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Questions About Descending Grades

Yahoo Message Number: 86330
I am sure this issue has been discussed and I searched a bit, so please don't flame me for bringing it up again.
We have been flatland drivers for most all of the three years we have owned our 03 Intrigue (400 ISL with 2 speed Jake). This year we will be traveling in the west and will be facing more grades.
I don't mind going up, but am not a big fan of coming back down. I find that on 6%+ grades even with the Jake on high and the trans automatically dropped to 4th, I will gain speed. (I would prefer to come down at about 50mph.)
I tried once in the east to nearly stop at the top of a grade and put the trans into 3rd before starting down. It held me back at a slower mph but it seemed the revs quickly became too high and I got an engine warning light. (I wasn't watching my temps, but it wasn't that long of a grade.) I had to use the service brakes to further slow my speed to lower the rpms, but I was in a lower gear hopefully to avoid the use of my service brakes.
So, help me out a bit here. What was I doing wrong and how should I best deal with steep down grades? Thanks.

Gary

03 Intrigue 11509

Re: Questions About Descending Grades

Reply #1 –
Yahoo Message Number: 86335
I have been driving in the west for three years and have finally gotten over coming down steep grades. The first thing is to turn off the cruise control and make sure the Jake brake is turned on. . Then I will downshift to 4th or third if needed the engine will mostly hold the speed to around 45. I may have to touch the brakes every 30 seconds to one minute but I don't have to ride the breaks . After coming down a really steep grade like the one just south of Ashland the engine and transmission are cooled down to 100 degrees.

Brian O'Day
ePRINT
9970 SW Greenburg RD
Portland, Oregon 97223
(503)684-2679

Re: Questions About Descending Grades

Reply #2 –
Yahoo Message Number: 86337
Gary,

It sounds to me like you are doing the right things. The Allure with ISL that I had performed perfectly when used as you described. It may be that you are much bigger and heavier than my 33' coach was.
I suspect that your engine brake may not be working on high. When I would descend any of the interstate highway grades a 4th or 5th gear and high engine brake would control my speed fine. On the us or state highway grades a 3rd gear approach was needed. I suggest that you get your engine brake checked by Cummins.

Good luck.

George in Birmingham
'03 Magna 6298
George in Birmingham
2003 Magna 6298

Re: Questions About Descending Grades

Reply #3 –
Yahoo Message Number: 86339
Gary

Another thing to keep in mind is that the indicator panel for the Allison is only showing the target gear not necessarily the actual gear. Often when you let off the throttle descending a grade the indicator will show 4th but the actual gear may be 5th. Often you have to punch the service brakes a couple of times to get the rpms low enough for the Allison to down shift to 4th.
The Allison is always in control of what gear it is in. The driver can only request a gear but the Allison will not go there unless the Allison determines that doing as you ask is not going to over rev the engine. It can and will up shift any time the rpms get too high going down grade in your requested gear even if it requires shifting into neutral. Having the Allison up shift on a steep downgrade is not pleasant experience.
Controlling your speed using the engine break in conjunction with the Allison is all a matter of controlling the rpms. Also remember that braking action provided by the engine brake increases with rpm in any given gear range. The max can be obtained just below the engine red line.
The old adage to go down the hill in the same gear you went up has some merit but has a couple of problems. First it is much more applicable to large trucks with standard transmissions and second the other side of the hill may not be the same grade. It is really only good for your return trip.

Don Seager
Ex CC Owner

George wrote:

Gary,

It sounds to me like you are doing the right things. The Allure with ISL that I had performed perfectly when used as you described. It may be that you are much bigger and heavier than my 33' coach was.
I suspect that your engine brake may not be working on high. When I would descend any of the interstate highway grades a 4th or 5th gear and high engine brake would control my speed fine. On the us or state highway grades a 3rd gear approach was needed. I suggest that you get your engine brake checked by Cummins.

Good luck.

George in Birmingham
'03 Magna 6298

Re: Questions About Descending Grades

Reply #4 –
Yahoo Message Number: 86341
Gary,

The rule of thumb is what ever gear you need to climb in is the gear you go down in. I usually select one gear lower, put on my 4 ways and enjoy the ride. 50 mph might be to fast on mountain grades.

