Coasting downhill with Pac Brake

[Wagonmaster2]

I have a 2004 Meridian Cat C-7 330HP coach with the Pac Brake.   I know with the Pac turned on as soon as I step on the brake peddle the tranny starts downshifting toward 2nd gear,  but without touching the brake peddle I can't tell that the Pac Brake is really doing anything toward slowing the coach any.

While coasting down hill can any one with this Pac tell if it does anything by turning it on/off during the decent?   I have done this several times and can't detect any slowing motion at all.   I do keep the Pac oiled at all the suggested points with regular Pac lubricant a couple times a year and can move the rod in/out during this lubricating so I know it's not frozen up.

My next trip to a Freightliner shop I want to get them to change the downshift point to either 3rd or 4th gear instead of 2nd gear, which seems way to low.   Haven't decided yet which gear I want.

[SargeW]

With a coach as heavy as yours, you will not notice a significant amount of difference with the Pac Brake.  They do work, and work well for what they are designed to do.  If there is a concern if it is working, you should be able to short the switch at the brake and watch the actuator arm close.  If there is any doubt, have the brake taken off and tested to make sure that the butterfly valve is closing as tightly as it should. 

As far as changing the shift points, it may not be safe to do so.  Especially if the Pac Brake is not working as well as you expect, the only retarding action would be the trans downshifting, holding you back.  Going down a significant grade you will want all the holding power you can muster.  Just my .02

[SCVJeff]

It's not a stunning difference, but you can tell the difference by finding a long coast, let the Allison get to it's happy speed and gear, LOCK into the gear in manual, then switch the exhaust brake on and off. Now all that switch is doing is running the Pac itself.

[Jim Dick]

Allen,

The pac break works in conjunction with the engine/transmission. It will slow the coach if it is within the parameters of the rpm's of the engine.

The engine/transmission are smart enough to know when not to downshift to the next gear for fear of harming either one. 

Watch your engine rpms. When the exhaust brake is on, the rpms should not rise above normal. With it off, you may see a rise above the acceptable amount.
 

[Gary RV_Wizard]

You should feel some difference, but it won't be marked until the speeds - and the actual gear - gets lower.  In 5th or 6th gear, there is very little effect on speed, but as you  get down to 4th and 3rd and 2nd, you will feel it more.  Help the PAC by applying the service brakes firmly but briefly to get speed down where the tranny can downshift to a lower gear.

I recommend that you do NOT change the target gear to a higher one. That would defeat the purpose of the PAC brake at lower speeds.  The target gear can't be too low, because the transmission will never reach it unless speeds get low enough anyway. By targeting 2nd gear, you get firm brake assist once speeds get down around 20 mph. If the target was 4th gear, you would get no additional brake effect below about 35-40 mph.

It is possible your PAC is inoperative. With the PAC engaged and descending a steep grade, brake down to 35-40 mph and coast a few seconds, then switch the PAC off. You should feel an upshift and an increase in speed.  If you do not, have it checked.

[Ned]

I know with the Pac turned on as soon as I step on the brake peddle the tranny starts downshifting toward 2nd gear,  but without touching the brake peddle I can't tell that the Pac Brake is really doing anything toward slowing the coach any.

It sounds like your exhaust brake is programmed for latch mode.  When it's turned on, it won't engage until you press the service brake pedal.  That engages the exhaust brake and preselects the lower gear on the transmission.  Press the accelerator and the exhaust brake disengages and the transmission shifts to the highest gear.

[Jim Logan]

  Allen

   I have a 2006 Country Coach and it is set up so when the cruise control is on the exhaust brake will take over when the coach get to 5 mpg over the set point or I use the service brake. But if the cc is off the exhaust brake will take over as soon as I let off the gas pedal.

 

[ArdraF]

I also would not recommend changing the target gear to a higher one.  If you ever drive in the western mountains you WILL want the lower gear!  There are at least three downgrades in California and Nevada that require use of the service brake even with the Pac or Jake engaged because the grades are steep enough and long enough (10 miles or more) that it simply cannot hold back a 40,000-lb motorhome and a toad.  We had a Pac in our previous motorhome and a Jake in our current one.  The Jake has two target gears, high and low, and with our mountains in the west we tend to use the lower gear instead of the higher gear.

