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Exhaust brake always on?

24K views 43 replies 19 participants last post by  dmaxjr 
#1 ·
I'm not really understanding this exhaust brake... I drive in the hills often and when I'm headed downhill and even tap the brakes, the truck downshifts and the RPMs soar. If I turn ON the exhaust brake while this is occuring, nothing happens; but if I then turn it back OFF, the truck upshifts immediately.

So basically, when I want to roll/pick up speed downhill I have to quickly ON-OFF the exhaust brake to get it to upshift. Otherwise it will hold a low gear and RPMs will go crazy high if I give it gas.

Is this normal? Sometimes I just wanna rolllllll downhill...
 
#12 ·
Hill Descent assist is only for low speed up to 20 mph used in steep off road applications.
The Exhaust brake is a function of the variable turbo to close exhaust under zero throttle, creating more cylinder pressure to slow you while towing or descending hills at any speed that the Torque convertor is locked to aid in the hold back..
The grade braking is when transmission downshifts, locks TC and uses engine compression to slow.
 
#3 ·
Are you referring to this?:

"Normal Mode Grade Braking is
enabled when the vehicle is started,
but is not enabled in Range
Selection Mode. It assists in
maintaining desired vehicle speeds
when driving on downhill grades by
using the engine and transmission
to slow the vehicle"

If so what's the difference between this and the exhaust brake? Functionally, they appear identical. It also appears there is no way to disable this feature. When there is traffic on a down grade, I don't like the truck revving to >4k while I try to lightly accelerate back up to speed!
 
#4 ·
Yes that is what I was referring to


I do not know the difference of that and the exhaust brake. I've never driven a diesel to know how it works.
They are entirely different I know, but I don't know why/when one is better than another.

You are correct, the system you outlined above cannot be disabled.
 
#5 ·
...

You are correct, the system you outlined above cannot be disabled.
My understanding, but we don't have a lot of long hills around here to test this on, is pressing that long, skinny pedal to the right of the brake pedal will override the hill descent feature.
 
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#7 ·
I think the exhaust brake is used as a supplement to the grade braking. Therefore, if you're not towing, downshifting is enough to maintain speed and the exhaust brake won't do much even in tow/haul mode. As you add weight and downshifting isn't enough, the exhaust brake will apply more force.
 
#11 ·
In T/H mode WITH a trailer behind you. Otherwise Normal Mode Grade Braking is in effect.
 
#10 ·
Right.
 
#13 · (Edited)
As far as a know, the exhaust brake uses the variable nozzle geometry in the turbo to create back pressure. The nozzles can close up pretty tight so that there is very little cross sectional area for the exhaust to flow through and thus it makes the engine work harder to push that exhaust out. It also creates higher velocity going into the turbine which in turn creates more boost and helps force even more flow through the engine creating even more back pressure. It takes hp to spin the turbo and when you engine brake that horsepower is coming from the the trucks kinetic energy.

My guess is if you have torque pro or some similar obd monitor you will see that boost is higher with the exhaust brake on while coasting downhill.

When I test drove the dmax I tested the exhaust brake out on a very steep hill, like 9% grade and found that it didn't do a lot at lower rpms but once you get to 3k+ you could tell. Since you are not injecting fuel and making heat while engine braking it requires a lot more flow to get the turbo spun up which is why it doesn't kick in until higher rpm.

I also wrote a tune for my Jeep after seeing this feature and got the same effect. In the Jeep's case I cannot turn it on and off with a switch so I set it up so that it only closes the vanes above 2600 rpm and 0 throttle. I was able to achieve about 50% more pressure(3psi of boost vs 10-12psi) while engine braking. I think the newer vnt turbos are better than the one in my jeep

Sent from my Nexus 6 using Tapatalk
 
#14 ·
I know exactly what you are talking about and it is somewhat annoying at times. Its like it decides on its own when to downshift and revv the piss out of the engine. I have found that if you put it in M and + up to 6th and leave it there when you drive, it will coast for you.
 
