Pacbrake

Pretty much everything on the bottom side of the coach: Steering, wheels, tires, brakes, suspension
Limier
Posts: 17
Joined: Thu Mar 12, 2020 2:46 am

Pacbrake

Post by Limier »

My Pacbrake has decided to work intermittently. It was working fine up to a week ago, then decided to sometimes work and sometimes not.
It is permanently wired on, previously when I touch the brake pedal, it kicks in, and the gearbox display panel signals a shift down to 2.
Now, it kicks in sometimes, and works, or kicks in for a couple of seconds only, even at highway speeds.
Mechanically it seems fine, if I activate the solenoid manually the compressor kicks in and the arm moves. I haven’t tried this at highway speeds, I have no other driver. The brake lights seem to be OK too, eliminating the switch there as a possible cause. I can’t find a loose connection in the visible wiring, and the hidden wiring is a mishmash underneath, wires disappear into looms, and the colours at each end are different, the wiring from the brake pedal end is red, black, or white, and the solenoid actuator wire is green.
Over 30 years some of the wiring has been redone, so I
Is there maybe a control box somewhere that maybe interfaces with the gearbox? Or is the gearbox kick down actuated from the same solenoid? There seems to be a back wire disappearing in that direction.

Otherwise I’m stumped, if anyone has suggestions I’m all ears.
Regards

Bob Side

‘93 Serengeti. C8.3-250hp Cummins. 6 speed Allison. Oshkosh Chassis
New Zealand. Converted to drive on the left.
TDJohn
Posts: 552
Joined: Sun Oct 27, 2019 11:34 pm

Re: Pacbrake

Post by TDJohn »

Your likely issue is the switch on the brake pedal, that generates the signal to the transmission computer, is going bad. A possibly bad ground or feed wire connection, or there is an issue inside the transmission control module, like a poor solder joint that is causing an erratic signal.
For the preselect 2nd gear to work, the transmission gets a signal from the switch on the brake pedal. On your model year coach, I don't know where the location of the VIM (vehicle interface module) or TCM (trans control module) would be located. Allison should have diagrams showing where the signal wire would be connected, or you could trace the wire from the switch that is located on the brake pedal.

I'm surprised that the Pac Brake does not have an off switch, that would drive me batty. It would not be hard to install a switch to have the option to turn the PB off if desired.

If your PB gets the signal from your brake switch that operates the brake lights, I would see if someone can drive behind you to see if the brake lights flicker or work intermittently.
John
'95 Serengeti, Cummins C8.3-300
Allison 6spd.
Limier
Posts: 17
Joined: Thu Mar 12, 2020 2:46 am

Re: Pacbrake

Post by Limier »

Thanks John, the switch seems to work when stopped but
as you say it could be erratic especially if movement causes vibration. I’ll have a closer look at it.
Regards

Bob Side

‘93 Serengeti. C8.3-250hp Cummins. 6 speed Allison. Oshkosh Chassis
New Zealand. Converted to drive on the left.
wolfe10
Posts: 222
Joined: Sun Oct 27, 2019 4:12 pm

Re: Pacbrake

Post by wolfe10 »

Limier wrote: Fri Mar 03, 2023 3:10 pm Thanks John, the switch seems to work when stopped but
as you say it could be erratic especially if movement causes vibration. I’ll have a closer look at it.
So, does the Allison shift pad change from "6" to whatever gear is your exhaust brake preselect-- often 4th or 2nd?

If not, the very first step in the "apply the exhaust brake is not functioning.
Brett and Dianne Wolfe
Ex: 2003 Alpine 38'. Ex 1997 Safari Sahara. Ex 1993 Foretravel U240
Moderator, FMCA Forums 2009-2020
Chairman, FMCA Technical Advisory Committee 2011- 2020
Moderator, http://www.dieselrvclub.org/ (FMCA chapter) 2002-
ProCycle
Posts: 135
Joined: Sat Oct 03, 2020 1:28 pm

Re: Pacbrake

Post by ProCycle »

wolfe10 wrote: Fri Mar 03, 2023 4:43 pm So, does the Allison shift pad change from "6" to whatever gear is your exhaust brake preselect-- often 4th or 2nd?

If not, the very first step in the "apply the exhaust brake is not functioning.
When my Pacbrake is engaged it always tries to shift down to 2nd if it can. Can the programming be changed? it would be so much better if it only went down to 3rd.
Jeff
1997 Ivory Edition 3740 Cat 3126 Allison
wolfe10
Posts: 222
Joined: Sun Oct 27, 2019 4:12 pm

Re: Pacbrake

Post by wolfe10 »

ProCycle wrote: Thu Mar 09, 2023 6:51 am
wolfe10 wrote: Fri Mar 03, 2023 4:43 pm So, does the Allison shift pad change from "6" to whatever gear is your exhaust brake preselect-- often 4th or 2nd?

