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Vibration/Power Loss 1500-2000 RPM Under Load

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While on a 988 mile trip to Denver I noticed that our class A DP started vibrating from the rear while under load from either a hill or acceleration at the point that the engine was under load. While climbing the pass at the Eisenhower tunnel the check transmission light came on and threw a "d1 P0796" code on the Allison console when I read it (Solenoid "C" Stuck Off). When this happened the rig was under heavy load in 4th gear on it's way up the hill and shaking/vibrating while trying to climb. We immediately took it into a large service center (Transwest) where I gave them all of the details but they were unable to reproduce the issue and obviously since I cleared the code in order to restore transmission function, their reader saw no diag codes. They said fluid seemed clear and full, the Allison Transmission console says the filter is ok, although it is probably due for a change (we purchased this rig used a year ago). On the way back from Denver to California the exact same issue happened, the vibrating any time there was load at 1500-2000 RPM (changing gears or getting out of that range stops the vibrating). I'm taking her in tomorrow to have a truck/rv service shop take a look tomorrow but would like to get opinions or experience anyone has had with this issue and what the resolution might have been. The shop did not have our jeep attached to it when they looked at the rig so I suspect the weight of the Jeep makes the issue more pronounced/reproducible.

Here are the rig's details:

2011 Holiday Rambler 36PFT, 28,800lbs
Flat Towing 4000lbs Jeep Wrangler
Allison Transmission (3000 I think)
MaxxForce 10 Engine

Issue Details:

- Seems to only vibrate/shake from the rear end at 1500-2000 RPM and only under load and any gear. When not under load she'll run in the 1500-2000 RPM range just fine but the second load is introduced the shake usually will show up and can range from a quick vibration to a pronounced shake that seems to match the engine RPMs in some way.

- When the vibration happens it seems like the engine loses some power.
- I can be going up a hill just fine in 5th gear and it sounds like the engine has plenty of power to continue then all of a sudden almost like a switch, power seems diminished and the vibration will happen. Letting off on the pedal (usually pedal is at the floor if going up hill and this happens) will get rid of the vibration at the expense of momentum loss.

- Changing gears when the vibration to a lower gear like 4 -> 3 puts the RPMs above 2k and stops the vibration.
- When changing gears from 4 -> 3 to stop the vibration usually the rig can handle staying between 2000-2500 RPMs up most inclines but I will feel an occasional slow pulse of power loss for split seconds that isn't pronounced at all. It's like the moment you feel when you are just about to go the other direction on a swing, like a slow pause.

- Will shake if pedal to floor when trying to pass on relatively flat land if the RPMs are in the above range.
- Seems to get worse the longer I drive. Shutting off the engine for 15-30 minutes does nothing for it if I'm on a long trip. Usually within about 2 hours the vibration issue will be far easier to reproduce.
- Seems worse or easier to produce when flat towing Jeep.

- Transwest inspected the drive line, the fluid, codes and found no faults/issues. Could not reproduce without Jeep connected, or at least the service tech couldn't detect it. I can usually detect the issue without the jeep as well or even in the first legs of the trip when it's not acting up as much but it is faint. I just know what the feel is when it's doing it regardless of intensity.

Some of the things I plan to have them look at/change:

Transmission fluid to Transynd
Trans filter change
Oil Change
Fuel Filter Change
Inspect Injectors
Possibly change out whatever solenoid is throwing the DTC
Transwest suggested it could be transmission valve body

Any ideas or experiences that might match and what to look for to resolve would be such a help. My fear is I won't be able to reproduce it for the place I'm taking it too or they will run wild speculating what it is and I'll have rebuilt both the engine and transmission before the issue is resolved.

If you got this far reading, thank you, and please HELP!


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Welcome to the FMCA Forum.

First thing I would do (before other non-preventive maintenance items) is change fuel filters.  Your description follows restricted fuel supply IF when you downshift to a lower gear and symptoms go away you also back out of the throttle (since that would also call for less fuel flow). If the above 2000 RPM is also at WOT (Wide Open Throttle) and no symptoms, it is likely not a fuel restriction, as that condition calls for even more fuel.

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Thanks for the welcome!

Interestingly enough, I can downshift with the pedal all the way down, it will shift, and vibration stops without letting off on the pedal at all. Engine switches to the higher RPMs like a champ.

Of course if it is a steep incline and the downshift/momentum loss heads back into 1500-2000 rpm territory it will happen again even in the downshifted gear.

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Be. Careful about who you take your MH in particular If it is a Allison problem. I can tell you first hand that many service vendors recommended by the coach manufacturers are not competent to work on M H’s.  I did receive excellent service from W.W. Williams, they are nationwide in most major city’s.  These guys are PROFESSIONAL GRADE. On engines of all types, electronics, Allison and other brand tranny’s.

If you have a Freightliner Chassis, don’t assume shops listed on FL’s 24/7 App are competent, they are not all up to par.


Good Luck


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