Cummins Shifting 5th gear to 4th with Exhaust Brake On

[Sasquatch]

I've been driving the 2500 Laramie mega cab Cummins for about a week and love it. I had rented a 2500 big horn with the diesel last summer and towed about 1,500 miles, so knew it would be a good truck. During that trip I almost always had the trailer hitched, so didn't really get a sense of the shift patterns driving without a load.

I'm pleased overall with the shift points without the exhaust brake on - seems relatively smooth. But with the exhaust brake on, I notice a consistent bucking/jolt as I slow down and the truck downshifts from 5-4. Happens every time, so I think it's designed to do that. Have others experienced this on a consistent basis? Without the EB on, the 5-4 shift is smooth as ever.

Also, I have noticed when city driving it's sometimes more enjoyable to drive by locking out the 6th gear - otherwise I sometimes feel like I'm lugging the engine.

Thanks in advance

 

[Toyotaguy]

I was wondering this same thing. I’ve stopped using the exhaust brake even though I really like it because of the 5-4 shift and worrying about the transmission wear. I’m probably just being paranoid though. Looking forward to the educated responses


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[Sasquatch]

Didn’t think it was just me - must be designed that way for sone reason.

 

[Toyotaguy]

I’ve noticed when not using the exhaust brake the truck shifts 6-5-3-1 but with the exhaust brake it shifts 6-5-4-3-2-1. I’ve changed my mind and decided to keep using it, the engineers are smarter than me and wouldn’t make the truck shift like this if it really was detrimental to the longevity of the trans.


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[jeffn]

I would not rule out haphazard engineering (exhibit A: CP4) but watching the exhaist brake gauge in action shows the truck exerting only about 50% of the available HP when braking under load. The 3k RPM under braking bothers me a bit but even without understanding everything that is going on with the engine I will assume that it is a different type of mechanical stress than under acceleration.