ArdraF

[Mavarick]

X-3 on leaving the gearing headed for 2nd. Between WA state and AK through the Yukon there are 10% + grades and I was very happy to see the 2nd gear staying engaged keeping my rpm's up. It makes all the diff in the world when you are descending a mountain pass.
I pretty much have the same rig and I agree with Ned that your programming is set to latch. This programming can be changed by the dealer if you want without changing the gearing etc on the tranny.
As an example, If I am driving on the flat with the brake switch turned on and just let up on the throttle enough to stop accelerating, I can find a center position of the throttle to coast. If I let up a bit more the pacbrake starts to engage and you will feel a definite slowing of the MH as the rpms increase. At this point I can decide to keep the brake engaged and the rpm keeps increasing and then of course the trans will start downshifting, or, apply a little throttle and the pacbrake will open back up and I will resume coasting. I think this is the operation you are looking for.

[Jim Dick]

Our coach came targeted for 4th gear. Had thought about reprogramming it but they wanted too much money. I just manually downshift if needed. Works fine and didn't cost a penny.

[Jim Godward]

Actually we like the latch mode as it allows us to choose when we use the braking effect of the system.  We leave the exhaust brake switch on almost all the time.  When we start down a steep hill we touch the brake pedal and have the braking effect.  If on the hill it levels out for a while a touch of the accelerator and the braking goes away.  Later if we needed to brake again, a simple touch on the brake pedal and we were being braked again.

The old way was to turn the exhaust brake off and on again each time we needed it.  I think the latch method works much better.

[Jim Dick]

Jim,

Mine is also programmed with the cruise control.  If I leave it on when in cruise, it will sense when the speed increases to +3 mph at which time it will activate. Once the speed drops, it will release and the cruise continues to control the speed. Very nice on a long downhill run.

[Tom]

We have a foot switch to operate our PacBrake. It just takes a light touch of the left foot to apply the PacBrake (not a 'latching" operation), and it can be applied at any accelerator position. That means that the driver can override the condition where the PacBrake requires zero accelerator, which in turn causes the transmission to downshift; There are times when you don't want that to happen.

Cudos to Jerry, and especially Ardra, preventing me from returning the foot pedal to the Monaco parts trailer at a rally when I didn't want to install it myself. They convinced me to keep the foot pedal, return to the coach, and have a Monaco tech install it (free). Our regular driver and I think it was a good decision to listen to Ardra 

[Jim Godward]

Mine is also programmed with the cruise control.  If I leave it on when in cruise, it will sense when the speed increases to +3 mph at which time it will activate. Once the speed drops, it will release and the cruise continues to control the speed. Very nice on a long downhill run.

Unfortunately I used a BrakeSwitch to get my "latch" mode.  The programing is cheap enough, it is the 300+ mile round trip that was the killer.  I really should have had it done during all those trips to Billings but didn't seem to have the time or inclination with all the medical stuff going on.  Maybe at a rally some time.

[Wagonmaster2]

Yes, I do have the "latch mode",  I had it changed to that during one of my trips to the Freightliner shop.   I know what the Pac Brake does with either the cruise on/off and what happens with the Brake turned on/off,  other than being able tell if the exhaust part is really making any difference.     I don't like when the down shift finally reaches clear down to 2nd gear and the RPMs hit above 2,500 every time I touch the brake going down a grade.   In those instances I usually turn the Brake off and manually downshift to the gears I want.   Usually 3rd or 4th gear is enough with an occasional touch of the brake even on the Passes in Colorado, which I do a couple times a year on vacation.   I'm sure the way things come from the factory new is appropriate for new drivers who aren't familiar with driving a heavy vehicle but for most accomplished drivers it seems to be too radical and severe.   I generally do all my shifting manually going up the grade instead of waiting for the tranny to finally downshift and end up doing the same going down.

It seems like I remember reading some Posts in the past where members did change the downshift point to a higher gear and was very satisfied with the arrangement.   It would be nice to be able to change it then if it wasn't satisfactory be able to change it back without paying through the nose at a shop to have it done.   Probably the reason I haven't done it after driving the coach for 8 years.

[Ned]

The transmission won't actually shift to 2nd gear but will only downshift to the lowest gear that keeps the engine RPMs from getting too high.  As the vehicle slows down, it will downshift to lower gears, eventually getting to 2nd but you'll be going pretty slow by then.  I see no advantage to programming a higher preselect gear on the Allison.  Read Gary's excellent explanation above on how this works and why you shouldn't change the preselect gear.