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Manual transmission downshifting

Yahoo Message Number: 90264
I am a new owner and read the posts regarding transmission shifting and now have a question. When I am on a steep grade (in the right slow lane with trucks) should I be manually downshifting the transmission, or just keep the pedal to the metal to maintain speed?

Thank you,
John

2000 Allure 30526

Re: Manual transmission downshifting

Reply #1 –
Yahoo Message Number: 90275
John, with most any diesel pulling a heavy load up a grade it is always desirable to keep the engine rpm's up and that is usually best accomplished by manually downshifting. Keeping the rpm's up (maybe 1,800 - 2,000 depending on the particular engine) helps prevent overheating.


Gary 07 Allure 470 #31578

Re: Manual transmission downshifting

Reply #2 –
Yahoo Message Number: 90279
I wrote about this in the last week or two. Planning on going to Reno from San Jose CA and plan on following this advise. I think the summit is around 7000 feet.

Wally Falch

Wfalch@...

Re: Manual transmission downshifting

Reply #3 –
Yahoo Message Number: 90290
Driving a DP motor home is just like driving a diesel semi and is quite different than a gas engine. You should find and become familiar with the torque and HP curves for you particular model of engine. If you look at it closely you will see that the maximum torque is generally achieved at a relative low rpm, many times around 1300-1400 rpm. Torque above this rpm will drop off quickly, even though you will have higher HP at higher rpm's.
What this means is that ideally under a heavy load (like climbing a hill) you should pick a gear to keep the rpm close to the max torque(or just slightly above). This may be 4th gear, or even 2nd or 3rd gear on a very steep 6% or 7%+ grade. Pushing the "pedal to the metal" will generally not get you to the top of the hill any faster in a diesel. When you rev a diesel up way high under load, you can feel the power drop off and you will not really go any faster (due to there not being enough torque at that higher rpm to pull the weight, especially up a hill)
I don't really like the way my Allison tranny shifts on hills, so I always shift manually. As I approach a mountain pass in 6th gear, I watch the rpms and manually downshift as they drop down near the max torque point. If it is a very steep hill (or you have a very heavy load) then the engine won't be able to hold that rpm and it will continue to drop, forcing you to downshift again. When you can keep the rpm's near the max torque (or slightly above), you have found your gear for that hill. Don't worry about what your speed is, it is what it is.
As far as the cooling question, unless your engine/cooling system has some issue, your engine should be able to run all day long at maximum torque. You should watch your temperatures (engine, transmission, turbo) and if you see overheating, obviously pick a lower gear and slow down. Your engine will also be the most fuel efficient at the max torque rpm. Pushing the rpm's significantly higher will generally just waste fuel.
Speaking of hills, it is worth pointing out that what goes up, has to come down! :-)
You should approach every mountain descent with even more caution than the incline. Generally you should go down a pass in the same gear as you went up it. You should pick this gear before descending and let the jake brake do its job. Always use the jake brake! If you pick the gear correctly you should be able to descend virtually any pass without even touching your service brakes. If you have to hit the brakes on the descent to slow down, then you are in too high a gear and could be in a very dangerous situation. The problem is that with heavy loads the service brakes heat up very quickly and do not cool down quickly meaning that they will not be able to stop you. If you cannot slow down, then you will not be able to shift down to a lower gear. This usually results in a accident.
Out west (I live in Utah) we have quite a few very steep and long mountain grades. 6%-8% grades are common on the interstate and much higher than this on state roads. On our way back from our latest trip we came upon a semi accident that had just happened near the bottom of a 6% grade on I-70 in eastern Utah. A trucker hauling water had not gotten in the correct gear and could not slow down enough to gear down. Near the bottom of the hill he decided to bail out. He grabbed a pillow and jumped out of the driver side door about 100 yards before the semi went over the edge. Clearly it saved his life! He was taken away in an ambulance but did not have life threatening injuries. I'm not sure how fast it was going when he bailed, but plenty of speed. There were two runaway ramps on the pass, he missed the first one and had the accident before making the second one.
Here are a couple pictures: Picture1 Picture2 Picture3 It was a stark reminder of the massive forces at hand and a good reminder to be very vigilant and safe in our travels.

