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PAC Brake Issues

Yahoo Message Number: 114003
We are the new owners of a 96 Allure with the Cummins 8.3 and Allison 3060. Our single stage PAC Brake (exhaust only) doesn't seem to be operating correctly. On the highway, if I turn the single position rocker on, the PB immediately activates, whether I'm on the throttle, off throttle, or on the brakes. Shouldn't it only turn on when I m ether off throttle or one he brakes? Perhaps telling, the PB will activate when stopped, even in neutral (confirmed by watching the piston open and close from the engine bay). So that's issue one.
Issue two is that it on the road going down a mild grade, the transmission surges. If I'm at say 55-60 in 6th gear and activate the PB, the transmission will properly preselect 4th gear. However, when it downshifts to 4th, it will only hold it for a second and then up shift back to 5th. It will then continue hunting between 4th and 5th with no appreciable slowing. If I end up using the service brakes, it will ultimately hold 4th and stay there.
To me this seems like the PB isn't working correctly with the transmission computer. Any thoughts?

Re: PAC Brake Issues

Reply #2 –
Yahoo Message Number: 114015
With the ISl 400/425 and ISC etc. They will hold a gear to 2500+ rpm with out fuel being applied. Other wise the shift point with the pedal to the metal is about 2150 rpm. I had Allison reprogram my 3000 transmission to normally seek 2 nd gear, because we drive out west a lot in the Rocky Mountains and over the Coastal and Sierra Mts. Your really don't et as effective engine braking with an exhaust brake as you do with an engine brake. So with your 8.3 liter you may also sufferuing from exhaust brake failure. It should e checked every year and serviced bi-annually. TWI 2004 Intrigue 11731

Re: PAC Brake Issues - UPDATE

Reply #3 –
Yahoo Message Number: 114032
Thanks to everyone for the input. After some more real world testing I found out I was wrong on my initial assessment. The surging is actually the transmission pre-select going from 4, back to 6, and then back to 4. That's what's causing the bad behavior. If I manually downshift to 4 and then turn the PB on, it works perfectly and slows the coach down. Based on this, I'm guessing I have a TPS sensor issue or other gremlin within the Allison control. I'm going to see if there are any good Allison mechanics around the Bay Area to take her to.

Re: PAC Brake Issues - UPDATE

Reply #4 –
Yahoo Message Number: 114035
It sounds like your unit is set to pre select 4th when the PB is on. If it is jumping when yuor foot is off the gas pedal, look for carpet or something that is preventing the gas pedal from coming all the way back to no throttle.

Mikee

Re: PAC Brake Issues - UPDATE

Reply #5 –
Yahoo Message Number: 114044
It sure sounds like mine was acting. Contacted Clark Power (big mistake).First thing they did was replace the TCM at a price of thirty three hundred dollars. It did not help. Finally drove it to their place in Greensboro N C some 150 miles away. The guy down there did figure out what the problem was.I had the transmission fluid changed at a truck dealership who claimed to be an Allison repair center (they weren't) .It seems that people that don't know that much about Allisons are prone to mess a 10 dollar gasket up. With that said, take your unit to a Allison transmission service center and have them check the pressure on the ports .

On Monday, February 12, 2018 2:28 PM, "RiseOsiris@... [Country-Coach-Owners]"  wrote: