is NOT always right in this application. It can't be...it cannot anticipate anything, it can only react to something that has already happened. You have eyes that can look ahead and see things that are about to happen. You CAN anticipate, whereas the computer cannot.

This was the best $60 I ever spent on our coach. In my opinion, they should come from the factory programmed this way.



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