Good Luck!
Dan

2006 Allure 430 with tag
#31344

Re: Manual transmission downshifting

Reply #4 –
Yahoo Message Number: 90291
Donners Pass on I-80 between Sacramento and Reno is a serious pass and very beautiful! You are right in that the pass is just over 7000 ft which is not all that high, except that the starting point in Sacramento is only at about 100ft! It is quite a long pass also.
I have traveled this pass many times and have seen many broken down rigs along the way, it is to be take seriously! It is beautiful in summer but equally as pretty in the winter...enjoy your trip and stay safe!
Are you by chance going to Reno for the annual Rib festival over Labor Day? We will be there, we make the trek from Utah every year...great food! :-)

Dan

2006 Allure 430 with tag
#31344

Re: Manual transmission downshifting

Reply #5 –
Yahoo Message Number: 90308
Quote
Quote from: love_a_road_trip"

Dan,

My complements on a very constructive post. I have driven diesel coaches (3) over 200,000 miles and have always been able to competently handle the grades of some of the worst. Your contribution to this important topic will be very helpful to many of the lower mileage owners. I would, however, take a small exception to the comment regarding going down the hill in the same gear as the one going up the hill. I have heard others make the same comment which assumes, many times in error, that the downside grade is similar to the upside grade. If the upside is say 5% and the downside is 8% you might want to reconsider that "rule of thumb." Conversely if the upside is 8% and you are in third gear and the downside is 5% you might find that fifth gear with the jake on is more appropriate for the descent. The point being nothing beats thinking, know the concept but use your head.
Lyle Wetherholt
04 Intrigue 11740

> Driving a DP motor home is just like driving a diesel semi and is quite > different than a gas engine. You should find and become familiar with > the torque and HP curves for you particular model of engine. If you look > at it

Quote from: John Sweeton

>

> I am a new owner and read the posts regarding transmission shifting > and now have a question. When I am on a steep grade (in the right slow > lane with trucks) should I be manually downshifting the transmission, or > just keep the pedal to the metal to maintain speed? > >

> Thank you,
>

> John

> 2000 Allure 30526
>

Re: Manual transmission downshifting

Reply #6 –
Yahoo Message Number: 90333
X's 2 on the good write up Dan, and an appropriate caution from Lyle about using the thing between the ears to adjust to the appropriate gear for the down side of the hills too:)!
We're 'rookie's' on the Diesel side of RV'ing. We have 2 years and 8 months, with a grand total of about 11K add to our coach under our control. (We'll work to significantly add the miles on this coach over the next decade, as be hope to be on the road quite a bit more:)!).
It was on this board, that a 14 months after owning and driving this rig. Many Rocky Mountain 'hills' in our 11K of miles, and I did lots of reading on our ISL's HP/Torque curves to determine which gear to 'gear down to' when climbing hills. (Felt I had a good grasp of that...) But just about 14 months ago I learned that what I had thought I understood about the Jake (come on, how hard can it be. Off/High/Low - just one button to play with:)!), when I learned I had missed two key things about the Jake's interaction with our coach:

1) Did not think the Jake worked with cruise control on. WRONG. Once you creep up to over 5 mph over the set cruise speed, the Jake will kick in. But, only if you are not over the speed that it can safely kick in at. (If cruise set at 65, and you coast down a hill up to 70, sure enough it will not kick in as the speed is too high. But if at 55 on cruise, and you coast up to 60 - it does kick in...

2) Default for our coach, is Jake to jump the Allison down into 4th gear. Sometimes I wanted to have 3rd or 2nd. So, I would hit the down button on the transmission, and nothing would happen. Learned on this board (or via PM's, and confirmed from a visit to Cummins Cal Pacific), that even thought transmission gear is reflecting '4th' gear with Jake engaged - the transmission brain still thinks it is in the 'previous selected' gear. Usually this was 6th. So, to drop down below 4th gear, you have to hit 'down' three times to get into 3rd (6-3=3rd). Or down 4 times to get into 2nd (6-4=2nd gear).

