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Old 06-14-2021, 05:26 PM   #1
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2021 Force engine brake issue (Detroit DD8)

I have a new 2021 Force with Detroit DD8. The engine brake functions much differently than my Cummins powered Isata-5 did. The new Force engine brake, regardless of Hi or Low setting, has a rather unpleasant habit of downshifting the transmission, bringing the RPMS up to a seemingly too high, say 2700-2800 rpm. It sounds radical, and operates radically going from a pleasant gently braking action I am accustom to, to a dramatic and noisy operation. I'd rather it didn't down shift at all. Not sure what this is about, but as it is, I'd rather not even use the engine brake, except in the most mild of needs. Perhaps a setting? IDK. I'd be curious of other experience in the Freightliner M2s with Cummins power how their engine brakes operate.
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Old 06-14-2021, 07:33 PM   #2
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The cummins ISL downshifts as well.
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Old 06-14-2021, 08:05 PM   #3
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The cummins ISL downshifts as well.
Yes, but it's pretty gentle and doesn't result in engine revs like he describes. That could be just the way the Detroit motor operates. Maybe just use it in Low mode.
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Old 06-14-2021, 11:14 PM   #4
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I always use it in low mode.
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Old 06-15-2021, 07:05 PM   #5
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Originally Posted by Invisibleman View Post
I have a new 2021 Force with Detroit DD8. The engine brake functions much differently than my Cummins powered Isata-5 did. The new Force engine brake, regardless of Hi or Low setting, has a rather unpleasant habit of downshifting the transmission, bringing the RPMS up to a seemingly too high, say 2700-2800 rpm. It sounds radical, and operates radically going from a pleasant gently braking action I am accustom to, to a dramatic and noisy operation. I'd rather it didn't down shift at all. Not sure what this is about, but as it is, I'd rather not even use the engine brake, except in the most mild of needs. Perhaps a setting? IDK. I'd be curious of other experience in the Freightliner M2s with Cummins power how their engine brakes operate.
What transmission do you have? If it is an Allison the problem can be corrected by reprogramming. They should be able to change those shift points and engine brake inputs.
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Old 06-16-2021, 05:39 AM   #6
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My engine brake is smooth along with smooth tranny downshifting. The rpms seem to max out at about 2k.

I read something in the Cummins manual about max rpm and being surprised about how low it was. Maybe it was 2500? Not sure...
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Old 06-16-2021, 07:49 AM   #7
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I have an email into the local rep, see if that is normal.

Don't forget though...the Ram is exhaust brake only. DD8 is an engine brake, so you are comparing apples to oranges.
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Old 06-16-2021, 09:57 AM   #8
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The chassis is governed at 2600 RPM's, so if it is downshifting to higher, then that very well could be an issue.

As I recall on the cummins... if you are going downhill it will brake, but once you hit the brake again it will downshift (Or am I thinking of the Ford tow/haul mode?)
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Old 06-16-2021, 04:25 PM   #9
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I don't think I have found an area of any manuals that deal with the DD8 engine brake specifically, so not sure how it is "supposed" to work. Initially, however, it seems to work similarly in feel as I am used to on the Isata-5 cummins. I do not remember my Isata-5 every down shifting while using engine brake, at least not as a braking action. And if you pushed the brake peddle, I thought is released the cruize control which in turn released the engine brake. But not 100% certain of that recollection. I do know, however, my DD8 much too aggressively down shifts even on the most gentle of engine braking, and that's even in LO. I've never even tried HIGH mode. I've driven quite a few trucks with Jake brakes and never recall any transmission downshifting. I typically only rely on engine or exhaust braking for cruising downhill grades where you don't want to drag the brakes. Any other purpose and the brake pedal is my choice. I wish there was a way to completely switch the down shift function off. It downshifts normally with the engine brake off. It should downshift the same whether the engine brake is on or off. I figure this is probably just a programming issue.
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Old 06-17-2021, 11:05 AM   #10
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Why not try HIGH and see what happens. Maybe the switch is backwards.
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Old 06-17-2021, 12:08 PM   #11
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I could, but I wouldn't want this aggressive high RPM on HI or LOW. Trust me. This is way to aggressive a down shift. I wouldn't be surprised if it's above recommended RPM ratings.
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Old 06-17-2021, 12:12 PM   #12
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And, is this designed to be an active transmission braking system, or is it an engine breaking system? I can't find anything about the engine/Freightliner combination.having automatic transmission, transmission breaking. And, I'm not planning any trips for the next few weeks for doing any experiments.
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Old 06-18-2021, 07:34 PM   #13
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Anything new on this? I guess not or it would be here. I'm just trying to keep the subject alive.
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Old 06-19-2021, 05:50 AM   #14
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I could, but I wouldn't want this aggressive high RPM on HI or LOW. Trust me. This is way to aggressive a down shift. I wouldn't be surprised if it's above recommended RPM ratings.
My Cummins 8.3 does the same downshift your complaining about. I don’t like it either. If anyone knows of a programming fix for this I would be interested. I admit I have not researched this at all. On a side note, mine does not engage with the cruise control on. I have to touch the brake to disconnect the cruise for the jake. Brake to engage.
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Old 06-19-2021, 11:23 AM   #15
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The DD8 and the ISL 9 are true "jake" brake engines. The cummins 6.7 in the Dodge chassis is an exhaust brake sometimes referred to as a PAC brake.