If not, the very first step in the "apply the exhaust brake is not functioning.
When my Pacbrake is engaged it always tries to shift down to 2nd if it can. Can the programming be changed? it would be so much better if it only went down to 3rd.
Yes, an Allison dealer can use their programmer to select any gear of your choosing for the "exhaust brake pre-select gear". I had mine re-programmed to 5th. More driver control, but more driver input needed when coming to a complete stop.
Brett and Dianne Wolfe
Ex: 2003 Alpine 38'. Ex 1997 Safari Sahara. Ex 1993 Foretravel U240
Moderator, FMCA Forums 2009-2020
Chairman, FMCA Technical Advisory Committee 2011- 2020
Moderator, http://www.dieselrvclub.org/ (FMCA chapter) 2002-
Limier
Posts: 17
Joined: Thu Mar 12, 2020 2:46 am

Re: Pacbrake

Post by Limier »

After much testing, I confirm the brake light switch is working every time. The viable wiring and connections seem fine, and I have installed a switch after the brake light switch in the Pacbrake circuit and run a new wire to the activation solenoid for the Pacbrake, this now works. But the transmission doesn’t downshift.

So I conclude there is a fault somewhere in the wiring up to or after the transmission control module, given that the Pacbrake normally doesn’t operate below a certain speed, sensed from the output speed sensor. As this wiring is mostly inaccessible without pulling out a lot of wiring and panels, I’ll leave further investigation until we return home in May.
Thank you for your suggestions, much appreciated.
Regards

Bob Side

‘93 Serengeti. C8.3-250hp Cummins. 6 speed Allison. Oshkosh Chassis
New Zealand. Converted to drive on the left.
brstough2004@gmail.com
Posts: 14
Joined: Sat Oct 08, 2022 10:47 am

Re: Pacbrake

Post by brstough2004@gmail.com »

As far as tracing the wires that "disappear" into a loom or harness, this is a very handy tool. Sometimes called "Fox and Hound".
Place the "oscillator" alligator clips on the wires, turn it on, then you can trace the wires throughout your RV with the "speaker/pointer? unit.
https://www.amazon.com/dp/B08T224QCC?ps ... ct_details
brstough2004@gmail.com
Posts: 14
Joined: Sat Oct 08, 2022 10:47 am

Re: Pacbrake

Post by brstough2004@gmail.com »

PAC BRAKE and air in general question(s):

I have a 1996 Safari Continental with 8.3 Cummins. There is a red indicator light on the dash that says "PacBrake" - but I've never seen it on...

I don't seem to have a switch to "enable" the PAC BRAKE.

From what I've gathered from my somewhat limited 'literature' that came with the MH, the Allison needs to be in a non-overdrive 'gear'? It is a six speed with both the 5th and 6th as overdrives (I think)

Continental has only been in my possession for six months. No long trips or campouts as of yet... Still getting to know it...


My other air-related question(s) have to do with: 1 Air Horn? Button? Switch?? Pull rope?? I have found the fuse labeled "Air Horn Pump", but no idea how to activate it...

I figured out how to inflate the air bags on the suspension - there is a manifold (with four regulators) fed by a quick-connect... So I tried applying air from a compressor - and that works - So I need to find a 12V compressor..

So, where does the air come from for the Pac Brake??

(Sorry, lots of questions, and not in a great order...)

Brian
wolfe10
Posts: 222
Joined: Sun Oct 27, 2019 4:12 pm

Re: Pacbrake

Post by wolfe10 »

Brian,

I am not familiar with your year model, but most PacBrakes work the same:

Electric signal (switch) sends signal to the Allison ECU and two things happen-- the Allison starts downshifting "toward" its pre-select gear for exhaust brake operation. Often 2nd or 4th, but any gear can be programmed as the pre-select gear. No, it will NOT overspeed the engine!

The second thing is the signal to the PacBrake's air solenoid to open the air solenoid to close the brake butterfly.

Couple of easy checks:

Does the transmission shift pad change from "6" to some lower gear when you do turn on the switch (assuming you find the switch-- could even be floor mounted)?

Raise the bed and locate the PacBrake air cylinder. Disconnect/cut the positive lead and apply 12 VDC positive (plenty of them in the engine room) to it. You should see and hear the PacBrake snap closed.

Be aware that the PacBrake needs to be lubricated: https://pacbrake.com/product/c18037-superlube/
Brett and Dianne Wolfe
Ex: 2003 Alpine 38'. Ex 1997 Safari Sahara. Ex 1993 Foretravel U240
Moderator, FMCA Forums 2009-2020
Chairman, FMCA Technical Advisory Committee 2011- 2020
Moderator, http://www.dieselrvclub.org/ (FMCA chapter) 2002-
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