It is why I so enjoy checking this board, usually daily, because I learn something new weekly!!
I had read the manuals, retained some of it too. But it is the real world usage that really gets things to 'gel' for my brain. So when I learned I did not really understand how the Jake and Allison worked together, and with the cruise too - I went out and drove for an hour and tested these different conditions. Big grin on my face at the end,
Dan, Lyle - thanks again for sharing your knowledge, and good input on hill climbing/descending.

Best to all,
Smitty

04 Allure 31017
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: Manual transmission downshifting

Reply #8 –
Yahoo Message Number: 90344
I leave my jake brake on low 95% of the time which means it helps braking on any stop or slowdown. I only really shut it off when I want to truly coast.
I generally only use high when descending a mountain pass. I will pick my gear and switch the jake to high and monitor my speed. I have found that I can use the high/low setting as a half gear of sorts. For example, if I am in 4th gear with jake in high and I find it is a little too slow, rather than just upshifting to 5th, I will first move the jake to low and see how much the speed increases. If it isn't enough then I'll go to 5th/low jake. This lets me fine tune it a bit. Hope this make sense.
I don't find a great amount of difference between high and low, maybe 25% more braking in high? That is just a rough guess.
I will say that I found the placement of the jake rocker switch to be very troublesome! It was the middle switch in the second row. I could never easily find it without looking! The first thing I did was move it to the bottom row closest to the seat (right above the shifter block) I also moved my "tag up" switch to be right next to it, in the middle. Now I can find them without looking. Maybe a small thing, but crucial when you are driving and don't want yo take your eyes off the road.

Dan

2006 Allure 430 with tag
#31344

Re: Manual transmission downshifting

Reply #9 –
Yahoo Message Number: 90346
Dan, I also find the factory placement of the Jake switch not driver-friendly. How hard was it to relocate? Is there enough wire to move the switches around without extending them?

Thanks for this tip.
Gary

'07 Allure 470 #31578

Re: Manual transmission downshifting

Reply #10 –
Yahoo Message Number: 90347
One point I forgot to make regarding the jake brake is that I do leave it on low all of the time for safety reasons as well. If I am in a situation where I need to make a panic stop for whatever reason, the jake will aid in stopping, albeit a small amount. In a panic situation you won't have time to reach over and flip it on and sometimes in those white knuckle stops, every little bit of stopping power helps!

The other reason I leave it on is that I absolutely LOVE the sound! Check out this short youtube video. The killer sound starts at :22. I always grin when the jake kicks in!

Dan

2006 Allure 340 with tag
#31344

Re: Manual transmission downshifting

Reply #11 –
Yahoo Message Number: 90348
It was really easy. That style of switch snaps right out.
If I recall correctly I had to only extend 2 wires. Grabbed a soldering iron, some spare wire and a pair of strippers and had it done in short order.
I don't remember exactly what I moved around to make room for the jake and the tag, I can shoot a picture tomorrow.

Dan

2006 Allure 430 with tag
#31344

Re: Manual transmission downshifting

Reply #12 –
Yahoo Message Number: 90350
I always leave mine in the high position. If there is a steep grade coming depending on how straight the road, if its multi-lane, and traffic, I will slow down to about 60 as I approach the down grade and that allows the transmission to shift down to 4th with my foot off the accelerator to approach a 5% grade with the switch in low and almost 7% with the switch in high. If the grade is steeper then I slow down further to allow the transmission to shift into third. When in Colorado or anywhere I'm not familiar with steep grades I may shift into second to hold speeds around 30 mph on winding narrow roads. I've found by using Valvoline engine oil for diesel engines I almost gain 10 lbs of oil pressure, over Rotella and that makes my engine braking mush more effective. Especially when ambient temps outside are above 100 degrees F. When i bought my coach I had Gary from Allison in Cohburg reprogram my 3000 world transmission to automatically seek 2nd gear instead of the normal 4th. That way I can keep both hands on the wheel while navigating steep grades. TWI 2004 Intrigue ISL 11731

Re: Manual transmission downshifting

Reply #13 –
Yahoo Message Number: 90353
You've probably picked up it is via a rocker switch in the left 'command module' of switches.
ISL's and I think CAT 9's have 2Staje Jake engine compression assist. I think Big CAT's, like C13's can have 3 Stage Jakes. Not sure on ISC's. And i'm pretty sure this can vary by models. Some may have Exhaust Brake Assist.
On the Engine Compression Jake's 2 Stage will be 3 cylinder and 6 cylinder. 3 Stage will be 2, 4, and then 6 cylinder.
Reading the other posts, and also a search, is good reading on Jake's.