X-professional driver with over 600,000 miles.

Michael

05 Inspire #51381

**************************************** Michael Title

Halfmoon Bay, BC, Canada
CDN Cell: 604.741.3328

San Miguel De Allende, GTO Mexico MEX Cell: [52] 415.100.1543

Re: Questions About Descending Grades

Reply #5 –
Yahoo Message Number: 86342
I think that one other issue not touched upon is the weight of your toad. Even with a good air brake system like SMI, the toad is not being braked until you actual apply your service brakes. With engine braking you have another 3,4 or 5000 lbs pushing you down the hill which may well overwhelm the ability of the jake brake. I have a 3 speed jake brake and even on high coming down a steep incline I have to step on the brakes once every 30 seconds or so to bring down the speed. I have always surmised that since my toad is a Chevy Suburban which weights about 6000 lbs (close to 1/6 my coach weight), this is to some extent overwhelming the ability of the jake brake to completely control my speed. But after reading the posts, next time I am going to manually select a gear (say 4th in my Cat 6 speed transmission) and then put the jake brake on high (with no cruise control) and see what happens. I presume that like the Alison, my Cat CX-28 will not allow the engine revs to go higher than is safe without upshifting.

Joseph Burkle

2008 Country Coach Inspire 360 Founders Edition 43' quad slide tag axle 425 HP C9 pusher http://www. Wind-seeker.com/

Re: Questions About Descending Grades

Reply #6 –
Yahoo Message Number: 86343
Hi Gary - I'll add a bit more info to what the rest of the gang have provided already.
The ISL is a good engine, but is really not that large of displacement, and it is the compression ability of the engine, that provides the Jake Brake stopping assistance. The ISL being smaller, will not provide the same braking assistance as the Big CAT or ISM-ISX Cummins engines will. Most of the bigger blocks, have three stage brakes.
My ISL Jake in High, was not providing all of the compression it could. Cummins had to remove the valve cover, and make some adjustments to get High Jake working fully. (As I recall it, the valves were not fully closing, so some compression was lost.) This might be worth having checked on your ISL.
The last thing I just learend, talking with a mechanic at Cummin Cal Pacific, is that I did not understand how the Allison and Jake interacted with each other. As Don pointed out, the shown default 'target' gear is 4. It may not be the gear that you are in, depending upon speed. The other thing I learned, is the even though it shows 4, the Allison brain still thinks you are in Drive/6 Gear. So when I thought I was going from 4 down to 3, by hittig the down button one time. I was not. The Allison brain was going from 6 down to 5. So for me, if turning on the Jake wile in 6th gear, I needed to go 'Down Arrow' three times, to get it to go down into 3rd gear. (Four times for 2nd gear.) As Don and others pointed out, the engine and Allison brains work together, to protect each other. So you may not get down into 3rd, until the speed RPM combo is safe to do so.
I've gone out and tested this a few times now, and dang if I now feel much more braking while in High (with the fixed Jake) and going into 3rd or 2nd, depending upon grade. Still kind of strange to see the "4" Jake Default Target Gear displayed...
OK, really, the last comment:)! Remember when using the service brakes in combination with the Jake's, to brake strong and steady until you reach the speed you want to reach, then get you foot of the brake so it can 'cool' between the next need to slow down. (No riding the brake slow and gradual.) If I want to be ar 45 MPH on a steep grade, I'll start by slowing down at the top, hitting High Jake, then Arrow Down three times to grab third gear (watch the RPM's and you will get a feel for the gear and speed). Then if speed climbs say up to 50 MPH, I'll brake hard and steady until I hit say 40 MPH, then get off the brake and let the Jake hold it until it climbs back up again. (This was an example only, to ge the idea of Jake, Gear and Brake interaction.)
If you have a good hill in your area, go out and have some fun and see how this all works together.

Best of luck, have fun, and be safe:)! Smitty

04 Allure 31017 ISL370
04 Allure Sold to a nice new owner! Now enjoying a 07 Magna Rembrandt 45' ISX600 #6775
"We're ONDROAD for THEJRNY!" (Toad and Coach license plates, say Hi if you see us!)

