exhaust brake

[rudischaffner]

20066 36ft journey cat 350   when i go downhill should i leave the tranny in 6th gear and let shift on its own or should i shift down to 5th or 4th gear

rudi

[jleamont]

Rudi, let it shift on its own. The only thing I do is monitor engine RPM’s, if they climb too high I step on the brake to keep it in a safe range (Stab brake technique). 

Sometimes it gets a bit too close to redline for my likings.

[rudischaffner]

thanks

[richard5933]

When you say "go downhill" how steep of a grade are you talking about?

[obedb]

If you are headed down a grade that requires a lower speed limit for 18 wheelers, maintain that speed for your motorhome. I shift manually with our Cat powered  Phaeton with exhaust brake. Put the transmission in a gear that will keep the unit at that lower speed and within the Engine RPM limits. Some gentle brake application may be required. If you want the exhaust brake to do everything, you will probably be passed by the big rigs that are maintaining their required speed on the descent.

[huffypuff]

The exhaust brake and automatic downshifting is only there to assist you to slow your vehicle down.  You have to use your regular brakes if your RPM and speed gets too high.  

[rayin]

3 hours ago, huffypuff said:

The exhaust brake and automatic downshifting is only there to assist you to slow your vehicle down.  You have to use your regular brakes if your RPM and speed gets too high.  

Exactly! 👌 If I may expand upon your advice;

When engine RPM's are allowed to exceed the  braking red-line the ECM will  command the TCM to upshift, the operator has no control over that up-shift. When this happens you only have your service brakes to reduce speed and engine RPM's to the point the transmission will again downshift to a lower gear. It does not matter which gear you manually select, the ECM will not permit a downshift until the maximum RPM will not be exceeded in the selected gear.

For the ISC engine the braking RPM red-line is 2,800, vs the red-line under power, which is 2,500 RPM.

reference: https://quickserve.cummins.com/info/qsol/products/newparts/jacobs_md_nondodge.html

.  The RPM chart within the quickserve link shows the optimum RPM level to obtain the most benefit from your exhaust brake.

[Lenp]

Keep in mind the exhaust brake works better at higher RPM. Thus, the lower the gear you can safely get to the better the braking.

Lenp

[wolfe10]

Yes, the exhaust brake activation SHOULD cause the transmission to automatically downshift.  Look at the Allison shift pad when you engage the exhaust brake.  It SHOULD change from "6" to a lower gear.  4th and 2nd are common choices, but any gear can be programmed in as the pre-select gear by an Allison dealer. 

The "turn on exhaust brake" signal actually goes through the Allison ECU.

[manholt]

I have exchaust high/low break and engine break...used both in Quebec Providence, we had one 3 mile downhill at 27 degrees!  Linda took a picture of the sign.  

[wolfe10]

3 minutes ago, manholt said:

I have exchaust high/low break and engine break...used both in Quebec Providence, we had one 3 mile downhill at 27 degrees!  Linda took a picture of the sign.  

With both low and high, I suspect you have an ENGINE COMPRESSION BRAKE, not an EXHAUST BRAKE.  Exhaust brakes are either on or off.  With an engine compression brake you often have the ability to select either some or all the cylinders slowing you down.

The two are vastly different in how they work.

[manholt]

One lever says exhaust break hi/lo...the other says engine break.  2 separate rocker switches.

[wolfe10]

Well, either exhaust brake or compression brake can be casually referred to as an "engine brake".  Exhaust brakes are found on "smaller" diesels, with engine compression brakes being offered on larger engines, starting with the Cummins ISL (8.9 liters).

[five]

My DD 13 has a three speed compression brake.

[wolfe10]

58 minutes ago, FIVE said:

My DD 13 has a three speed compression brake.

Yup, that allows you to engage 2, 4 or all 6 cylinders.

[manholt]

OK, I'm so confused.  One switch say's on/off (M) the other say's Low/High (M), does that mean I have "compression brake?"  

[wolfe10]

Carl,

With the Cummins ISL it could be either.  Call Cummins with your engine serial number.  I am betting on an engine compression brake.

If

[hermanmullins]

If I am not wrong (sometimes I am not right) there are two types of systems. Pac Brake which is in the exhaust system and reduces the exhaust flow. And the Jake Brake is internal in the valve train. My Cummins 400 ISL has a Jake Brake.

