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Navy Flier · 12-08-2011, 01:41 PM

I have an 07EPA Cummins ISL in my motor coach. I have been driving it for 3 years and 29,000 miles and have finally gotten around to asking this question.
Cruising down the road I have the engine brake set in the low position. This motor has an rpm limit of 2200 rpm. When I use an exit ramp I let the engine brake do its work. All is well until the transmission shifts to 4th. At that time the rpms jump to 2400/2500 and do not go below 2200 until 35mph or less. Which means I either turn off the engine brake or exceed the engine limits. In this scenario it is manageable but irritating. The more important scenario is descending a grade. There are times that the grade is such that with the engine brake the transmission wants to settle in at 4th gear where the engine brake is not usable. However, this same type of grade is just steep enough that without the engine brake the coach is constantly accelerating. These type of grades also do not seem to lend themselves to my using 3rd gear as I now become an impediment to traffic. I am curious as to what techniques any others out there use in these situations.

Route 66 · 12-08-2011, 01:46 PM

I asked several Cummins distributors about that and they all said it was OK for the engine to exceed the RPM limit when using the Jake brake.

If the RPMs and speed get too high, the Allison will upshift.

On some hills, I had to use the service brakes to get the speed low enough for the Jake brake to be effective, then I could alternate between high and low with Jake brake.

RickO · 12-08-2011, 01:55 PM

There are a lot of different techniques when it comes to using the engine brake. I too talked with Cummins about this and was told the same thing as Dirk. The transmission will upshift if it revs too high.

My preference is to only use the engine brake when descending grades and I supplement with use of the service brakes to maintain the desired speed. I've tried leaving it on all of the time but found that I prefer being able to "coast" to manage my speed on relatively flat ground. I'd like to think that my fuel consumption is lower because of it but that doesn't seem to be the case.

Good luck

Rick

palehorse89 · 12-08-2011, 02:08 PM

i am just getting ready to address this issue, i am going to have a buddy of mine (w/ laptop and proper software) reprogram my ecm so it does not down shift with the engine brake. i just want to use my engine brake in any gear and down shift as i need to. most of the time at interstate speeds a long down hill grade justs needs the engine brake (pac brake) and not the transmission to downshift to 4th also. at this point i am not sure what options i will have but i will soon see.

Perry White · 12-08-2011, 02:27 PM

You'll be making a big mistake.

docj · 12-08-2011, 03:00 PM

Quote:

Originally Posted by palehorse89

i am just getting ready to address this issue, i am going to have a buddy of mine (w/ laptop and proper software) reprogram my ecm so it does not down shift with the engine brake. i just want to use my engine brake in any gear and down shift as i need to. most of the time at interstate speeds a long down hill grade justs needs the engine brake (pac brake) and not the transmission to downshift to 4th also. at this point i am not sure what options i will have but i will soon see.

I was about to do the same thing until I read an article on the Jacobs website which explained that the braking action of a compression brake is greatest when the engine is turning at its redline. As engine speed is reduced below redline the braking effect of the Jake brake is greatly diminished.

With the particular gearing on my MH, I can apply the Jake at speeds up to 65 mph and the Allison will downshift to 4th without taking the engine past redline. We spent the entire summer in the mountains of WY, ID and MT and with the Jake and manual downshifting, when needed, I was able to easily descend some pretty steep slopes. When exiting the Interstate I make sure I my speed is less than 65 (I normally cruise and 62-65) and I then apply the Jake to get my speed down to exit ramp levels.

Navy Flier · 12-08-2011, 03:34 PM

I have also talked to some Cummins people about this issue and get some strange answers. Some say 2200 can be exceeded with the brake on, others say no and some just give me a blank look. I have stopped using the engine brake exiting the interstate and have found that I really don't need to use the engine brake in that situation. With the proper anticipation slowing from 62 on an off ramp is not a big deal. I have also found on steeper grades I can start down at 60 and the engine brake will hold a consistent 55-60. I am going to talk to the Cummins customer service folks tomorrow about exceeding the red line using the engine brake. In the owner's manual it states in two places, in the explanation of the engine braking system, to never exceed the governed engine speed. I have always thought it strange that no where in the operator's manual is the maximum rpm of 2200 stated. By looking at the diagrams you can infer the limit but it is never stated and I have confirmed the limit with Cummins.
All of this discussion by me is because my previous coach had a Cat engine which had a PacBrake that I always used during deceleration and the engine never approached red line in any gear. Thanks to all for your views.

Harry B · 12-08-2011, 04:17 PM

My manual specifies 2200 rpm limit under load and 2800 rpm limit with no load (decelerating). So you are fine. I have an ISL400. Like your engine it will get up there decelerating. It up shifted when rpm tried to exceed 2800. (I had to see this to reassure my self) .

Steve Ownby · 12-08-2011, 05:31 PM

Chuck,

You are smart to have noticed this and worry about it. I had the same concerns and I called the Cummins tech service line. I have an ISM with a governed engine speed of 2100 and when the Jake was active I was not getting an upshift until 2400. The Cummins tech explained to me that the 2100 was in an "on throttle" condition and the 2400 in a closed throttle condition was acceptable. That was about 4 years ago & all is well.

palehorse89 · 12-08-2011, 07:29 PM

[QUOTE=Perry White;1028542]You'll be making a big mistake

can you explain please.....

Navy Flier · 12-08-2011, 08:42 PM

I do not know if they have changed the owner's manuals in the past few years but my manual has virtually no engine operating numbers of any kind other than a couple of oil pressure numbers. Using the engine brake is the only time the rpm ever gets close to the red line. In normal operation the rpm stays in the 1600 to 1800 range all day.

Road Ranger · 12-08-2011, 09:09 PM

The performance of the PacBrake has been a concern for me, too, because it will upshift as the revs climb while going through the Rockies. I understand the difference as the PacBrake being an exhaust retardation while the JakeBrake is an engine retardation. Maybe I have too much of a load on what with the MH and the motorcycle in the trailer. This is my first diesel engine so maybe it's a normal functionality in a diesel - it certainly has much less deceleration than a gas engine in downshifting. I hate the smell of burning brake material, however, so I'm watching and listening for anything that will make my PacBrake hold the rig back better.

Five2o · 12-08-2011, 09:33 PM

I have an ISL 400 Cummins....I turned the Jake onto high when I bought the coach 42k miles ago and have never turned it off.

dons2346 · 12-08-2011, 11:23 PM

[QUOTE=palehorse89;1028781]

Quote:

Originally Posted by Perry White

You'll be making a big mistake

can you explain please.....

All I can say is you don't "have your buddy" do anything to your transmission/engine system unless he is a Cummins or Allison tech. If anything goes wrong, it is not "your buddy" that will be picking up the tab.