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Old 09-17-2018, 07:20 AM   #1
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Downhill Shifting

Still trying to learn how to properly/safely get down hill with minimum breaking driving a 98' HRE 37' DP/dolly w/toad/275 Cummins/Allison 6 speed/exhaust break. I like to go slowwwwww downhill but find myself using the breaks to much to keep the rpm's under 2800 and if I give it some gas to keep the rpm's under I start picking up speed. A lot of the videos talk about manually down shifting but then my rpm's start climbing. So if some of you more experienced drivers can give me some tips on how to stay slow with minimum breaking while keeping the rpm's down that would be great. We are full timing so there's going to be more mountain driving coming up and I need to do it a lot safer for ourselves and all the others on the road.
Thanks in advance for all responses,
Jim & Diane
98' HRE 37'
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Old 09-17-2018, 07:30 AM   #2
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Ok, you say you are going slow. But the EB won't hold your speed. What do you need to do? Stand on that foot valve! Get speed down! Drop a gear. If the RPM starts to climb, repeat. If you are on the brakes more than about 10% of the time, you are going to fast.
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Old 09-17-2018, 08:05 AM   #3
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Originally Posted by ScoobyDoo View Post
Ok, you say you are going slow. But the EB won't hold your speed. What do you need to do? Stand on that foot valve! Get speed down! Drop a gear. If the RPM starts to climb, repeat. If you are on the brakes more than about 10% of the time, you are going to fast.
That's how to do it!
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Old 09-17-2018, 08:16 AM   #4
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Are you sure your Exhaust Brake is working?
If you are starting off slow and have exhaust brake on and downshifted. You should be getting some braking. Assuming that your not towing an over weight load without brakes.
What type of % grade?
What speed are you at when starting out downhill?
What gear are you in starting out?
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Old 09-17-2018, 09:27 AM   #5
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Just some food for thought, but the PO of the coach I bought had the EB replaced because it had rusted completely through and was not doing its job. I mentioned it to my mechanic and he said it happens on older coaches like mine. Might be worth checking.
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Old 09-17-2018, 09:51 AM   #6
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Go down hill in the same gear you went up. If the unit downshifts to 3 or 2, put it there manually and go down. EB will help - use brakes ONLY if required.
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Old 09-17-2018, 10:31 AM   #7
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Per Camp Freightliner, brakes on hard for 3 seconds, then off for 5 seconds to allow them to cool. Do not ride them!

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Old 09-17-2018, 11:33 AM   #8
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Per Camp Freightliner, brakes on hard for 3 seconds, then off for 5 seconds to allow them to cool. Do not ride them!

Skip Y
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Loc: Mich
Freightliner must use special brakes on RVs to get 60% duty cycle.
Using service, Jake, or exhaust brake, it takes more brake to keep speed between 20 and 30 than to keep it 10 to 20...
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Old 09-17-2018, 11:53 AM   #9
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Quote:
like to go slowwwwww downhill but find myself using the breaks to much to keep the rpm's under 2800

You don't need to manage RPMs - the engine & tranny ECm will handle that fine. It will upshift by itself if it gets too high. Just manage the speed and let the ECM handle the gearing



If you come over the crest of the hill at your desired downhill speed, you should be able to maintain that speed using the Freightliner method cited above. Brake firmly for a few seconds, then back off for several. You don't need to time it - just a simple pattern of firm brake and no-brake, whatever it takes to hold your speed.
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Old 09-17-2018, 11:55 AM   #10
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Your Cummins ISB, 275 HP engine is designed to safely operate at 2,600 RPM, delivering 660 lb/ft of torque. That is different than when your have the exhaust brake engaged and the engine is not under power, during this period the engine can safely exceed 2,600 RPM's, in fact higher RPM's improve exhaust braking effect.
The Allison TCM and Cummins ECM "talk" to each other during operation to prevent damage to either from overspeeding the engine.
When the engine RPM's exceed Cummins safe limits the transmission will upshift by itself to reduce engine RPM's to a safe level again. This is where the driver must pay close attention, for now the service brakes are the last effect against increasing downhill speed to an unsafe level until the driver can drop enough speed for the transmission to downshift again and provide more speed retard.
I hope that makes sense and sounds logical.