#15 ·
@Newfaner

As evidenced in the manual:
"Normal Mode Grade Braking is enabled when the vehicle is started,
but is not enabled in Range Selection Mode. "
 
#16 ·
I would think that even the 'normal' grade braking must use the turbo to generate some backpressure. By default a diesel has no way to give you any engine braking. Engine braking is a result of restricting the airflow either into or out of the engine. A gasoline engine has a throttle plate to restrict airflow through the engine (aka. air pump). Before the variable geometry turbos were common, a simple butterfly valve (like a throttle with a pre determined amount of leak) was installed in the exhaust to provide an exhaust brake.

The full size GM trucks utilize a 'transmission brake' that is part of the allison for what feels like engine braking. I am not sure exactly how the allison system works. There are aftermarket kits to make the turbo on the full size behave the same way as in our trucks so that you get true engine braking as well. The two combined will slow a 16k trailer in very short order.

The variable geometry turbo is also used to generate back pressure for the EGR system as well. In the cummins world they use this function of the turbo to light off and control the DPF regen burn.
 
#17 ·
Sorry, to clarify: I have a Duramax LT. It doesn't have a tow/haul mode, just the exhaust brake on/off switch on the console. To those who have tried it: does it invariably send the RPMs way up? I also can't tell a difference in sound between it on/off. I'll try to have someone shoot a video this weekend.
 
#18 ·
I have an LT also. The exhaust brake switch also functions as the tow/haul mode switch. You can not operate them separately and if you have a diesel you have both functions.

There will not be a loud noise from an exhaust brake. It is not a decompression brake... aka Jake brake. They are functionally quite different. A decompression brake vents cylinder pressure to atmosphere on the compression stroke. That is why they are so loud. An exhaust brake simply restricts the exhaust path. If anything it would quiet the exhaust.
 
#19 ·
Are engine RPM actually being increased or is the RPM increased because it has down shifted.

I've read repeated posts indicating "the RPM is fast".....Not sure you you do that without changing gears.
Educate me.

For example, back in 1983, if you shifted the vehicle in 2nd gear the RPM was increased. No wait, my 2008 does the same thing. Nowhere did I "increase the RPM" but rather the RPM matched speed/gear selection.

I'd like to know if something electronic in fact increase the RPM.
 
#20 ·
The RPM increase is from a downshift of the transmission. If the road speed is slow enough and you tap the brakes a second time it will downshift another gear and give you even more braking effect... of course the RPMs increase again correspondingly.

I got a lot of experience with this on my 2000+ mile trip to the Rockies last week while pulling a trailer.
 
#21 · (Edited)
I was towing my 23' travel trailer and tried the exhaust brake (tow/haul - exhaust brake switch on) on moderate downgrades and it didn't really do jack. Other than the downshift and higher revs, I couldn't tell a difference. It really doesn't help slow the truck and it doesn't crank up the exhaust note like you would expect. I would hope for a little "Jake Brake" noise to affirm that it is working.
 
#22 ·
I was towing my 23' travel trailer and tried the exhaust brake (tow/haul - exhaust brake switch on) on moderate downgrades and it didn't really do jack. Other than the downshift and higher revs, I couldn't tell a difference. It really help slow the truck and it doesn't crank up the exhaust note like you would expect. I would hope for a little "Jake Brake" noise to affirm that it is working.
Well again they are using the vnt mechanism in the turbo to provide a little extra engine braking. The turbo was not designed with this function in mind but it does make a difference.

Plus, the engine is small, even with the exhaust brake it can only provide so much stopping power. My ecoboost is only a 3.5L and one of the few complains people have while towing is that it doesn't have as much engine braking as the 5.0 v8. It simply doesn't move and compress enough air.

Sent from my Nexus 6 using Tapatalk
 
#26 ·
Did some experimenting yesterday. If you put it in manual and click up to 6, it drives much better. Keeps the trans from down shifting on touching the brakes. Noticed slightly higher rpms when in manual with the exhaust brake on, so it must be doing something...
 
#27 ·
I experimented over the weekend as well. I ran the same set of hills multiple times to try and find differences. I found that the amount of engine braking was highly dependent on the angle of the grade!

Exhaust brake on, shifter in "D"...

