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Old 10-05-2006, 03:28 AM   #1
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A couple of weeks ago I asked about the Jake brake coming on automatically on a down grade while in cruise control - opinions were mixed.

We have had some long, long 4-6% grades in Wyoming and South Dakota and had plenty of opportunity to test the situation.

What I found was that the Jake brake will indeed activate when you reach a speed differential (set vs. actual) of about six to seven mph.

It was also interesting to note that in light winds and a very long down grade we apparently have so much wind and rolling resistance we would only build up speed of four to at the most 10 mph over our cruising speed of 65 mph. I never want to take the coach over 75 so that's where we put our foot down (or let the Jake put its foot down )
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Old 10-05-2006, 03:28 AM   #2
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A couple of weeks ago I asked about the Jake brake coming on automatically on a down grade while in cruise control - opinions were mixed.

We have had some long, long 4-6% grades in Wyoming and South Dakota and had plenty of opportunity to test the situation.

What I found was that the Jake brake will indeed activate when you reach a speed differential (set vs. actual) of about six to seven mph.

It was also interesting to note that in light winds and a very long down grade we apparently have so much wind and rolling resistance we would only build up speed of four to at the most 10 mph over our cruising speed of 65 mph. I never want to take the coach over 75 so that's where we put our foot down (or let the Jake put its foot down )
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Old 10-05-2006, 04:29 AM   #3
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No better way to find out for sure than test it

Thanks for posting the definitive answer
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Old 10-05-2006, 06:29 AM   #4
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John,

That's OK out there but do not try it in the Ozarks when you are not on an interstate. Down hill grade of anywhere between 6% and 11% and not normally posted - narrow 2 lane roads - no shoulders - and there is ALWAYS a curve, either half way down or at the bottom. When you top the crest, that is the slowest you will go unless you are on the service brakes. I did not have the guts to use the cruise control but the Pac Brake was never turned off. Not exactly a relaxing drive.

I will have to do a test with the Pac Brake one day. I do not remember it coming on when I was in your situation but I could easily be wrong.

The good news is that the mountians are not as high as the Rockies.
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Old 10-05-2006, 11:59 AM   #5
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">What I found was that the Jake brake will indeed activate when you reach a speed differential (set vs. actual) of about six to seven mph. </div></BLOCKQUOTE>

Hi John,

Thanks for the info. I guess I miss led you with my reply to your question, since I have not allowed the speed to increase by 7 MPH over my usual set speed of 62-63 MPH. I will give it another test as you now have done as I am curious if it still works the same. I had my Jake reprogrammed to not come on unless I depress the brake pedal vs full release of the throttle. This allows me to coast when I release the throttle.
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Old 10-05-2006, 01:34 PM   #6
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I wonder if this is only a feature of the newer rigs with a Jake brake. My 1999 Itasca doesn't work this way....or at least I've not had it in a situation to test as John has.

While my JB is engaged with the cruise on, it tries to bring the vehicle to a stop. Otherwise it works perfectly. Is this a new feature or should my 99 have this as well??
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Old 10-05-2006, 03:38 PM   #7
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mach428:
I wonder if this is only a feature of the newer rigs with a Jake brake. My 1999 Itasca doesn't work this way....or at least I've not had it in a situation to test as John has.

While my JB is engaged with the cruise on, it tries to bring the vehicle to a stop. Otherwise it works perfectly. Is this a new feature or should my 99 have this as well?? </div></BLOCKQUOTE>Don't have a clue - six years is a long time in product development evolution.
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Old 10-08-2006, 09:24 AM   #8
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Mach428,

I think I'd call Cummins or CAT and ask them. Seems to me your cruise control should allow you to cruise with the JAKE in the "ON" position. Mine does.

I think you might just need some sort of minor adjustment.
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Old 10-09-2006, 01:27 AM   #9
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John:
Our '03 Journey with the CAT 3126 and exhaust brake works in a likewise manner. The exhaust brake WILL come on in cruise going down hill, depending on speed, etc, it might not if you started down the hill at a speed too high for the Allison to down shift.
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Old 10-09-2006, 02:05 AM   #10
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I guess I had a case of brain fade. I re-read the post by John C. and agree; that's the way mine works too.

Ahhh, getting old is so frustrating, but I guess it is better than the alternative!
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Old 10-15-2006, 03:54 PM   #11
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I see the word JAKE brake used and exhaust brakes. Cummins and Cat do not have a Jake system only used buy GMC and some large truck engines and most are used in the 450 to 500 HP engines. A Jake system closed down engine valves as needed to slow down unit. Exhaust brakes close down the hold exhaust pipe to slow unit down. I have only found Jakes on couple motorhomes with 500 HP engines.
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Old 10-15-2006, 06:37 PM   #12
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by John Blue:
I see the word JAKE brake used and exhaust brakes. Cummins and Cat do not have a Jake system only used buy GMC and some large truck engines and most are used in the 450 to 500 HP engines. A Jake system closed down engine valves as needed to slow down unit. Exhaust brakes close down the hold exhaust pipe to slow unit down. I have only found Jakes on couple motorhomes with 500 HP engines. </div></BLOCKQUOTE>John - nope.. we have the honest-to-goodness Jacobs compression brake with a low/high switch. Low position is three cylinders, high position is all six cylinders acting like a giant air compressor.