Basically the low setting will set one cylinder to be a "compressor" and the "Hi" setting will set 2 cylinder to "compressor" thus the term compression braking.

An exhaust or Pac brake blocks the exhaust and creates back pressure to the engine. Less efficient for braking purposes.

Regarding the RPM limits. The engine governor is based on throttle position which would translate to demand. Governor will control the "demand" to a certain max RPM. However, in braking or downshifting the governor will not necessarily control max RPM and with improper use of the transmission gear can very likely over rev the engine. (Risk is engine block venting).

With the Allison transmission that is in the M2 the programming should be tailored to the installed engine characteristics. Worth having checked by Freightliner service facility.

While having it checked you could also have it programmed to have the jake break function while in "Cruise control" I have mine set to 3 mph above cruise speed and it automatically maintains cruise setting on the long downhills using the engine braking. Sweeet!

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Old 06-19-2021, 12:21 PM   #16
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Just a thought as I worked at a DDC/ATD distributor for 20 years, is the Allison trans shift pad in performance or economy? If neither setting makes a difference the engine settings can be changed as well as the trans settings. If you can’t figure it out go to your local Detroit Diesel / Allison DISTRIBUTOR…note i said distributor, the work on the engines and transmissions only not the rest of the chassis like a DEALER. There is a huge difference.
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Old 06-19-2021, 12:30 PM   #17
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What I was used to on the Isata-5 was, set the cruise to say 70, with brake "on". Cruise did its thing flat, up or down. Never touched a thing, within reason. The engine brake would hold as necessary down the long down hill grades, and throttle would come up on the uphill side. Never had any downshifting in those situations. It does sound like this is pretty likely a simple settings issues. The question will be of who to establish the settings and whether this is a warrantee issue. I can't see it not being, as the engine brake, in my opinion, is unusable as-is, and is throwing the engine into high revs than can't be good. I'm sure BC will back with more info soon enough.
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Old 06-19-2021, 01:04 PM   #18
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What about the Allison shift pad being in economy or performance mode? It does have an Allison correct? Anytime we had a complaint like this we always warrantied it. That is why I told you there is a difference between a dealer and distributor.
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Old 06-19-2021, 01:18 PM   #19
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Yes, Allison TRV. I don't know about any performance or economy settings on the shif pad.
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Old 06-19-2021, 03:37 PM   #20
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So I am assuming you have the Allison shift pad and not the Stalk that FCCC uses. Here is a pretty good video Allison put out to help everyone understand how the pad functions work. There describes the performance and economy modes. I would make sure its in economy mode and test the engine brake again. I have seen cases where that drops the trans 1-2 gears depending on road speed when trans is in performance mode. Also what i have found on the trans side was the TCM was programmed to drop 2-3 gears when engine brake is activated. Hope this helps you out….

https://youtu.be/NNa7FO_9VXk
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