And then I may be off base.

Herman

[richard5933]

18 minutes ago, hermanmullins said:

If I am not wrong (sometimes I am not right) there are two types of systems. Pac Brake which is in the exhaust system and reduces the exhaust flow. And the Jake Brake is internal in the valve train. My Cummins 400 ISL has a Jake Brake.

And then I may be off base.

Herman

Then there's also the related, but different, retarder - placed inside the transmission but essentially has the same function - slowing down the vehicle without having to use the service brakes.

[rayin]

3 hours ago, manholt said:

OK, I'm so confused.  One switch say's on/off (M) the other say's Low/High (M), does that mean I have "compression brake?"  

That describes an engine/compression brake, turn off/on , then select hi/lo depending on downgrade % of braking force required.The engine brake is much more powerful than any exhaust brake, and it is the one that makes the loud BRAaaaak sound when slowing. Essentially what it does is turn the powerful diesel engine into an air compressor - in reverse, and has the same HP rating for retard and propulsion.

Often wish I had one.

[manholt]

Cummings 425, I'll do as Brett recommend.  

Ray, mine does not sound bad, Linda has been behind me and knows when it's on, say's not like the sound a 18 wheeler makes.

[five]

5 minutes ago, manholt said:

Cummings 425, I'll do as Brett recommend.  

Ray, mine does not sound bad, Linda has been behind me and knows when it's on, say's not like the sound a 18 wheeler makes.

Carl, on our coaches the compression brake, when working, also turns on the brake lights.

[ispjs]

My engine brake also activates the brake lights when I'm using it (which I think is a good idea).  Last year there was some Class A owner unhappy with this option and was trying to figure out a way to turn off the brake light feature (different forum).  I guess a commercial truck driver followed him down a grade and then asked him at a nearby fuel stop why he was riding his brake all the way down the hill, or something of that nature.

I have noticed more and more newer 18 wheelers with this feature now though, so apparently Freightliner and others are not just putting it on "custom" chassis.

[richard5933]

1 hour ago, ISPJS said:

My engine brake also activates the brake lights when I'm using it (which I think is a good idea).  Last year there was some Class A owner unhappy with this option and was trying to figure out a way to turn off the brake light feature (different forum).  I guess a commercial truck driver followed him down a grade and then asked him at a nearby fuel stop why he was riding his brake all the way down the hill, or something of that nature.

I have noticed more and more newer 18 wheelers with this feature now though, so apparently Freightliner and others are not just putting it on "custom" chassis.

With the computer tech onboard nowadays, you'd think that they can figure out a way to have the engine brake apply the brake lights according to the amount of braking action being provided. If the system is just holding a steady speed then no brake light, but when the engine brake is causing deceleration the system recognizes this and applies the brake lights. This would stop the brake lights from being on all the way down the mountain, but would still provide warning to other driveways when appropriate.

[jleamont]

9 hours ago, manholt said:

Cummings 425, I'll do as Brett recommend.  

Ray, mine does not sound bad, Linda has been behind me and knows when it's on, say's not like the sound a 18 wheeler makes.

You most likely will not notice excess exhaust noise due to the Diesel Particulate Filter, in a sense you exhaust is clogged all the time thus reducing exhaust output noise.

 

13 hours ago, hermanmullins said:

If I am not wrong (sometimes I am not right) there are two types of systems. Pac Brake which is in the exhaust system and reduces the exhaust flow. And the Jake Brake is internal in the valve train. My Cummins 400 ISL has a Jake Brake.

And then I may be off base.

Herman

YUP, you are correct! Monaco in our years ONLY opted for Compression brakes on the ISL-400 and larger on their higher end coaches. The Windsor had an exhaust brake standard with an option for compression brake. 

 

2 hours ago, ISPJS said:

I guess a commercial truck driver followed him down a grade and then asked him at a nearby fuel stop why he was riding his brake all the way down the hill, or something of that nature.

Believe it or not on a Commercial vehicle illuminating the brake lamps with an engine brake is an option and its an option you wont see often, RV's it appears to be standard. It is certainly programmable should someone want it turned off if the engine is electronic.