Smitty

04 Allure 31017
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: Manual transmission downshifting

Reply #14 –
Yahoo Message Number: 90355
Because of the way the cylinders are split on the ISL every once in a while depending on the grade and the rpm there is an unusual oscillation I don't like when running on 3 cylinders. That why I prefer the high position using all 6 cylinders.
TWI 2004 Intrigue 11731

Re: Manual transmission downshifting

Reply #15 –
Yahoo Message Number: 90362
Hello:

When I am driving I need all the help I can get especially if I see a down grade coming up on an unfamiliar highway.
So, I stuck a little dot of Valcro on the the jake brake switch (the rough part of the Valcro duo) and when i need to rock the Jake on I simply feel for the Valcro without taking my eyes off the road. Works well for me.
Also I put the matching soft circle on the toilet flush button. When company visits our bathroom I just say "find the Valcro" to flush. Works well for me.

I think both ideas were Herb's (moderator). Thanks, Herb.

Shirley OBrien
Allure 06 #31290

Re: Manual transmission downshifting

Reply #16 –
Yahoo Message Number: 90372
Hi All,

I often use my engine brake on hills and sure enough my CAT transmission will usually stay in 4th gear but sometimes I want to up shift to 5th gear and no matter how often I hit the up arrow nothing happens. What am I missing here? Thanks,

Al Kirkendall
2009 Veranda 300
Serial # 52197

Re: Manual transmission downshifting

Reply #17 –
Yahoo Message Number: 90380
Dan,

Would you comment on Jack vs Exhaust. I'm a retired tour bus driver and have always manually shifted up and down. However, coming from a Jake to an Exhaust I have found the braking not as efficient. What are your thoughts?

Michael

05 Inspire 51381

**************************************** Note: Do Not Use @...
email: met6195@...

Michael Title
B.C. Canada

CDN Cell: 604.741.3328

La Cañada, San Miguel De Allende, Guanajuato, Mexico

MEX Cell: [52] 415.100.1543
"To reach a port we must sail, sometimes with the wind and sometimes against it.
But we must not drift or lie at anchor" ~ Oliver Wendell

Re: Manual transmission downshifting

Reply #18 –
Yahoo Message Number: 90385
Michael,

The exhaust brake functions solely by plugging the exhaust pipe, providing back pressure. When a diesel engine idling down a hill, the energy used in compressing the air taken in at the intake stroke is mostly recovered when the piston goes back down after the compression stroke. This is why there is very little braking from a diesel engine on a downhill. When the Jake brake is on, the exhaust valve opens at top dead center, and the energy used in compression is lost, and the piston pulls agaist a vacuum on the way down. This is much more efficient than an exhaust brake--the braking power on our engine isalmost as much as the rated power.

Per

95 Affinity

Re: Manual transmission downshifting

Reply #19 –
Yahoo Message Number: 90387
There are actually several different types of engine brakes and it seems that everyone just lumps them together and calls them a Jake brake. Similar to all tissue being called "Kleenex"
The formal name is a Jacobs brake and came from the Jacobs company (the ones that made great drill chucks years ago) Apparently it was acquired and patented by Cummins in the 1960's.
The Jacobs type of engine brake opens a second exhaust valve on each cylinder right as the piston is at the top of the compression cycle and releases that air which causes a braking effect. It sort of turns the engine into an air compressor. The high/low/med switch activates it on some or all cylinders. This type of braking is the most efficient and can have almost as much braking force as an engine has power. It is generally the loudest as well.
The other type of engine brake is called an exhaust brake and is mislabeled a Jake Brake. It is a set of flapper valves in the exhaust manifold that restrict the exhaust leaving the engine. Sort of the same thing as putting a potato in the exhaust pipe. :-) Generally they are only 2 levels, high and how (3 cylinders or 6) They are not as efficient and do not give the same level of braking as a true Jacobs brake, but still work pretty well.
It seems to me that most motor homes have exhaust brakes, but I'm not 100% sure on this. My Allure is a ISL 400 and has an exhaust brake.
The recommendation to not use them on wet or slippery surfaces is to avoid losing traction. When the jake brake comes on it can cause a quick slowing of the drive wheels and they could easily slide if the surface is slick. Remember they only are on the drive wheels.