  • Steeper grades gave far more engine braking than was given on shallower grades.
  • There is also more engine braking when a trailer with brakes is connected compared to not. I did not try different trailer weights or one without brakes.
  • Engine braking was slower to disengage after a steeper grade or while trailer connected.

Exhaust brake off, shifter in "D"...

  • I found some engine braking after applying the service brakes a considerable amount on grades that were steeper.
  • Couldn't detect engine braking on shallow grades even with considerable service brake application.
  • Light service brake application on steep grades didn't seem to engage engine braking.

Exhaust brake off in manual shift mode...

  • I found pretty much NO engine braking unless I shifted down to M2.
  • When in M2 it was a significant amount, similar to steep grade with exhaust brake switch on.

I would conclude from the above that the ECM is monitoring the inclination of the vehicle and whether a trailer is connected. It also might be monitoring vehicle load (weight), but I didn't investigate that.

I also got to thinking that this 2.8 DOES have a throttle plate on the intake for generating a bit of lower pressure for EGR to function. It could be possible that they are using it to provide some engine braking effect as well. I think I will investigate this some more and see if I can get some help coming up with the codes to monitor the "throttle plate" from my scanguage or borrow a scan tool from the dealer for a few minutes.
 
#29 ·
With an automatic transmission downshifting will ALWAYS be a 'component' of producing engine braking. Let me see if I can explain a bit and that might help you rephrase your question to something I can maybe test for...

As you likely know, engine braking comes from the engine's "resistance to turn" due to restricting the flow of air through the engine. The three common methods to achieve this are an intake brake, an exhaust brake, and a compression brake. I won't go into compression braking (aka jake brake) since that doesn't apply to any of the engine options available on our trucks. In its most basic form a diesel engine has none of these. This is still the case for many off-road diesel applications. On a gasoline engine there is a built in intake brake. We call it the throttle plate. Until recently, for purposes of EGR on turbo applications, diesels had no throttle plate. In contrast to restricting the intake, diesels have commonly used an exhaust brake. In its simplest form this is simply a throttle plate in the exhaust system that provides up to around 80% blockage. The exhaust brake on our trucks actually comes from the variable geometry turbo being used to restrict the exhaust flow. This is actually very effective.

The other component to engine braking is how the driveline is linked or coupled to the engine. Unlike 'most' manual transmissions, automatic transmission have one or more over-running clutches. All modern automatic transmissions that "I" am aware of (I have never been inside any of the 8 speeds) have an over-running clutch for all gear ratios that are 1:1 or less. I haven't looked at the specs or parts diagrams for the trans in the Colorado but I highly doubt it is different. An over-running clutch does just as the name implies. It allows the output of the clutch to turn faster than the input, but never slower. This is what allows a transmission to coast when in the old equivalent of "drive" or higher. In the twins, 4th gear is 1:1 and both 5th and 6th are overdrive ratios. That means when the transmission (actual trans, NOT shifter) is in any of those 3 gears you will not have ANY engine braking (regardless of engine type or size) since the over running clutches allow the output shaft to turn at any speed equal to or higher than the input of the over-running clutch.

How far you downshift will have a direct effect on how much engine braking you experience. In a lower gear (say 2nd) the road speed tries to turn the engine faster than in a higher (say 3rd) gear. The restriction to airflow only wants to allow so many cubic feet per second to pass through. If you turn the engine faster it is trying to move more air than when it turns slower, therefore you get more engine braking effect in the lower gear. The balance is to NOT try to turn the engine at such high RPM that you brake something.

So as you can see, simply engaging the exhaust brake while in 6th gear is doing the same thing as closing the throttle on a gasoline engine while in 6th gear and will have no effect since the over-running clutch lets the tail shaft of the transmission turn as fast as it wants to. The transmission must be in a gear that does not have an over-running clutch, but you ALSO must have a restriction to airflow through the engine. It does very little to be in a low gear on a diesel engine without some sort of restriction in the intake (EGR throttle) or exhaust (turbo used as exhaust brake) that is trying to restrict the flow significantly. Without the restriction it would be similar to downshifting a gasoline engine but holding the throttle wide open. There is very little restriction and until you get to extreme engine RPM there is little or no engine braking. It is really a combination of the lower gear AND the airflow restriction that produces the engine braking effect regardless of what 'mode' GM labels the function.

