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Lonnie & Bona · 11-12-2024, 02:13 PM

Can anyone offer the reason my Allison transmission jerks when downshifting while on cruise control? I have a 2001, Newmar, Mountain Aire which I purchased a year ago. The Mountain Aire has a ILS, 370 hp cummins engine. The only time it tends to jerk while downshifting is when the motor home is on cruise control. Other wise it shifts normally when cruise control is not being used and I manually control the fuel pedal with my foot. On flat land with no hills it is not a problem using the cruise control when the transmission does not need to downshift. This seems to happen mainly on over passes when the motor home reaches the top of the over pass and downshifts on the downward side to keep the downward speed constant. With the cruise control off, no problem with the Allison shifting on the downhill side. This has been happening ever since I purchased the motor home a year ago. Anyone out there have an explanation of what is causing this jerking motion???

d23haynes57 · 11-12-2024, 02:19 PM

Possible that as the cruise control is coming off the throttle, the exhaust or enigne brake is getting activated. Try disabling that brake.

Arch Hoagland · 11-12-2024, 03:05 PM

Has the transmission fluid ever been changed?

SafariBen · 11-12-2024, 03:06 PM

Quote:

Originally Posted by Lonnie & Bona

... This seems to happen mainly on over passes when the motor home reaches the top of the over pass and downshifts on the downward side to keep the downward speed constant...

DO NOT turn off the exhaust brake. When you're almost to the top of the pass, shift the trans to the gear it's in, or the lowest it was in on the climb, and then wait till you're to the bottom and can go faster to shift back to a higher gear. Yes, you'll feel a little funny going over the pass at 2,200rpm, but you will be in the right gear before you need it. I'll say a jerk when the cruise goes into engine braking mode is normal for that era transmission. If you want to avoid it, use the brakes to slow down, and manually shift to the proper gear so it doesn't have to do a full load downshift.

d23haynes57 · 11-12-2024, 03:53 PM

I think if you're going over hills where it takes real effort and downshifting to go up and braking is needed to go down cruise control should not be used.

jacwjames · 11-12-2024, 04:08 PM

It might be your engine causing the problem, when was the last time you changed your fuel filters.

Have you used the shift pad to check fluid level?

Have you looked to see if there are any codes using the shift pad?

wolfe10 · 11-12-2024, 04:15 PM

Lots of varied suggestions.

Have you used the Allison shift pad to check for DIAGNOSTIC CODES?

I agree if what is happening is you have a downshift at the same time as exhaust brake application, it could be harsh.

If this is the cause, a good "work around" is to "down arrow" and then apply the exhaust brake. That way all the "decel" does not happen at the same time.

Lonnie & Bona · 11-13-2024, 01:32 PM

The oil and filters have been recently changed. The next time out, I will try disabling the brake and/or shift down to a lower gear. I got a feeling one of these two suggestions will solve the jerking issue. Thanks for the tips. I should have thought this thru better myself. Sometimes it is just easier just to ask for a beginner.

Dutch Star Don · 11-14-2024, 12:49 AM

Your coach was not designed to operate the cruise control and exhaust brake at the same time, like newer coaches. The minute you turn on your exhaust brake switch, the cruise control should turn off and not play any part in the downhill driving.

It sounds like you have an issue with possibly a throttle pedal or throttle position sensor. The short answer, leave the cruise off when descending a grade and using the exhaust brake.

d23haynes57 · 11-14-2024, 05:23 AM

For my 04 Tropi-Cal the dealer was able to configure it so that once the cruise control switch was on, even if not active, the exhaust brake would not activate until the brakes were tapped. Then stepping on the throttle would release it. This was desirable to me as it allowed for no throttle coasting. I/m hoping I can do the same for the 08 Nimbus. The nimbus has the transmission retarder with the multistep lever. Very easy to activate it. I leave it off until I want it.