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Old 10-27-2020, 07:36 AM   #1
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Hard jerk when the engine brake is on and brake is applied.

We have a 2018 Tiffin 33aa, cummins eng. and eng break(exhaust brake?}

This occured yesterday after driving about 2 hours coming home. I flipped on the
eng. brake and when I applied the foot brake there was a hard jerk in the entire motorhome.

This occured about a year ago and I decided that the brake light current had something to do with this while towing a car behind the rv. Seems then that disconnecting the brake light to the tow solved this problem until yesterday coming home.

Any thoughts would be appreciated. FL chassis

Bruce
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Old 10-27-2020, 10:04 AM   #2
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Did the transmission downshift at that moment?
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Old 10-27-2020, 10:54 AM   #3
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Did the transmission downshift at that moment?
This^^ was my first thought as well. How fast were you traveling when you felt the jerk? You could have coincidentally flipped ON the engine brake, applied the foot brakes, just when the coach speed was appropriate for a downshift. Did you notice any increase in rpm at the time of the jerk?
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Old 10-27-2020, 11:12 AM   #4
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Did the transmission downshift at that moment?
At the right( wrong in the case of a harsh engagement / shift ) speed , my trans will go down two gears; with the exhaust brake trans downshift program ; then the word HARSH , just barely covers the feeling.
I've learned that at certain speeds , to manually downshift , before the exhaust brake engages to hold the program to a single gear down.
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Old 10-27-2020, 12:25 PM   #5
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The previous replies completely cover what happened.
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Old 10-27-2020, 07:29 PM   #6
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Did the transmission downshift at that moment?

I think it did. The jerk was so sudden and unlike normal that I did not think about the shift of the tran.

The chassis battery if not fully charged affects the operation of the back up camera and so I wander if the state of charge of the chassis bat. may have something to do with it.

When we camp, the trips are seldom over 75 miles. Thinking the chassis bat were charged when shore power present I though chassis bat would be charged. Not the case for our coach.

Checking the bat when plug in, I found the chassis bat to be 12.4v and house to be 13.6v. So this may be a factor that could affect the electronics of this system.
thx for any of your thoughts. Oh yes, the speed was around 45mph or less.

thanks for your help.
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Old 10-27-2020, 07:39 PM   #7
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Have your alternator output checked and chassis batteries load tested , should be 13.2 minimum when running 14.2 with low batteries.
Low volts can affect all electronics.
Run the Allison trans check for codes to see if low volts comes up .

BTW ; at 45 mph with light throttle , my tans would be in 5 th gear , exhaust brake activation would cause a downshift straight to 3rd.
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Old 10-27-2020, 07:45 PM   #8
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I plan to check the rv out without my tow behind it and see what happens again.

I probably need to read up better on operation of the exh. brake, but may we leave the exh. brake engaged all the time. What do you do? Had a HR Neptune and I engaged the exh brake only when needed-never had this jerk.

After this occurred the first time I slowed down to about 30mph and engaged exh brake then I applied the brake-the jerk occurred again. Needless to say, I left exh brake off the rest of the way home. Jerk seems to occur when exh and foot brake are used at the same time. that is no jerk when exh flipped on but when brake petal is applied

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Old 10-27-2020, 07:55 PM   #9
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I plan to check the rv out without my tow behind it and see what happens again.

I probably need to read up better on operation of the exh. brake, but may we leave the exh. brake engaged all the time. What do you do? Had a HR Neptune and I engaged the exh brake only when needed-never had this jerk.

After this occurred the first time I slowed down to about 30mph and engaged exh brake then I applied the brake-the jerk occurred again. Needless to say, I left exh brake off the rest of the way home. Jerk seems to occur when exh and foot brake are used at the same time. that is no jerk when exh flipped on but when brake petal is applied

Bruce
Personally, I put the engine brake on before I need it. Maybe as I crest a mountain the engine brake goes on before I pick up any speed. It's asking a lot of the engine brake to actually slow you down in a quick manner and that's not really what they are designed for. Think consistent slow descent with an engine brake.

My guess is the brake line current was totally a coincidence. Have you been running around with no brake lights? That may have its own set of problems.

Good luck - I hope it is just these simple things and not something bigger.
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Old 10-27-2020, 07:59 PM   #10
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mine shifts hard into 2nd when decelerating with the engine brake on (vgt exhaust brake for the detail oriented) and standing on the brake while going down a grade, approaching a turn that requires slowing for instance. Sound familiar? Nothing to worry about, but definitely noticeable.

I only use the exhaust brake when descending grades, and when I want to roll on out on a long straight, I turn it off so I can coast without slowing. So in mountain driving, which is very common for me, I’m turning it on and off frequently. I also often take over shifting when I’m not satisfied with the programmed shift strategy. It’s nice to take a break and let it shift in its own, but there are many situations where that is just suboptimal.
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Old 10-27-2020, 08:03 PM   #11
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I don't leave the exhaust brake on full time , because it will activate on rolling hill where I'd prefer to coast ( even on cruise where activation will shut cruise down) . Some chassis set ups won't engage cruise if exhaust brake is on.
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Old 10-27-2020, 08:52 PM   #12
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I check the voltage of chassis bat. 12.7v
connected bat charger and I will take a look in the morning.
We use our coach a lot. But for camping on short trips. So in the past 2 yr I think the chassis bat have become anemic. My excuse, I assumed the chassis bat were being charged when on shore power. I know it is not now and will make it a habit to put a charger on the chassis bat as well as the house bat. Hopefully these 2 plus years old bat will come back to life for us.
I'll report back tomorrow
thx
Bruce

Have your alternator output checked and chassis batteries load tested , should be 13.2 minimum when running 14.2 with low batteries.
Low volts can affect all electronics.
Run the Allison trans check for codes to see if low volts comes up .
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Old 10-28-2020, 10:11 AM   #13
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Some exhaust brakes activate as soon as you take your foot off the accelerator, while others are programmed to activate when the brake pedal is pressed. Do you know which method yours is set for? It's a chassis programming option, but usually set to the motorhome makers specification. The exhaust brake on switch merely allows it to activate - it doesn't actually start braking until conditions are met.



I ask because you may be experiencing an immediate downshift as the exhaust brake comes on. At a certain speed or rpm, the brake activation may be harsh.
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Old 10-28-2020, 04:24 PM   #14
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My exh brake I think begins as soon as the foot is removed from the accelerator petal.

I did find the chassis bat to be low. So I have it charging today to check things out after we get through with ANOTHER hurricane in south Alabama.

thx
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