Cummins 400HP 3000 trans downhill questions

[oldryder]

2005 Winnie Vectra with Cummins 400HP 3000 trans. 33,000 GVW. Haven't had it on any long downhills.

Can someone recommend a write up and/or youtube that covers the correct use of trans and engine brake and also what to expect. I have used the engine brake on existing from the interstate and am always surprised how high it drives the engine RPM (2200 - 2500) as it downshifts. I want to be sure its working correctly.

thx to anyone taking the time to offer advice.

mark in MN

 

[Ksouers]

The engine brake or Jake brake has no control over engine RPM (save for slowing down the rig) or the transmission shifting. The transmission is completely under mechanical or computer control, so it should stay within a safe RPM range. Do you have a separate foot control for the engine brake or just an on/off switch and applied only when you are completely off the throttle?

I've only used a manual transmission, either 9, 10 or 13 speed, so I've already told what I know about automatics. But use the Jake any time you are on a long grade and think you need to apply service brakes. It is safe to use both, if necessary. It is also safe to use the Jake continuously until you are safely at the bottom of your hill.

Take a practice trip across Tennessee on I-40 between Nashville and Knoxville. It's beautiful this time of year. If you want longer and steeper grades try Monteagle on I-24 out of Nashville going to Chattanooga, that's a bit steep though. Jellico on I-75 from Kentucky going to Knoxville is also very pretty this time of year. Great views of the valley the entire climb to the top. Very long shallow drop into Knoxville.

Of course, anywhere in West Virgnia, Maryland across the Roller Coaster (I-68), or Virginia offers spectacular views and plenty of opportunity to try the Jake on various hills.

Kevin

 

[UTTransplant]

I have a 360 HP Cummins with the Allison 3000 transmission. On my engine the max torque (used for downhills) is about 2400 RPM. So it is actually about right for what you are seeing. The transmission has the ability to manually downshift, and that’s what I do 90% of the time rather than letting it do it on its own. I can see into the distance and know what’s coming up while the Allison can only work on instantaneous information. Slow your speed and downshift to keep the RPMs in that range. BTW, somewhere buried in the Cummins online documentation is a torque curve to let you know where your sweet spot of RPMs lie. Oh, and I leave my exhaust brake on all the time except in rain and snow.

 

[SargeW]

When I activate my engine brake it will automatically downshift the trans with 2nd gear as the target. It does so as the maximum engine braking is achieved at near max RPM for the motor. The higher the RPM, the more back pressure it creates, which slows the motor and rig. If I know a steeper grade is approaching I will manually downshift to 5th gear to get the rpms up to start the braking process a bit sooner.

On my rig the way that the cruise control "cancel" button is set up, it will also engage the engine brake, even if the cruise control is not currently set. This allows me to drive the rig in most road conditions without ever needing the brakes. That way they stay cool in case there is a need for a sudden hard brake.

I also use the same process if a significant incline grade is approaching. The higher rpm helps the Cummins maintain speed better with less lugging.

 

[Robert K]

Is this an exhaust brake or a jake brake

 

[oldryder]

Auto trans and if I have the terminology right it's an exhaust brake. No pedal, just a switch of "low" or "high" which is either 3 cyls of braking or 6.

 

[ChasA]

That is an engine brake, aka jake brake. An exhaust brake is either on or off with no selection for cylinders.

 

[Gary RV_Wizard]

As others have described, the transmission cannot over-rev the engine, either under power or braking. The tranny computer and engine computer cooperate to prevent that and optimize for the situation. The Allison 3000 will up/down shift as needed, regardless of the gear selected as the "target gear".

 

[Laura & Charles]

A helpful suggestion I got here and use often: When you crest a steep upgrade, use the same gear going down as you just used going up. Also, I have the engine brake on nearly all the time.
As Marty said, when our engine brake is on, it works in concert with the transmission and downshifts automatically. The way you pitched your question makes me think yours works that way as well.
(And as Gary said, Allison won’t let Cummins over rev.)

 

[oldryder]

THX EVERYONE.

 

[Gary RV_Wizard]

When you crest a steep upgrade, use the same gear going down as you just used going up.

Technically sound advice but no longer very applicable. In general the gear going up was chosen by the automatic transmission and the gear going down will be chosen by the engine brake programming. You can manually downshift, of course, but the Allison may well ignore it. The transmission considers the gear selector to be a "target" gear rather than an absolute requirement. With a sophisticated tranny such as the Allison 3000, you merely need to slow down at the crest of the hill to the speed you would like to maintain going down and then coast over the crest. With the engine brake switch on, the brake activates and the transmission selects the preprogrammed gear (typically 4th) for the downgrade. On steeper grades you may need to assist by using the service brake occasionally, but avoid "riding" the brake pedal unless the grade is so steep that constant service brake use is required. You can also downshift to a lower gear, but you will have to brake to a low enough speed to enable the transmission to actually engage that gear. It will not do so if the speed is high enough to over-rev the engine.

 

[Laura & Charles]

Technically sound advice but no longer very applicable. In general the gear going up was chosen by the automatic transmission and the gear going down will be chosen by the engine brake programming. You can manually downshift, of course, but the Allison may well ignore it. The transmission considers the gear selector to be a "target" gear rather than an absolute requirement. With a sophisticated tranny such as the Allison 3000, you merely need to slow down at the crest of the hill to the speed you would like to maintain going down and then coast over the crest. With the engine brake switch on, the brake activates and the transmission selects the preprogrammed gear (typically 4th) for the downgrade. On steeper grades you may need to assist by using the service brake occasionally, but avoid "riding" the brake pedal unless the grade is so steep that constant service brake use is required. You can also downshift to a lower gear, but you will have to brake to a low enough speed to enable the transmission to actually engage that gear. It will not do so if the speed is high enough to over-rev the engine.