Gary and I were typing at the same time, sorry for the redundancy.
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Old 09-17-2018, 09:20 PM   #11
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Your Cummins ISB, 275 HP engine is designed to safely operate at 2,600 RPM, delivering 660 lb/ft of torque. That is different than when your have the exhaust brake engaged and the engine is not under power, during this period the engine can safely exceed 2,600 RPM's, in fact higher RPM's improve exhaust braking effect.
The Allison TCM and Cummins ECM "talk" to each other during operation to prevent damage to either from overspeeding the engine.
When the engine RPM's exceed Cummins safe limits the transmission will upshift by itself to reduce engine RPM's to a safe level again. This is where the driver must pay close attention, for now the service brakes are the last effect against increasing downhill speed to an unsafe level until the driver can drop enough speed for the transmission to downshift again and provide more speed retard.
I hope that makes sense and sounds logical.


Gary and I were typing at the same time, sorry for the redundancy.
I have the same engine, tranny and close to the same size as the OP. I didn't have any issues in the passes in CO hauling a 22' trailer with a load right around 5Klbs. using the gear and jake. sounds like the jake brake or something isn't right
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Old 09-18-2018, 06:32 AM   #12
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What gear do you climb up hill in? That should be same gear or one lower going back down the other side. That is an old rule of thumb for trucks in the day. Now you go down the hill in maybe 1-2 lower gears as motors are much more powerful than old days.

Engine should be doing most of the breaking. As said maybe not in a lower enough gear. If not slow down and drop another gear. When under 35 mph put your flashers on to warn people you are slow moving.

Maybe exhaust brake is rusted again worth taking a look at. Tell us gears going up and down and we can see how it compares.
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Old 09-18-2018, 07:28 AM   #13
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There are a few really good responses in this thread. And to restate what has been said:

Get the Jake Brake checked out. On my last 1997 Discovery coach I found the Jake Brake was frozen in the open position.

Unless the Allison Transmission has had its programming altered, if the Engine Brake is activated, the program will automatically select the best gear for conditions, without any operator input at all. Going down hill, the transmission will select the best gear for the exhaust brake to work at maximum efficiency.

BUT, as Ray,In indicated
Quote:
When the engine RPM's exceed Cummins safe limits the transmission will upshift by itself to reduce engine RPM's to a safe level again. This is where the driver must pay close attention,...
an operator must be very aware of this. If the coach is picking up speed and the transmission upshifts to protect the engine, the coach could actually increase speed down hill unless you use the service brakes properly.
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Old 09-18-2018, 07:54 AM   #14
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Nobody said it yet, but is your dolly and toad braking for themselves? If not, you have 4000 pound pushing instead of pulling, and that's a big problem. I rigged a simple test of my exhaust brake. I could not convince my wife to go back in the bedroom as we are going downhill, crawl over the bed and open the engine cover to look at the exhaust brake valve to see is it had closed upon activation. "Oh, it's too hot and messy?" (whatever) So, with a piece of fence wire and a piece of thread, I rigged a telltale. The fence wire is attached to the exbrake valve and is about 12" long to dissipate heat, with a loop in its free end. I tied the thread to that loop and secured the other end of the thread to the bulkhead. The I went for a drive without the grumpy wife, and used the exbrake. Sure enough, it pulled on the thread and broke it, proving the valve is moving. I feel a lot of braking from my exbrake, but I could not tell how much came from the exbrake and how much came from the automatic downshift that occurs at the same time, therefore, could not tell if the exbrake valve was moving. This telltale proved the valve is moving.


And yes, it's spelled brake, not break.
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