General Boards > Technical Support
Pacbrake misbehaving. Help?
Bill Sprague:
I apologize for writing a long post, but my Beaver and I are under stess!
For some reason, Beaver forgot to put a Cat in my 2004 Monterey. I have a Cummins ISC 350. As a newbie when I bought it, I didn’t know better. Except for a $600 fuel filter, the drive train has been flawless for almost 80,000 miles. Even though I don’t have a Cat, maybe you can still help me!
After having a lot of fun at Jim and Gerda Bull’s rally in Grants Pass last week, we drove north on I-5 towards Ellensburg for a Jazz Fesitval. For the first time in 80,000 miles, the Pacbrake malfunctioned.
On the first downhill grade, I switched on the Pacbrake. The cruise control disengaged, the RPM kicked up and I felt the braking effect. Then, for no apparent reason, it up shifted, the RPM dropped and the braking effect went away. A few seconds later, the Allison down shifted again and the braking effect returned.
Puzzled by the abnormal behavior, I put the Aladdin engine page in the primary scan. On the next downhill grade it did it again. I had engaged the Pacbrake with the switch, the Aladdin showed that “2” was selected and it down shifted to “4” from “6”. After about a half minute, the selected gear changed from “2” to “6” , the Allison shifted and the braking effect went away. Then it reselected "2" and went back to normal down grade operation. The abnormal pattern repeated itself on the next couple of downgrades.
Befuddled, I phoned Marty Schenk on my (hands free!) cell phone to brainstorm. I don’t know if he knows anything, but I had him on speed dial (grin!). The chassis voltage was fluctuating from 13.8 to 14.1 and my chassis batteries are 7 years old so I had convinced myself the computers were not happy with that. Marty talked me out of that idea. We agreed that a shut down and restart at the next rest stop might re-boot the computers. He also remembered a thread on the Cat board about a faulty relay.
I pulled into the next rest stop, shut down, did the usual walk to the back of the motorhome, returned, restarted and pulled out to attack the next hill. On the next couple of downhill grades it repeated the same bad behavior. The re-boot didn’t work.
It occurred to me that, instead of letting the Pacbrake do it, I could tell the Allison what to do with the arrow buttons on the keypad. On the next downhill grade I engaged the Pacbrake, verified that the cruise control canceled, verified that “2” was selected, watched the shift to “4”, pushed the down arrow 4 times (to back up the Pacbrake signal) and waited. It worked normally. There was no “hunting” of the “selected” gear. At the bottom of the hill I disengaged the Pacbrake, pushed the “D” button to end the “2” selection and reengaged the cruise control. This worked consistently on the next couple of downgrades.
To further the troubleshooting, I tried switching the Pacbrake off and on a down grade while the transmission was selected to “2” with the arrow key. I could feel the braking effect change confirming the Pacbrake valve was operating.
Soon, I was out of the hills of southwest Oregon and am satisfied that I can continue to drive safely by backing up the Pacbrake by downshifting with the arrows.
My theory is that the Allison is working properly as long as it gets the a command from either the Pacbrake or the arrow keys. I think that the Pacbrake may be sending an initial downshift signal to the Allison, but then the signal drops out. That suggests the wiring from the dash switch is OK to the Pacbrake, but not to the Allison.
What sends the signal to the Allison? Does it come from the dash switch, the Pacbrake or a relay? Where should I look for the simple stuff like wires and fuses? What other ideas might you have? Where are the best Pacbrake technicians in the Northwest?
Thanks in advance!
Bill
PS: I’m looking forward to seeing a lot of you in Harrisburg!
Gerald Farris:
From your description, it sounds like a bad relay is the most probable cause for your problem. A fuse or wire will very rarely cause an intermittent problem like you described, although a poor connection will cause a similar problem, but a poor connection will almost always get much worse quickly from arcing when it starts.
I do not know were the components are located on your coach, but they should be in the LF electrical bay.
There will be Pac Brake technicians at the FMCA rally in Redmond if you need their services.
Gerald
Bill Sprague:
Gerald,
Thanks for the suggestions. Unfortunately I got "cross scheduled" and won't be at Redmond. I'll survey the electrical bay and check my files for a wiring diagram.
On today's drive heading west down I-90's Snoqualmie Pass the Pacbrake worked perfectly. While using it, it occurred to me that something lightly touching the accelerator would have caused the Pacbrake to do what it did. Since I know better than doing that with my foot, it may have been that something like the GPS power cord was bumping part of the accelerator. I should know better than that too! Some dust, dirt or loose dog fur could be keeping the pedal from fully returning to the idle position. If that turns out to be the case, I feel pretty dumb!
Whatever it is, I'm still on the hunt.
Larry and Heidi Lee:
Bill, I experienced a similar condition when I purchased my 05 Monterey a couple years ago. The rig would slow down going down steep grades but would upshift and I would have to slow back down with the service brakes so much that I was overheating the brakes. Something was wrong! I finally figured out the air solenoid that actuates the Pac brake was stuck so I wasnt getting any braking assistance at all! I removed the solenoid, cleaned it with some WD40 and it's worked perfectly ever since.
Bill Sprague:
[quote author=]Bill, I experienced a similar condition when I purchased my 05 Monterey a couple years ago. The rig would slow down going down steep grades but would upshift and I would have to slow back down with the service brakes so much that I was overheating the brakes. Something was wrong! I finally figured out the air solenoid that actuates the Pac brake was stuck so I wasnt getting any braking assistance at all! I removed the solenoid, cleaned it with some WD40 and it's worked perfectly ever since. [/quote]
Thanks for the advice. Pacbrake makes packages their own synthetic lube for that. They say it should be lubed at every service but I've noticed that in my seven annual oil changes I've had to ask for it. Now I have a bottle of my own. Although it seems to have healed, I'll give it a couple of drops.
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