For a better graphical explanation of jake brakes - look here: http://www.youtube.com/watch?v=RZwsmqPXYvs
Dan

2006 Allure 430 with tag
#31344

Re: Manual transmission downshifting

Reply #20 –
Yahoo Message Number: 90388
Hi Dan,

It sure gets confusing, especially since Exhaust Brakes are also made by Jacobs/Jake. So people say they 'Have a Jake', and confuse this with the 'compression' brake of the old Jacobs that you described so well. The exhaust, in my layman's terms, is 'post turbo'. The 'exhaust brakes' get more confusing, as they can function differently between normal turbo vs Variable Vane Turbo - so KISS this is not, when you go thru the many years, models and types of what is all lumped together in 'engine braking' discussions.
I add this link to a Wiki, as it also does a good job of explaining Jake/Compression, and then has links at the bottom to Exhaust Brakes too...
http://en.wikipedia.org/wiki/Compression_release_engine_brake
I was surprised to learn your year ISL400 had Exhaust Braking assist, as I thought the Allure on up, had the Jake 2 Stage Compression Braking. (Our 2004 Allure (with 2003 CAPS ISL370) has Jake 2 Stage Exhaust Compression.)

Good thread all of this stuff!

Best to you, and all,
Smitty

04 Allure 31017
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: Manual transmission downshifting

Reply #21 –
Yahoo Message Number: 90394
Well I'll be danged! Smitty is right and I am wrong...my ISL 400 does have a Jacobs compression release brake!

This is how I figured it out, hopefully someone else can benefit from this.

Has everyone with a Cummins engine registered their engine on the Cummins Quickserve site? End users can do this. All you have to have is your serial number.

Cummins QuickServe Online
This is a great resource that gives you specific info for your engine based on the serial #. It has all the service bulletins, warranty info, maintenance intervals etc. It also has the engine dataplate that is like it's birth certificate. Mine was built in July 27, 2005 ( it just had a birthday!)

Here is a link to the dataplate for my engine, for reference. Cummins engine dataplate
You will see that there is a CPL#, mine is CPL8718 This stands for "controlled parts lists" and is specific to your engine configuration.

Now if you jump over to jakebrake.com you can get info that tells you which engine brake is included in CPL8718

Jake Brake Application Wizard
Enter your engine Cummins CPL# and it shows you which Jacobs product you have. Then in the parts dropdown you can get specific info on that part.
It works for other engines as well, CAT, Detroit etc. You just enter different info for each manufacturer.

So my engine has the Jacobs 490 compression engine brake. (link to pdf) It shows that I get about 200HP of retarding power at 1700 rpm. I assume this is on high. Not too bad!

There is also a wealth of other info there, parts kits, maintenance info, etc. Their FAQ is also very interesting.
Thank you for questioning me Smitty, you were right! It forced me to flex and learn something new!

I hope this helps someone else as well.

Dan

2006 Allure 430 with tag
#31344

Re: Manual transmission downshifting

Reply #22 –
Yahoo Message Number: 90674
When going up a steep grade and the transmission is doing a lot of upshifting/downshifting hunting for the right gear, I usually downshift to a gear that will keep the engine rpm's around 2000. This keeps the rpm's steady and in a range where the engine is more responsive. This also helps to keep the engine (CAT C9) from heating up too much.
Vince Mercurio
2006 Inspire

Re: Manual transmission downshifting

Reply #24 –
Yahoo Message Number: 90826
What engine do you have? It looks like from a couple google searches on CPL1096 that you have a Pacbrake Exhaust Brake.

This thread from IRV2 talk about them and an upgrade that PacBrake has for it with more back pressure.

PacBrake Exhaust Brake
This page shows an upgrade for it with higher pressures.

http://www.ocdiesel.com/Pacbrake-C40020-Exhaust-Brake-Kit-p/pac-c40020.htm
Do you have a high and low switch, or just an off/on?

Dan

2006 Allure 430 with tag
#31344