Originally Posted by Dutch Star Don
The justification for not using the cruise control in the mountains really isn't valid. These aren't trucks with multiple gears, they have a six-speed transmission and no matter what you do, you're not going to be able to hold steady at a certain speed by upshifting or downshifting. Mashing the pedal to the floor is not really a good plan either, you can't feel the engine response.
With that said, you can safely shift from 6th to 5th manually without any issue, while using cruise control. As many said, your Allison is pretty smart and knows when to shift......EXCEPT..... it can't see an upcoming grade and doesn't know when to shift until you start to lose speed.
On many grades, I drop to fifth at the base and then let the Allison do the rest. If you want to increase speed a little before the grade, you can push the cruise "UP" button to bump it up a little. I use the cruise control 90% of the time I'm driving because with a diesel pusher and fly by wire throttle, there is NO sensation as to what's going on with the engine.
Even better, I use cruise control going down grades. My coach has a three-speed engine brake and I set it and let it do its thing. If I want to go slower or faster, I change the cruise speed. The engine brake engages at 1-2 mph over my set speed.
Lastly, a couple of people on here are talking about manually downshifting on older coaches to keep them from overheating. The OP has a modern DP and will not have the heating issues.
Below from 2021 Tiffin Manual.
• Normal driving – best fuel economy
o Select “D” and “Mode On”
o “Mode Off”
o For mountain driving, select lower gears to maintain 2000+ engine RPM
• Hill climbing on hot days
o Keep RPMs high to cool engine
Driving Tips with the Allison 3000MH or 4000MH Transmission:
The points at which shifts occur depend upon predetermined speeds and other operating conditions. A
transmission “shift calibration” includes several sets of shift points used according to current or anticipated
operating conditions, such as engine or transmission fluid temperature. You can change shift schedules using
the MODE button.
The transmission control module (TCM) includes the capacity for two separate and distinct shift calibrations,
one for use in “Primary Mode” of operation and one in “Secondary Mode.”
Primary – This shift schedule is typically used for all normal vehicle operations.
Secondary – This is an alternate shift schedule that the TCM uses upon request. This is operator-
controlled using the MODE button.
When you are driving under normal road conditions, the DRIVE mode is recommended for the best
performance and fuel economy. The MODE switch should be set to ON for economy mode, but MODE off
should be used when climbing hills and when extra performance is required.
The display screen on the shift control pad will indicate the highest selected gear for the transmission. When
mountainous or up-and-down terrain conditions are encountered, you should manually select a lower gear,
preferably lower than 5th gear. This can be done at any road speed by pressing the down arrow repeatedly until
the desired gear is indicated in the window of the shifter pad. When your road speed decreases to a safe point,
the transmission will downshift at a higher RPM than normal. This will decrease the use of overdrive while
pulling hills, which can result in excessive heat build-up in the transmission, and keeps the engine operating at
peak horse power and performance.
When ascending a grade, maintain engine speed to within 400-500 RPM of governed engine speed. Governed
speed will be 2200 RPM on the Cummins ISL engine model. Road speed may decrease, but the engine will be
at its peak in the power curve.
It is especially pertinent to monitor your water temperature gauge when climbing steep grades. Keep in mind
that it is not uncommon for the temperature to increase, especially in hot weather. If the gauge reaches the end zone
zone or if the temperature warning light on the gauge panel should come on, reduce your road speed, shift to
the next lower gear and keep your tachometer within 500 RPM of engine governed speed. In many cases this
will stabilize the water temperature. If the temperature gauge continues to rise, pull to the side of the road and
shift the transmission into neutral. Bring the engine RPM to 1,700—2,000 RPM until the temperature drops
down into the normal range. This should occur in a relatively short period of time. If the temperature gauge does
not begin to drop, stays in the red zone, or continues to rise, shut down the engine and allow it to cool. After the
engine is allowed to cool check the fluid level in the reservoir and add coolant if needed.
A good “rule of thumb” for descending grades is to never use a higher gear than was used to climb the same
or similar grade. Try to keep the engine within 500 RPM of governed speed. This will give the best engine
braking and reduce the need to use the service brakes. Select a gear that will keep you at a safe speed with
minimal brake application. Never ride your brakes when descending a grade since excessive brake heat will
build up and your brakes could fade, leaving you with little or no braking power.
Your vehicle is equipped with an engine brake. The engine brake will assist in slowing your vehicle on a
downhill grade. With the engine brake switch in the ON position, release the accelerator and depress the
service brake to activate the engine brake. When the engine brake is activated the transmission will pre-select a
lower gear to aid in braking. This is indicated by a “2” in the left hand pane of the transmission shift selector.
The transmission will begin to down-shift as soon as the road and engine speed will safely allow. This will
produce a slowing effect and will remain engaged until either the exhaust brake switch is turned off, the
accelerator is pressed, or the engine speed drops to 800 RPM. If your initial speed is high, you may have to
step on the brake to slow the vehicle before the transmission will down-shift from 6th gear to 5th gear. This is
Always select (N) neutral on the transmission shift pad prior to turning off the vehicle engine.
The above from from 2021 Tiffin Manual