Re: Questions About Descending Grades

Reply #7 –
Yahoo Message Number: 86344
Gary,

I have almost the same coach as you, '03 Intrigue w/400ISL. I find the Jake brake so ineffective that I thought it wasn't working. Even after I tested everything and satisfied myself that it was working, I took the coach to Cummins Coach Care and had them check it. They verified that it was working and said that the coach is too heavy for the brake. That confuses me since any truck that engine goes in would probably be heavier than the coach. I have found that starting the descent with very slow speed and the transmission in 2nd gear, then upshifiting as RMP dictates until I'm in 4th gear, then using the brake to keep the rpm's slow enough for 4th gear gets me down with the fewest number of clinches.
My previous coach (SOB) had a Cummins 300 with a six speed Jake that would throw you forward in the seat and bring the coach to a stop. I sure miss it. (just the Jake, not the coach!)

Greg Jones

'03 Intrigue 11571

Re: Questions About Descending Grades

Reply #8 –
Yahoo Message Number: 86345
The brake horsepower of a Jake brake is related to the size of the engine. Your 9 liter engine will probably have somewhere near 300 bHP while your coach has the weight of a 43 foot coach with tag and suburban. A 12 liter engine will have somewhere nearer to 400 bHP. That probably makes you one gear lower on a down grade than a similarly sized coach with a larger engine. Unless you wish to use your service brakes more frequently.

George in Birmingham
'03 Magna 6298
George in Birmingham
2003 Magna 6298

Re: Questions About Descending Grades

Reply #10 –
Yahoo Message Number: 86351
Lot's of great info in the responses! Thanks
My coach weighs 35,600 lbs with full fuel and water. My toad is a Ford Focus wagon which even with cargo carrier, golf clubs, and other stuff probably is only around 3,000 lbs. So I am generally running around 38,000 lbs for the total rig. Maybe that is a bit on the high side for the 2-stage engine brake capabilities of an ISL.
I think I will have the jake checked by Cummins, but I do seem to feel two stage action to it.
I also need to find some hills and without a lot of traffic so I can just experiment more -- high and low jake, different gears, real vs indicated gears etc.

Yes, I do only jab the brakes and don't ride them.

Gary

03 Intrigue 11509

http://boonedocksllc.blogspot.com/

Re: Questions About Descending Grades

Reply #11 –
Yahoo Message Number: 86352
I have a 05 inspire with a c9 which does not have a jake brake, it uses an exhaust brake and holds most any grade even in cruise.

Later,
Derrel

Re: Questions About Descending Grades

Reply #12 –
Yahoo Message Number: 86354
We purchased a 2003 Allure 3 yrs. ago from a flatlander and he didn't know the jake was not working. Spent 2 days at cummins Sacramento and they could not find the problem. Packed up and went to Guaranty in Junction City and believe it or not a mechanic named Jake had it fixed in 1 hr. The brake never worked from the factory. Now it will hold on a 6% grade without ever touching the foot brake,

Jerry

36' 2003 Allure #30897

Re: Questions About Descending Grades

Reply #13 –
Yahoo Message Number: 86356
Thanks for cthe omments George... I presume it relates to Gary's original question as well as my own. though the numbers are different. But I am a bit confused... You said "Your 9 liter engine will probably have somewhere near 300 bHP". I had never really understood the differences between rated horsepower and Brake Horsepower, so I looked it up online and came up with the following distinctions.
Brake horsepower (bhp) is the measure of an engine's horsepower before the loss in power caused by the gearbox, alternator, differential, water pump, and other auxiliary components such as power steering pump, muffled exhaust system, etc.
The key difference between BHP and HP, is that when BHP is measured, the engine torque is determined by applying a brake to the flywheel as opposed to using a torque converter, like in the case of HP.
The Wheel HP number is normally around 20% less than the Brake HP, because the transmission and other drivetrain components (and sometimes the installed exhaust and intake components) eat up some of the power.
In either case it sounds like BHP should be a higher number than HP as measured at the dyno, and to confuse matters further Dyno measurements typically in cars are not the same as manufacturers claims. Nevertheless Caterpillar and Country coach rate my 9.3L engine at 425 HP (and 1350 ft lbs of torque)... and in fact it is vastly more powerful than the Cummins ISB I had in my previous coach which was only rated at 350 HP (w/ 650 ft lbs of torque.) The power and acceleration difference is palpable even though my Country Coach weighs 50% more than my previous coach. It probably has more to do with doubling the torque than the HP.
So I am not sure how or why you come up with a theoretical 300 bHP for my CAT 9 when it is rated at 425 HP by the manufacturer. Seems like, according to the definitions above the BHP would be higher... and the horsepower at the wheel would be a bit less especially since the side radiator I know for a fact eats up some of that horsepower.
In any case I am sure you are correct that the larger CAT engines can apply more deceleration with engine compression than my 9.3, but nonetheless, I think my assumption is still correct that the 6000 Suburban in tow will more easily overwhelm the jake brake than would the weight of the coach alone, since the SMI brake system only applies proportional braking to the toad when the service brakes are applied and not when the jake brake is applied. What I will test later this spring when my wife is driving the Suburban separately and towing our sailboat will be to test the jake brake on high down a steep descent with only the weight of the coach in play.

