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PacBrake Intermittent Failure to Engage Problem

February 21, 2015, 11:04:02 am

Greetings - I am at a loss as to how to proceed and hope someone here with the same engine and transmission setup might be able to provide further direction. I have a 1997 American Eagle on a Spartan Mountain Master chassis with a purely mechanical Cummins c8.3 engine and Allison 3060 world transmission. The issue is that my PacBrake will intermittently fail to work when its activation switch is depressed. When the intermittent problems occurs, the Allison transmission also will not downshift (as programed) to fourth gear. Additionally, the dash instrument light that would ordinarily illuminate "PacBrake" also does not come on.

As an FYI, I learned my PacBrake does not have a PacBrake brand/supplied throttle position switch, but is instead controlled by the Allison throttle position switch which connects to the throttle linkage via a wire cable.

Here is how I have attempted to trouble-shoot this intermittent problem. I have checked all fuses and tested the dedicated / specific pacbrake relay . I also confirmed 12 volts is making its way back to the 12 volt relay located in a fuse compartment at the rear of the coach and just below the Allison transmission control module when the ignition is "on" and the pacbrake activation switch is also "on". My coach does not have an Allison "vehicle interface module". I also checked the two plug-in connectors going into the transmission control module and found them to be well sealed with no corrosion. I also checked the pacbrake itself (again) by first unplugging it just before the air solenoid then applying direct 12volt power to the air solenoid after airing-up my coach and it worked each and every time it was energized.

I next visually inspected the Allison transmission throttle position sensor and its movement upon the Bosch P7100 fuel injection pump throttle linkage and it appears normal. ONE BIG QUESTION is whether or not the problem is related to the Allison throttle position sensor since the transmission shifts smoothly / just like "normal" even when the pacbrake will not engage/operate. Logically it would seem that if it shifts properly, the TPS is working properly, but electrically that may not be the case??? Suggestions for further trouble-shooting would be most appreciated! THANK YOU

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Re: PacBrake Intermittent Failure to Engage Problem

Reply #1 – February 21, 2015, 12:50:41 pm

While my PacBrake is not connected to the transmission, I noticed last November that it was not engaging. I had to clean the arm and its connections thoroughly and relubricate well with their lubricant and it came back to life.

Make sure you're using an appropriate lubricant. WD40 won't do it.

Craig

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Re: PacBrake Intermittent Failure to Engage Problem

Reply #2 – February 21, 2015, 12:56:32 pm

Jeff,

Go to the relay (probably in under dash area) where the signal from the exhaust brake switch and signal from throttle come together.  Suspect the fault is either there or in the wire from there to Allison ECM.  The ECM then sends the signal to the exhaust brake solenoid.

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Re: PacBrake Intermittent Failure to Engage Problem

Reply #3 – February 21, 2015, 05:01:59 pm

Jeff, my U270 has the same setup as yours ,but there is a special switch on engine to control the on ,off to the PAC brake solenoid . I have a picture but don't seem to be able to post it on the new format.
Regards,Irwin

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Re: PacBrake Intermittent Failure to Engage Problem

Reply #4 – February 24, 2015, 10:44:03 pm

Jeff,did you get your problem figured out with the PAC Brake
Regards,Irwin

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Re: PacBrake Intermittent Failure to Engage Problem

Reply #5 – February 27, 2015, 06:53:45 pm

Sorry for the delay in providing an update.  I do not yet know if I have permanent "fix", but here is what I have done thus far.  Checked and rechecked all electrical connections and fuses, checked the dedicated relay, checked for transmission trouble codes (none) and finally replaced the throttle return springs as recommended by an Allison technical bulletin regarding my C8.3 as well as the 5.9 that addresses intermittent pacbrake operation.  So, bottom line, I should learn whether or not I have a permanent fix during our next adventure.  One last thing...  An Allison rep with whom I spoke stated my throttle position sensor is not the problem since the transmission always shifts properly.  THANK YOU all for your help and input!

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Re: PacBrake Intermittent Failure to Engage Problem

Reply #6 – February 27, 2015, 08:20:52 pm

I have attached a Spartan file showing my setup...  Perhaps it may help someone in the future...

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