PAC-BRAKE FAILURE upon heat/stress

Hello All ..... I am looking for help, hoping that someone has experienced the same problem. In short, the coach is 96 American dream 300 w/ banks and allison md3060. The pac brake system works fine in normal driving conditions, but fails after a strenuous climb. (just when I need it coming back down!) After 'something cools off enough', the proper signals are sent and the shifter screen reads the normal '2' and the system works by shifting down and exhaust brake in place. I have gone to allison and others and no one has experienced this possible 'heat' related problem that shuts down the pac brake signals. Accelator Linkage has been replaced as well as ecu relays to no avail. Allison (2) says there is nothing in their system that would react to overheating and therefore shut down the pac brake and shift down system. Allison said the trans is running alright and referred me to Cummins as there might be some engine communication to the shift control module affected by heat. The gauges show heating up on hills but not in dangerous range. Again, the pac brake system ONLY fails after a difficult pull uphill. Thank you for assistance .....
Re: PAC-BRAKE FAILURE upon heat/stress

If I am understanding this right when it fails the touch pad does not indicate 2 correct? If so I would say TPS. (Throttle position switch) I see this is a 96. Do you know if it is an electronic or mechanical engine? If it is mechanical there is a TPS conected to the linkage. They have been known to fail, stick or break. I would look there first.
Good Luck
Re: PAC-BRAKE FAILURE upon heat/stress

Thank you for your info. My mechanical cummins c-series 8.3 has had the throttle switch replaced and as I mentioned before the unit always works as it should strangely until I pull a strenuous hill and then turns off until what I can only describe as a cooling off period and then works ok again. I was also wondering if the throttle switch had any control over the shift control ecu via sensor or something , that is, does the ecu sense the switch energized and then signals the transmission to shift down. If not, then it would seem that the ecu is not starting the ball rolling in the first place. Either way, I was wondering what the 'hill climbing' or engine/trans heat had to do with anything>
Thanks again for your input. GE
Re: PAC-BRAKE FAILURE upon heat/stress

The tps sends a signal to the ecu for shifts. Also, if it doesn't return all the way for what ever reason the brake can't work because the ecu senses to much throttle. It is cable operated and it is possible when the engine is hot the cable could be sticking. Try lubing the cable and conections at the linkage and see if it helps. Good Luck
Re: PAC-BRAKE FAILURE upon heat/stress

After rereading your thread, everything works as normal except the pac brake doesn't engage when hot.
There is an air solenoid that engages the pack brake and it gets it's signal from the shift box.
I'd look around there. Jumper the air solenoid to see if it works or binds up under high heat- same with the pac brake.
Hope This Helps.


Re: PAC-BRAKE FAILURE upon heat/stress

u may have some type of warpage going on while pulling a hill ,, and it may be binding the flap that closes off the exaust ,, but it don't make any since why the trans wont down shift ... my cummins is pretty close to the yr of u'rs ,, mine is a 97 ,, but i installed a true Jake brake sytem on mine ,,, it's loud when it is on but it does the job ,, and also my trans does not down shift ,, it stays in the same gear until i manually shift it ,, but i have a regular lever shifter and not a keypad,,,, hope this is some insight ,, and welcome to the forum ,,
Re: PAC-BRAKE FAILURE upon heat/stress

Hello Bush70, and all, Thanks for your concern I want to apologize for my ignorance, but i was wondering how the tps sent it's signal as it appears to only activate the solonoid for the pacbrake air system, one it itself is activated by the idle speed position. Is it because of a closed circuit that is sensed by the ecu? Or maybe the ecu sends it's sign/power first to both the tps and trans to shift down at the same time one it's sensors have been satisfied by the program criteria (ie speed, in gear, etc) Kinda like the chicken and egg. I bypassed the non-signal to the tps in the past and manually worked the exhaust brake, but obviously that did not help with the signal being sent to shift down. Sorry for the problem, but all i've gotten from allison and others is tps and linkage replacement, no answers. Thanks again GE