Joseph Burkle

2008 Country Coach Inspire 360 Founders Edition 43' quad slide tag axle 425 HP C9 pusher http://www. Wind-seeker.com/

Re: Questions About Descending Grades

Reply #14 –
Yahoo Message Number: 86360
Ditto on my 2003 36'Allure w/ISL 370 after Cummins fixed the Jake engine brake.

Larry 30856

Re: Questions About Descending Grades

Reply #15 –
Yahoo Message Number: 86361
I've heard two schools of thought when applying the service brake going down long, steep grades. One approach is that the service brake is to be applied firmly to bring the coach speed down, then release the service brake and let the Jake work until the speed creeps back up, repeating the use of the service brake. This is the technique I've used. The second approach is to lightly apply the service brake all the way down the long grade at, say, 3% to 5%. I've noticed truckers almost universally have their brake lights on going down steep grades, indicating they may be using this technique. I've been told this second approach will not heat up the brakes, but effectively controls the speed.

I'd appreciate any insights others may have on either technique.

Paul Onerheim
2004 Magna

Re: Questions About Descending Grades

Reply #16 –
Yahoo Message Number: 86362
On Many rigs the "brake lights" come on with the Jake brake or Pac brake, so the brake light does not necessarily mean the service brake has been applied.
Riding the brakes will almost certainly heat up the brakes imnsoh opinion.

Bob wexler
SOB

Re: Questions About Descending Grades

Reply #17 –
Yahoo Message Number: 86363
Joseph,

My comments were related to Braking HP. Braking HP is a specification related only to engines equipped with compression brakes. Cat can give you that precise number which will be somewhere near the 300 that I estimated. The ability of an engine to brake a load is less than it's ability to accelerate a load. I think my use of the term bHP created the confusion.
I agree with you that the your towed weight is part of the weight you need to stop thus effecting your ability to control speed on a down grade.

George in Birmingham
'03 Magna 6298
George in Birmingham
2003 Magna 6298

Re: Questions About Descending Grades

Reply #19 –
Yahoo Message Number: 87823
Idiot alert. Do not read the epilogue to my original post if you don't want to read about how I am an idiot......
As I indicated I would be, I have been running around in a lot more grades than in previous travels. In experimenting a bit, it became clear to me that I had fooled myself between the high and low settings of the jake. I was somehow convinced that the low setting was high and vice versa. As such I had just flipped the switch to what I thought was high which was really low and left it there and then only to complain that the jake wasn't working as well I wanted.
A few days back I went down 4 straight miles of 6% grade. Dropping the trans into 3rd, and with the jake in the TRUE high setting, I never topped 40mph on the way down. I actually had to get back on the accelerator a couple times.
I now am convinced my jake works just fine and will hold me back well enough on almost any grade I encounter as long as my operator error doesn't get in the way again.
Thanks for all the advice I received, but what I really needed was "Jake Brakes for Dummies".

Gary 03 Intrigue #11509

Questions About Descending Grades

Reply #21 –
Yahoo Message Number: 86333
Whatever gear you go up come down in the same gear.

Dave Rousey Jr.

Former CC Technician
Styleworks RV Repair &
Bus Restoration LLC
4762 Long St.
Sweet Home, OR
(541) 543-7983
daverjr4@...