Hard Down shifts

[Reagan Sirmons]

I have never posted but read all of the discussions on various issues. At the OKC meeting on the morning of the presentations at United Engines, there were several comments made regarding hard down shifts. Since I have an older modeled coach (1987 Newell with a hopped up Al Unser 500 HP ++ Detroit Diesel on an Allison transmission), I thought my issues were unique. Nonetheless, two weeks ago, we were made aware the "drive axle torque bar" was very worn to the extent both ends were totally lose. I had another torque bar completely built and installed. It made a tremendous difference in the downshift response and made the total shifting pattern smoother. The downshift is still very firm but the jar is gone. If anyone wants to pursue this further, I will be glad to explain what we did.

Reagan Sirmons
Newell2
Pres

[JIM KELLER]

Reagan, What is the " Drive Axle Torque Bar. " What does it do.

[Jon Wehrenberg]

Radius arm?

[Reagan Sirmons]

The drive axle torque bar on this coach is connected to the 3rd member (REAR END) and the chasis frame. It is located directly in front of the drive axle and transends the space across the coach in front of the axle. Mine is 30 inches in length from the frame bolts to the through bolt connection. on the member. The bolts on the frame are on the drive wheel side and are 1 inch bolts requiring strong power tools to remove. The bushing attached to the member has a bolt larger than that. The bar weighs about 35 to 40 pounds.

In talking to Newell in response to your question, they stated the torque bar is designed to keep the drive shaft, the transmission and the rear end centered. In essence, it stabilizes the entire rear end by helping to keep it aligned. If it is weak, the coach will be shifting with the DRIVE SHAFT AND TRANSMISSION out of proper alignment. In addition, the coach will "lean" in corners more than it should and the drive shaft will experience significant snap when a downshift occurs and it is out of alignment. I can attest to these specific experiences. At Stewart and Stevenson some 48 months ago, they stated the downshift on the transmission was firm but they would not adjust it to a smoother downshift due to potentially more wear on the assembly. They did not recognize the weakness of the torque bar at that time so it simply progressed and became worse.

The bushings on this device are enormously strong and the torque bar itself must not be adjustable and therefore it is to welded once the correct length has been detrermined.

Once he said it keeps the alignment correct, it all made sense.

I had the torque bar made here in Houston at Heitman. It is made of component parts but must be ordered based on the pattern unless you seek to order from the manufacturer (Prevost). It cost $308 vs the initial estimate of over $600 I received. It took the mechanic 5 hours round trip to remove and re-install the device in the warehouse where we keep the coach. The coach must be reasonably elevated (18 to 24 inches) for access as you would expect. My air bages did the trick with two 20 ton jacks and two frame supports for safety.

I hope this helps.

Very best,

Reagan

[JIM KELLER]

Thanks for the great response.

[merle&louise]

Reagan,

Thanks for explaining the "drive axle torque bar" problem and the resulting hard down shift problems.

I experienced some hard down shifting about 6 months ago. I contacted Marine Systems (local DD dealer) here in Houma, and they asked me if anyone different had driven the coach. I answered that my son-in-law had driven it. They said that the tranny computer "adjusts" to the way one person accelerates/decelerates and that was the reason for the hard down shift. It sounded like BS to me, but sure enough IT STOPPED the hard down shifting and the problem hasn't resurfaced. I questioned why it didn't do the hard down shift when I first purchased the coach.

I think that I will have Newell check the drive axle torque bar next time I am in for service. I just don't buy that bunk about a different driver causing the tranny to down shift hard!

[Jon Wehrenberg]

Two comments......Reagan's coach is of a vintage that did not have the DDEC brain to "learn" the driver's habits. I also think that in Tuga's case it might have been an anomoly because in order for the transmission to change shift patterns I believe Allison has to "deprogram" the learned shift pattern and then the shift patterns have to be relearned by driving a certain number of shifts before it resets itself.

[truk4u]

I think I remember a procedure the operator can do on the 6 speed that resets the learning. Something about with engine off, key on, so many presses with the throttle or something like that. I believe it's in the book.

[JIM CHALOUPKA]

Please err on the side of caution and do not press the throttle with the key on and the engine not running.

I read in some Prevost manual or.... awhile ago that doing so is Verboten and will cause BIG trouble, so until or unless you can prove me wrong, don't do it!!!!!!!!!

JIM