Understand the point, and maybe I’m micro-managing (have had more than one employee level that as a complaint about me), but I’ve crested grades and, if I don’t manually select 4 or 3, the downhill run exceeds the high rev limit of the gear I climbed with and I end up using the service brakes to get slowed to the point where it will downshift to my desired gear.

The folks behind me often don’t seem to care for my “MO”, but pleasing them is relatively low on my priorities.

 

[Gary RV_Wizard]

I’ve crested grades and, if I don’t manually select 4 or 3, the downhill run exceeds the high rev limit of the gear I climbed with and I end up using the service brakes to get slowed to the point where it will downshift to my desired gear.

Then slow down more at the crest. Do your service braking before you start the descent. It's a "pay me now or pay me later" thing.

But yeah, what you have experienced is what most people do. They have the pedal floored trying to get up the hill at a good speed and forget to back off as they near the crest. Like there is no downhill on the other side. It's a matter of your mindset and planning just a little bit ahead.

 

[mel s]

That is an engine brake, aka jake brake. An exhaust brake is either on or off with no selection for cylinders.

I believe the correct terminology is:
Compression Brake for a Jake Brake.
Exhaust Brake for a PacBrake

See: Difference Between Jake Brake and Exhaust Brake | Difference Between

And/or this video:

 

[A Traveler]

You have a “Jake Brake” which is correctly called a compression brake. I have the same power train In my coach.

Freightliner programs the Jake to provide 100% of available compression braking 100% of the time. That’s why the transmission downshifts so aggressively as you slow down.

But what if you don’t want 100% braking all the time? What if you want just a little gentle braking for an exit ramp, or a slight downgrade? As far as Freightliner is concerned, you’re out of luck. They program the Jake this way because they think that most motor home drivers do not know how to properly use a Jake Brake, hence the 100% all the time programming. This is not my opinion. A Freightliner exec told me this In person.

The absolute best mod I ever did to my coach was to visit an Allison shop and have them change the programming so the transmission does NOT downshift when the Jake engages. It took about 20 minutes and cost $50.

I can now use the Jake all the time. The only time it is turned off is when the engine is still cold after the morning start-up. Once she’s up to 180 degrees, the switch goes on and stays on HIGH the rest of the day.

The transmission will still downshift as I slow down, but it does so down at 1,000 RPM, where the braking effort is more gentle. If you need maximum braking for a very steep hill, or a downhill stop, you can always tap the down button on the transmission control four times to target 2nd gear, and the brake will then give you 100% braking for this stop. Once you set the tranny back to “6” the new, more gentle programming is once again in effect.

 

[Gary RV_Wizard]

Freightliner programs the Jake to provide 100% of available compression braking 100% of the time. That’s why the transmission downshifts so aggressively as you slow down.

Not so on my 2004 Freightliner with ISL engine. The jJake is selectable for 50% (3 cylinders braking) or 100% (all 6 cylinders). And it came pre-programmed to target 4th gear, so not really aggressive. Back in the day they used to target 2nd gear, which is great for really steep hills but overkill for much of anything else. With 4th gear as the pre-programmed braking gear, you can still downshift to 3rd or 2nd if you apply some brake pedal to get the speed/rpms down to a safe range for that gear.

The larger engines (ISM & ISX) have 3-speed Jake brakes.

 

[SargeW]

My 2017 Allegro Bus still has 2nd gear as the target gear when I activate the engine brake. It really doesn't matter much though, as the trans won't downshift until the RPM's are acceptable for the next lower gear.

I am not on a Freightliner chassis however, instead a Tiffin Powerglide chassis. I always assumed that Allison and Cummins set the shift points and engine brake parameters anyway.

 

[Larry N.]

Not so on my 2004 Freightliner with ISL engine. The jJake is selectable for 50% (3 cylinders braking) or 100% (all 6 cylinders). And it came pre-programmed to target 4th gear, so not really aggressive.

My 2007 Beaver was exactly the same (Cat C-13 engine, Allison 4000), and while the 3-cyl selection was fairly mild (similar to the VGT on my Ventana with 360 HP Cummins and Allison 3000), the 6 cyl selection was enough to keep me no higher than 60 mph going down the grade (I-70) from the Eisenhower tunnel to Silverthorne (6%-7% grades) -- wish my Ventana VGT were that effective.

The Ventana VGT (Variable Geometry Turbocharger) DOES target 2nd gear, but never gets there unless I slow way down, which I do when the need arises (longer, steep), vs the 4th gear on the Beaver.

 

[A Traveler]

Gary, my comment about "100% braking all the time" was meant to describe the shift pattern, not whether you have the Jake Brake switch in the low, medium or high position.

If your transmission targets 2nd gear when you engage the Jake Brake, you've got the "100% all the time" setup.

 

[Gary RV_Wizard]

I always assumed that Allison and Cummins set the shift points and engine brake parameters anyway.

Sort of, but the the chassis builder typically chooses from a set of pre-made shift profiles and specifies what is desired for the particular application. Might even specify their own unique set. And the coach builder or a previous owner might have had Allison re-program to a different shift profile.