I might be wrong but I think most chassis/Cummins ISL engines are spec'd with the Jacobs brake for competitive purposes by motorhome manufacturers.
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Old 10-15-2006, 08:27 PM   #13
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I see the word JAKE brake used and exhaust brakes. Cummins and Cat do not have a Jake system </div></BLOCKQUOTE>

If it is a Jake brake you need to call it a Jake brake. It has 2 settings High = 6 cylinder braking and Low = 3 cylinder braking. Almost never use High setting as it really breings the coach to a halt. Low or 3 cylinder braking still works better that my last coach which had a Exhaust brake. The Horizons and Vectras truely have Jake brakes.
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Old 10-16-2006, 04:42 PM   #14
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John,

I checked Cummins web site and see the ISL used a C Brake by Jacobs. This is a exhaust brake system only. Pacbrake and couple other exhaust brakes people use this system. A "Jake" system will have wires that come out of the valve cover and hook to a solenoid on engine valves. Engine on slow down are very loud and has the machine gun sound. I have never found this sound on any motorhome todate. Below is a good web site to read up on brake system.

On our unit we have exhaust and transmission retarded brake (done with oil). On a down hill run system will drop down to gear 5 if you need more hold back, down to 4. At gear 3 the exhaust brake kicks in and will stop you very fast. So we only need it in city travels to kill off speed fast if you need to.

Some systems close the exhaust valve part way on low and all the way on high. Lots of ways to slow engines down.

Jake Brake information

Check that engine and see if wires come out of valve cover. Also if you have a "Jake" you will not see a valve on exhaust pipe.

I would like to know.
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Old 10-16-2006, 06:50 PM   #15
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John Blue,
If you go to the Freightliner website and look at some of the specs for the XC chassis with the 400 hp Cummins you will see on the "Features" on the spec sheet that the "compression" brake is used which is a real "Jake" brake.

Here is the link for the Vectra: XC Chassis for Vectra

Here is the link for a list of manufacturers that use the XC Chassis: Manufacturers List using XC Chassis

I don't believe the CAT C7 300, 330, or the 350 hp engines are offered with the "Jake" brake but they use the Exhaust brake.

Ron
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Old 10-17-2006, 01:39 PM   #16
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by John Blue:
John,

I checked Cummins web site and see the ISL used a C Brake by Jacobs. This is a exhaust brake system only. Pacbrake and couple other exhaust brakes people use this system. A "Jake" system will have wires that come out of the valve cover and hook to a solenoid on engine valves. Engine on slow down are very loud and has the machine gun sound. I have never found this sound on any motorhome todate. Below is a good web site to read up on brake system.

On our unit we have exhaust and transmission retarded brake (done with oil). On a down hill run system will drop down to gear 5 if you need more hold back, down to 4. At gear 3 the exhaust brake kicks in and will stop you very fast. So we only need it in city travels to kill off speed fast if you need to.

Some systems close the exhaust valve part way on low and all the way on high. Lots of ways to slow engines down.

Jake Brake information

Check that engine and see if wires come out of valve cover. Also if you have a "Jake" you will not see a valve on exhaust pipe.

I would like to know. </div></BLOCKQUOTE>Let's try this one more time... We have the Jacobs brake and not an exhaust brake. We do not however have a transmission retarder like you do.
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Old 10-17-2006, 02:53 PM   #17
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Well they must have started that with the ISL engine because my 04 Vectra 40' 350 hp has an exhaust brake and as far as I can tell there was no Jake Brake option. Also the exhaust brake will not work with the cruise on. Activating cruise disables the exhaust brake.
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Old 10-17-2006, 06:21 PM   #18
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Bill Gotthelf:
Well they must have started that with the ISL engine because my 04 Vectra 40' 350 hp has an exhaust brake and as far as I can tell there was no Jake Brake option. Also the exhaust brake will not work with the cruise on. Activating cruise disables the exhaust brake. </div></BLOCKQUOTE>

Bill,

Yes, You are correct. The compression brake "Jake" started with the Cummins ISL 400 hp on the Freightliner chassis. All the lower hp engines use the exhaust brake.

On my Freightliner/CAT, it is like yours the exhaust brake is disabled with the cruise on and set. Just touch the brake pedal to disable the cruise and the Exhaust brake will work.

I have never checked to see if the exhaust brake would kick in if the down hill speed exceeded the set cruise speed as John Canfield has described.

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