Now the big question that "I" have on the twins, is what does GM use for what they call the "grade assist" function. Is GM using the turbo as an exhaust brake, though less aggressively, or are they using the throttle plate that is present in the intake for the EGR. That throttle plate doesn't appear to close as completely as the throttle on a gasoline engine. It just provides enough restriction to create a venturi effect to help suck in the EGR flow under increasing boost conditions. I plan to do some tests while watching the boost pressure to see if I can tell if the restriction is coming from the intake or exhaust side during what GM calls 'grade assist' conditions. As long as it works, it doesn't really matter, but I am a curious cat.

Newfaner... I am not sure if there is a way that I can test to see if the exhaust brake is being utilized at 100% before an additional down shift occurs. Is that what you intended to ask with your question? If I am not understanding your question correctly, please beat me on the head a bit so I can get on the same page. Sometimes I'm a bit thick.
 
#30 ·
My understanding is for normal grade assist, GM is using the transmission, i.e. downshift.
 
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#31 ·
Yes. Downshift and TC lock for normal grade braking. If you use EB/TH and/or Cruise Control you get both working together just like its big brother, hit EB button ,activate cruise and sit back ,relax ,enjoy scenery. You get full effect of the EB when a trailer is plugged in, without trailer the EB is far less aggressive.
 
#32 ·
Have found when playing with the TH/EB that even on level ground with no trailer the EB is pronounced and noticeable. Truck sheds speed way to fast to say that only downshifting is causing it... Even on flat ground my TH/EB will almost bring the truck to a full stop (down to 15-20km anyways). Obviously the less exhaust volume the EB has to work against the less effective it becomes; so under light load conditions it's harder to notice.
Works great! Love it in stop and go traffic on the 401. Looking forward to getting a trailer on this bad boy and putting it through its paces :)
 
#33 · (Edited)
Also noticed that if TH mode is engaged while you are already breaking the EB will not take effect. You need to lift off the brake and then reapply, at this point the EB will engage. Have yet to try this senario in reverse.
The transmission for sure shifts and hangs onto gears into a higher rpm in TH mode; not only would this help to stay in the powerband and make the truck more willing to get its load moving, but also provides a larger exhaust volume to the turbo vanes for a more pronounced braking effect... I believe anyways, I'm no professional ;)
 
#34 ·
Well everyone on this thread sounds like they know a heck of a lot more than I do on this as I know zero. My first diesel. I have used this exhaust brake on hills and notice the scenarios mentioned. My probably real lame question is: Does one engage this button only descending hills and then turn it off or does one leave it on regardless of whether you're accelerating or cruising, while driving .....?????
 
#35 ·
I have tried Tow/Haul mode with an empty truck not towing a trailer and found exhaust braking to be non-existent.

Per the owners manual: Tow/Haul is designed to be most effective when the vehicle and trailer combined weight is at least 75 percent of the vehicle's Gross Combined Weight Rating (GCWR). See “Weight of the Trailer” under Trailer Towing.
Tow/Haul is most useful under the following driving conditions:
. When pulling a heavy trailer or a
large or heavy load through
rolling terrain.
. When pulling a heavy trailer or a
large or heavy load in
stop-and-go traffic.
. When pulling a heavy trailer or a
large or heavy load in busy
parking lots where improved low
speed control of the vehicle is
desired.
 
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#36 ·
Empty I noticed very little change.
I put 1200Lb (20 60lb bags) of Concrete in the Bed and turned it on going down some small hills and it nearly brought the truck to a stop going down hill.

With our Travel trailer which is around 4000LB (have not had it weighed) The exhaust brake is amazing. 150ft before the exit ramps tap the brake and the truck revs up and slows right down for a nice easy exit with very little actual brake usage.

I did notice it seems to sense if you are just slowing a little or if you are trying to slow down a lot. Depending on how much you are trying to slow down it appears to use more or less RPM.
 
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