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Topic: Pacbrake not working on 1997 Cummins C8.3  (Read 1037 times) previous topic - next topic

Pacbrake not working on 1997 Cummins C8.3

Good morning from a "wishin-I-was a Foretraveler" who has enjoyed reading and learning from this forum's members for quite sometime now and would truly appreciate a little guidance regarding my non-responsive pacbrake.  Anyway, my wife and I are heading out today for a road trip and yesterday while driving the coach home from storage I noticed the pacbrake was not responding when I activated its rocker on/off switch to the "on" position.  I do have power to the switch as it's red "on" light illuminates, yet the transmission doesn't even downshift. 

When I originally bought this coach from the original owner, I had to replace the air cylinder (for the life of me cannot think of correct name of it at this moment) that opens and closes the pacbrake butterfly valve as it was shot. However, even when the pacbrake was not working back then, the transmission still downshifted. Has anyone encountered a similar issue and discovered the fix? THANK YOU VERY MUCH, Jeff
Jeff & Sandy
1997 American Eagle 40' Slide-less
2013 Toyota FJ Cruiser 4X4 Toad
Interested in Foretravel Coaches

Re: Pacbrake not working on 1997 Cummins C8.3

Reply #1 –
Even with my Pacbrake (the older model and at least ten years old) is working the braking is subtle and when I demonstrate the system to someone new I have to give them a heads-up when I'm going to apply it. So please don't be offended when I have to ask, are you sure it's not working? My Pacbrake has no connection whatever to the transmission (an Allison 643 lever-operated 4-speed) and the part about your transmission not downshifting makes me wonder whether the newer Pacbrakes have something more than mine.

With the light working all that means is that there is 12vdc to the switch but it's at least a little information. Go back and watch the Pacbrake unit itself while someone energizes it while the coach is standing still. Does anything move? Any "click" sound?

As subtle as the braking is, I'd hate to be without my Pacbrake.

Craig
1993 U225 36' Unihome GV with PACBRAKE exhaust retarder, Banks Stinger and Solar Panels.
Toad: 1999 Jeep Wrangler 2-door soft-top.

"No one has ever had to evacuate a city because the solar panels broke."

Re: Pacbrake not working on 1997 Cummins C8.3

Reply #2 –
Signal goes from PacBrake switch to Allison ECM and from there to the brake air valve.  The relay also need the "throttle closed" signal from the throttle switch. So check the relay, throttle switch and wiring between switch and Allison ECM.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Pacbrake not working on 1997 Cummins C8.3

Reply #3 –
Likely not related to today's question, but there is a cheap $7 diaphragm in the Humphrey valve.  It is hidden under a large nut on the end of the valve.  It is easy to remove to check whether the rubber part is still good.  As Brett suggested, check the the air cylinder is opening and closing.
Dave Cobb
Buckhorn Lake Resort The Club, #6202, Kerrville TX
check the map.  I do rent it out when I am traveling!
2001, U320, 36' #5887, in Kerrville, FT Club #17006, (7/23 to present)
2024 Jeep Grand Cherokee L, Summit, white
EX: 98 U295, 36' #5219, (mid door), (4/13-10/23)
EX: 93 U225, 36' (4/11 to 4/13)

Re: Pacbrake not working on 1997 Cummins C8.3

Reply #4 –
No, actually the first step is to get the transmission shift pad to indicate that the exhaust brake signal has been received.  Without that, there can not be a signal to the air valve.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Pacbrake not working on 1997 Cummins C8.3

Reply #5 –
If you have the 8.3 Cummings ,there is a extra switch that closes when throttle is in closed position. I had wire come off my switch and kept mine from working.
Regards,Irwin
Irwin

Re: Pacbrake not working on 1997 Cummins C8.3

Reply #6 –
Irwin.....any idea where this is located?
1994 U280, Build 4490
Deming, NM.

Re: Pacbrake not working on 1997 Cummins C8.3

Reply #7 –
Drove from Lake Conroe to Abilene today and unlike yesterday, it activated properly each and every time.  Assuming it must be a loose or corroded electrical connection.  Thank you for the troubleshooting suggestions and if I find the problem, I will post it and the solution.  Thank you again, Jeff
Jeff & Sandy
1997 American Eagle 40' Slide-less
2013 Toyota FJ Cruiser 4X4 Toad
Interested in Foretravel Coaches

Re: Pacbrake not working on 1997 Cummins C8.3

Reply #8 –
Quote from: Irwin –
If you have the 8.3 Cummings ,there is a extra switch that closes when throttle is in closed position. I had wire come off my switch and kept mine from working.
Regards,Irwin
That switch disables the exhaust brake when you push on the accelerator. The switch is located on the fuel pump. If the switch is not activated when you "give it the diesel" you stand the chance of blowing a head gasket by accelerating with the exhaust pipe closed off. The switch on my coach was missing ???, exhaust brake wasn't working (long story). I added a foot activating switch as described in my post Roland's exhaust brake foot switch (was Re: What did you do to your coach today).

Roland
1993 U280 4341
2010 Jeep Liberty
The Pied Pipers

Re: Pacbrake not working on 1997 Cummins C8.3

Reply #9 –
Twig, the switch on the Cummings 8.3 is located on the engine throttle bracket .I just recently replaced mine with new one . As Roland said it is used to make sure the PAC Brake is disabled anytime the throttle is not at idle.
Regards,Irwin
Irwin