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Old 09-27-2019, 05:30 PM   #1
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Hard downshifting

I have a pac break and it shifts it down to 4th. Often there is a significant jolt when it drops in and very infrequently it goes in with a bang. I have converted to Transend. Is this normal with Allison and a Cummins 350 ISC diesel engine?

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Old 09-27-2019, 05:53 PM   #2
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OH, Transend=Transynd tranni fluid. You had me going there for a while because I could not figure out what that was. Well, moving on...

OK, I have a Cat 330, Allison 6 speed and an exhaust brake. When I first bought this RV I had the same issues you are describing. It really sounded horrible and I didn't like it at all. What I did was learned to feather the exhaust brake to minimize the jolts. I press on the brake, the exhaust brake light comes on, and then I press on the fuel peddle just a little. That little bit of fuel seems to gentle the braking and reduce the harsh clunking.

Doing that has reduced the hammering to a minimum and I rarely hear or feel it anymore. It still occurs occasionally but not nearly as often.

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Old 09-28-2019, 12:37 PM   #3
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The Pac brake is only one part of the exhaust brake operation. If the dash switch is on, the TCU knows to control the Allison tranny and seek the gear that is programmed into it - usually 2nd or 4th. It does this when you remove your foot from the fuel pedal (and the coach is going faster than 10mph). The pac brake itself is not very noticeable when it activates. It's the combination of the pac brake and the Allison shifting that slows you down.

There has been a lot of threads on the stiff shifting of the 3000 series Allison. Some light reading for you:
Rick and Larrie Dee
1997 40' Newmar London Aire DP CTA 8.3 (Mechanical) 325 Spartan MM
Bringing her back to her glory.
'08 Jeep GC Overland.
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Old 10-04-2019, 06:09 PM   #4
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Thanks for pointing me to those links. That really helped
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