PAC Brake

[Mountain Aire III]

After having survived the mountains of the west coast I am reflecting on how the PAC Brake performed. I thought the brake is for helping slow you down on big hills. When I would activate the PAC Brake the RPM's would jump considerably and I also noticed that the gear indicator said it was in 2nd. Is this right? I was afraid the motor was going to blow since it would go into 2nd when you are trying to slow down from 65mph. Any advice on the correct way to use it or how and when it should be used is appreciated.

[Ronnie Bledsoe 2006 Dutchstar 43']

Your brake and engine responded properly. The engine RPM must be limited...when my RPM gets to 2300 I use the service brakes to slow the coach down which also reduces the RPM. In using the service brakes I do not apply steady pressure but feather it so the coach slows down and the brakes do not get heated. Others may drive differently but this is my method.

[Norm4015]

Your PAC brake seems to be working properly. When you turn it on it selects a "target" gear that is programmed into the ECM. Some motorhomes are programmed for 4th gear, but most are programmed for 2nd gear. Your transmission does not drop from 6th to 2nd gear, rather it starts shifting one gear at a time towards 2nd gear and will never actually drop to 2nd unless you get in the 20 mph range.

On a level highway at 60 mph you can apply the PAC brake and it will shift to 5th and about 45 mph it will shift to 4th and about 35 mph it will shift to 3th and about 25 mph it will shift to 2nd. All this time it will show the "target" gear of 2nd on the pad. (You engine an ECM programming will determine the actual shift points.)

When you apply the PAC brake it will show 2nd on the pad, but will actually drop to 5th gear and the engine rpm will increase. The higher the engine rpm - the more effective the braking. When the engine reaches certain rpm's the engine will either downshift to 4th gear and slow you more, or it will shift back up to 6th gear to prevent engine damage.

The engine rpm during PAC braking depends on the engine and generally a smaller engine has a higher rpm that a larger engine. A Cummins ISB may run as high as 2700 rpm while a ISX may top out at 2000 rpm. The good thing is all of this is computer controlled for optimum performance and protection of the engine and transmission.

The PAC brake will have very little braking if you are going too fast down a mountain. If you start down a 7% grade at 65 mph the PAC brake won't slow the motorhome down. I like to start down a mountain at the speed I want to finish that mountain. If I want to go down at 45 mph I start down at that speed and the PAC brake will hold the motorhome at about that speed. If I pick up a little speed I apply the foot brake one time forcibly to slow down to 40-45 mph and then let off the brake. If the PAC brake slows me too much I turn it off for a few seconds and then back on. Never keep you foot on the brake because it will overheat. Use the foot brake rather strongly to slow 5-10 mph, then let off so the brake can cool.

I have driven over the Rocky Mountains many times and I have learned to trust the PAC brake, engine, transmission and programming becaause they are smarter than me. I just relax and enjoy the scenery while the motorhome does all the work.

Hope this helps.

[Latitude]

Mountain Aire

Norm is 'Bang ON' with his info/explanation!!!!

Rick

[Mountain Aire III]

WOW, terrific explanation. I feel like I finally understand the PAC Brake. Thanks and now I look forward to hitting those big hills.

[becks]

Norm.......Your explanation was indeed very well explained. Good job!!!!

["007"]

Least we all forget I have added to QUICK TIPS. Thanks Norm

[Norm4015]

Quote:

Originally Posted by "007" Least we all forget I have added to QUICK TIPS. Thanks Norm

Happy to help.

[Larry3570]

When using my PAC brake I often wonder how much the butterfly valve in the exhaust line contributes to the braking as compared to the auto downshifting sequencer. Does anyone know the answer?

["007"]

Anyone have a answer for Larry.

[MonacoMama]

Quote:

Originally Posted by "007" Anyone have a answer for Larry.

Maybe we should move this to a different forum to get more answers???

[Shaun and Amy]

Quote:

Originally Posted by Larry3570 When using my PAC brake I often wonder how much the butterfly valve in the exhaust line contributes to the braking as compared to the auto downshifting sequencer. Does anyone know the answer?

