# Pac Brake on Cummins



## gene 409 (Sep 3, 2007)

Hi again with a tougher question. A couple of years ago we were coming home from a rod run towing our usual tadem enclosed trailer with car in and while coming down a step grade was using the pac brake. Once we reached the flats I turned the pac off but noticed the coach was struggling to make speed. What had happened was the brake had actually stuck. This went on for about 10 miles until we made the next small town. We drove around on a Sunday and about the time I was going to give up the coach started to run better. Slowly but surely it must of stopped retarding the cylinders and we were on our way. 

Today it doesnt do anything when the switch is flipped. My wife wants to take a trip over the Rockies and I know better than to attempt until this pac is fixed. Does anyone have any tips on something that may be relatively simple. I am pretty good with my hands and tools but do not profess to be a diesel mechanic. Maybe this is a big truck shop fix? Thanks in advance.


----------



## Guest (Sep 3, 2007)

RE: Pac Brake on Cummins

are u typing about the engine brake or u'r trailer brakes???

If it is the EB then u may have a prob in u'r trans module ,, most likely a relay sticking ,, for more info do u have the allison trans on u'rs and do u have a panel that shows what gear it is in ,, cause if it does not show a daown shift on the panel then u may have a prob in u'r trans controller,, but please post back on the type of trans u have and also about u'r trailer


----------



## gene 409 (Sep 3, 2007)

Re: Pac Brake on Cummins

Thanks for the response 730. The problem is not with a trailer but with the coach itself. It has a 6 speed Allison and it has the panel that shows what gear it is in as well. The panel reflects accurately the gear the coach is in. I looked at some PDF's online and saw the system most likely uses a selenoid to transfer the signal. I am hoping this is a relay or selenoid and not a need to dig into the top of the engine. The coach now has 60K and the problem started at 50 and one would think this would not happen this early but I guess from what i have learned about motorhomes is anything is possible at ANYTIME.  :blackeye:


----------



## Guest (Sep 3, 2007)

Re: Pac Brake on Cummins

it does sound like a relay prob ,, but to fix it u need the allison type scanner to scan the trans code and operation ,, my mobile tech's have see this prob before but ,, there was not much we could do for the customer (but we did not charge them for the call) we instead refered them to an allison trans dealer ,, any big truck company can do it ,, even cummins,,,
We do alot of work on rv's both onsight and at the shop ,, but we are limited to the chassis and eng stuff ,,, although we can fix any Cummins prob u have in our shop (sorry out of context of the post)


----------



## gene 409 (Sep 7, 2007)

Re: Pac Brake on Cummins

Thanks 730. The responses so far havent been what i was hoping for. The local cummins dealer said to dump the old pac and invest in a replacement for only 4 grand. I think I will fiddle around for a bit.  :angry:


----------



## Guest (Sep 7, 2007)

Re: Pac Brake on Cummins

I agree ,, there is something amiss on the EB system on u'r coach,, if i had it here i would do my damnist to find the prob ,, i think the Cummins dealer is blowing smoke up u'r a-- ,,, I myself feeel that u'r prob is in the controller ,, BTY did u ck the linkage to this item??? sometimes the linkage get rusty and it has to be freed up manually ,, i would also ck this and use some wd40 on it


----------



## gene 409 (Sep 12, 2007)

Re: Pac Brake on Cummins

Check the linkage....hmmm.....can you give me some pointers. Im so stupid I dont even know where to look.  :question:


----------



## dbarton291 (Sep 12, 2007)

Re: Pac Brake on Cummins

The Pac brake, or exhaust brake is normally a flapper that restricts the exhaust output flow when energized.  It may or may not be interfaced with the Allison controller depending on how the coach is wired.  Unless you've got a jacobs type compression brake, it's not integral into the top of the engine.

The simplest way to wire one is power to a switch, the output of the switch to a relay which energizes(moves or closes) the Pac brake.

To be most effective however, the switch on the dash connects power to an input wire on the Allison computer.  When the Allison computer sees power on the exhaust brake request line, it looks to make sure you are below a certain throttle level, then preselects to either 2 or 4 depending on its program, and grounds an output which closes the Pac Brake relay, energizing the Pac Brake itself on the engine, restricting exhaust flow and slowing the vehicle.  The preselect on the transmission keeps engine RPM up for maximum effectiveness of the Pac brake.

You need to make sure there's power to the Pac brake switch, the output of the switch connects ok to the Allison controller, the output from the controller goes to ground, the relay is good, and finally, the Pac Brake itself is physically OK.

Location of the relay and the Allison controller vary from vehicle to vehicle.  You need your vehicle's documentation (wiring schematic) to approach this in the most efficient way possible.  You can however, start from the wiring on the Pac brake and trace backwards to find the Pac brake relay, which then can be traced back to the Allison controller.  

Hope this helps.


----------

