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AttilaKara72 05-11-2013 08:45 PM

How does the Pacbrake work?
When I use my Pacbrake it seems that all it does is downshift the transmission to second gear and over revs the crap out of the motor.
Am I doing something Wrong? Am I missing something?
Engine brakes that I have used in tractor trailers work completely different.

336muffin 05-11-2013 08:50 PM

You are correct......true engine brakes(Jacobs) work in conjunction with the valves in the heads and work very well. Exhaust brakes are pitiful at best compared to them. Your Eb is working as designed.

CorLynn 05-11-2013 08:52 PM

Sorry if I'm not much help. I also have a PAC brake and it also down shifts , not sure too much more on it, I'm not the driver. My Hubby is. And as a passenger I can hear the motor downshift. I think?

Mr_D 05-11-2013 09:09 PM

When the exhaust brake is activated and you back off on the throttle the transmission preselects a lower gear, usually 2nd. The controls then downshift aggressively as possible, keeping the engine RPMS in check but still high enough to be effective. On and ISC Cummins the engine rev limit under throttle is 2,200 rpm, when the ebrake is activated the engine is allowed to go up to 2,400 on a closed throttle.
The higher the RPM the more effective the ebrake is.

JohnH59 05-11-2013 09:15 PM

Besides downshifting the engine, a flap is closed on the exhaust causing a lot of back pressure slowing the engine. Using the back pressure and transmission downshifts, it slows the coach considerably. Engine brakes are much stronger at holding back the engine, bid, the PAC brake does a good job on most coaches up to around 40'. It works great on our 40' American Dream.

Papa_Jim 05-11-2013 09:38 PM

You need to check to make sure that the exhaust brake is actually working. You can grab the flapper arm and see if it moves freely. If it doesn't, you need to lube it and get it freed up and working.

avfordguy 05-12-2013 12:56 AM

Ok I would like to ask a question on pacbrake operation. MY 99 dip with a 275HP ISB the pac brake would close at closed throttle,downshift,without touching the brake pedal. My 04 dip with 330 ISC has the pacbrake, it just works different! Pacbrake switch on, closed throttle, all it does is downshift until I apply the service brakes then the pacbrake closes.
I can feel the difference, and hear the difference in exhaust tone with the driver window open. I also noticed that with the pacbrake switch on, the green light (on the switch) only lights when I apply the service brakes. is this right?

Gorlininc 05-12-2013 01:00 AM

When you release the throttle pedal, you should feel and hear the exhaust flap closing the exhaust manifold. Once it slows down to a predetermined speed, then it downshifts to 2nd gear. Sounds like your exhaust flapper is stuck or not working.

EngineerMike 05-12-2013 01:53 AM

Attila- the PacBrake function is the two elements discussed above, a butterfly valve closing in the exhaust path to build up back pressure on the engine, and downshifting to put the engine into higher revs where the back pressure will be more effective. If you don't experience substantial braking, the butterfly valve is likely stuck & needs service. You'll like it once its returned to proper function.

Route 66 05-12-2013 05:32 AM

A Pacbrake stuck in the open position is the most common problem reported by our members.

AttilaKara72 05-12-2013 08:05 AM

What lube do you all recommend to lubing the Pacbrake?

palehorse89 05-12-2013 08:31 AM


Originally Posted by AttilaKara72 (Post 1564466)
When I use my Pacbrake it seems that all it does is downshift the transmission to second gear and over revs the crap out of the motor.
Am I doing something Wrong? Am I missing something?
Engine brakes that I have used in tractor trailers work completely different.

Here is some reading on this subject, I went through the same thing and did not like the factory program. I had it programed to stay in 6th (no down shift TO 4TH OR 2ND, still does its normal downshifting, just not the harsh downshift with the exhaust brake activation) and I have been very pleased for 2 years now. Even though it is programed to stay in 6th gear(no downshift w/exhaust brake) it still downshifts right down to 1st gear (5 mph) and kicks off.
On this thread this gentleman is asking about a engine brake on a ISL

src 05-12-2013 05:20 PM

how do u know if it working

palehorse89 05-12-2013 05:30 PM

When you let off the pedal and the transmission downshifts just press down on the throttle pedal a "wee bit", you should feel the exhaust brake go off, let completely off the pedal again and you should be able to feel it slowing you. When I mean a "wee bit" I mean just a slight amount, to much and your transmission will up shift and when you let off again it will downshift. There is a small window at the top of the pedal that you have to get use to to use it this way and test it. Give this a try in your next drive and experiment with what I have posted and you will see. Also , if you open your side drivers window you ought to be able to hear it come on especially if you are beside something for the exhaust brake noise to bounce off of.

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