I had a issue where the PAC brake on my truck stopped working. It would still downshift, but the butterfly valve would not move. Anyway, I did notice a difference. I have no specifics, but it was noticable. I repaced the bad one with a new PAC brake that has the two stages in it, and was extremely glad it was back.

[Norm4015]

Quote:

Originally Posted by Larry3570 When using my PAC brake I often wonder how much the butterfly valve in the exhaust line contributes to the braking as compared to the auto downshifting sequencer. Does anyone know the answer?

Larry, the answer is "it depends." If I had to venture a guess I would say the exhaust brake contributes up to about 80% of braking and the transmission about 20%. But they work in concert and the braking varies. The below applies to PAC brakes or exhaust brakes and not to Jake brakes or engine brakes, and to Cummins engines. I have no information on CAT engines, but I assume they are similar to Cummins.

Cummins ISB and ISC engines come with exhaust brakes, Cummins ISL engines come standard with exhaust brakes, but most RV manufacturers now order them with optional two-stage engine brakes. All engines larger than the ISL come standard with engine brakes.

The smaller the engine the higher the rpm. Also, the smaller engines provide less exhaust braking. The Cummins ISB tops out at 2800 rpm with exhaust braking and if the engine speed increases the transmission will up shift to drop engine speed and prevent damage. When it up shifts the engine rpm will decrease and exhaust braking will decrease. At 1000 rpm, which is just above idle, the ISB exhaust brake provides 30 braking horsepower, at 2000 rpm it provides 85 braking horsepower and at the max of 2800 rpm it provides 150 braking horsepower. So you can see between 2000 rpm and 2800 rpm the braking horsepower nearly doubles.

The Cummins ISC's braking horsepower is 1000 rpm (45 braking hp), 2000 rpm (120 braking hp) and 2600 rpm (170 braking hp). The Cummins ISL's braking horsepower is 1100 rpm (75 braking hp), 2000 rpm (250 braking hp) and 2400 rpm (320 braking hp).

From the above you can see to obtain the maximum braking you need to keep the engine rpm as high as possible. This may be difficult to do because the ECM selects the transmission gear for optimun braking and engine protection. But you can help it out by selecting a lower gear and using the foot brake to slow down until the transmission down shifts and the engine rpm increases.

There is nothing in the motorhome to indicate the exhaust valve has closed and is actually providing braking. The ECM sends a signal to the exhaust brake to close and that signal usually turns on a light on the dash indicating the exhaust brake is active. However, the exhaust valve is mechanical and it may stick open. The signal to the exhaust brake solenoid is happy and thinks everything is working correctly, but the valve is stuck and provides no assist to braking. The valve can also stick closed making the engine run like crap.

A friend with a Cummins ISC thought his exhaust braking was not working correctly and he went to a Freightliner shop. They connected a computer and took the motorhome for a test drive. The technician said the computer showed the exhaust brake was working. Later he stopped at a Cummins shop and they did the same test with the same results. Leaving the west side of Death Valley we hit a 9% grade for 9 miles and his brakes overheated. He came within 2 seconds and 300 feet of going over a cliff and I still have nightmares of what I witnessed. He was towed to a Cummins shop where they removed the exhaust brake and found it had never worked.

So an indicator on the dash only says a signal was sent to the exhaust brake and not that it actually closed. The valve can rust or seize in place and there are two things you can do to prevent failure. 1) Every time you drive use the exhaust brake at least once to move the valve. 2) Once a year lubricate the exhaust brake and manually move the value to work lubricant into the pivot points.

Many places sell the hight temperature lubricant and I buy PAC Brake lube. I lube mine once a year at oil change time. PAC Brake has a webpage that shows their exhaust brake and the proper lubricating procedure.

Sorry for the long reply, but for your question - the exhaust brake, if working properly, will provide between 10% and 80% of braking while the transmission will provide between 20% and 90%. Used properly the exhaust brake will provide 70-80% of the braking.

[Shaun and Amy]

Norm is correct, that in a motorhome you would not be able to really tell if it was working or not. I first, only knew because I can hear it working, because it's a regular truck. When I talked with a tech at PAC Brake they said you can test it by hooking the selnoid up to a battery. As you give it power you can hear the selnoid work and can watch for the piston to move the valve. This way you can determine which piece is not working.