Driving Safety: Mountain Driving

1.  When I let the Allison do its thing going up a grade, it tended to lug down to around 1000 rpm before shifting down. Is it best to let the transmission do the “thinking so to speak, or is it better to shift manually and keep the revs around say 1500 rpm which is where we usually run on the flat. The engine is an ISL 400 which has peak torque at about 1300 rpm.

Do not run any diesel at below peak torque RPM at high throttle positions– that is called lugging. Look up that number in your Cummins owners manual or contact Cummins.

2.  Should we shift down when going downhill? 

Your choice as to whether to be pro-active and shift yourself or let the Allison do its thing. I am more of the pro-active school of driving, as the transmission can only react to what happened well BEHIND your coach because of your momentum.

One piece of advice often given is to use the lowest gear going downhill that you used going uphill.   

That is a crude rule of thumb. Bottom line is to select a combination of gear and retarder setting that keeps your speed in check without the use of the service brakes. Service brakes should only be used to show you enough to select a lower gear. With the retarder, it is better for the drivetrain to use the transmission to select a gear that will keep engine RPM at least in the middle of your engine’s RPM range. This ensures that the water pump is turning fast enough to help dissipate the heat produced in the transmission retarder. Said another way, you heat transmission fluid less at higher engine RPMs and lower retarder settings.

3.  All of our driving before this trip has not only been on relatively flat terrain, but also at moderate winter/spring temps. Today was not only steeper but also hotter. The engine temp got up to about 220 on some grades (usually runs at 181 on the flat with moderate temps outside). How hot can the engine get before I should pull over and let it cool down? The engine did cool down to the 180s quickly after reaching the top of a grade.

A couple of things here. First make sure that you have cleaned the CAC (Charge Air Cooler, also known as the after-cooler and the inter-cooler) and radiator at least once a year. Dirt is a really good insulator and can keep your cooling system from performing as it should. Think about it– if you drove in a rainstorm that got the sides of your coach dirty, consider the amount of dirty water that got sucked into your CAC and radiator by the many thousands of cubic feet of air drawn through them by your fan. When that dirty water hits the hot metal, a lot of it evaporates. This leaves the dirt deposited where it can insulate the metal from the cooling air and also blocks the airflow through the small passages.

Running the engine at higher RPM/lower throttle settings lowers engine operating temperature. 

Consult Cummins for the highest safe operating temperature for your engine.

4.  Same question for the transmission. That temp is usually close to 180 when on relatively flat terrain in moderate temps. Today it got up to 240 while going down hills with the retarder on.  What is the usual operating range for the transmission? Again, it cooled right down after we reached the bottom of the grade.

As above, down arrow to higher engine RPM’s/lesser retarder settings to reduce the temperature in the transmission. With Transynd and the severe-duty fluid change schedule, occasional 240 is acceptable. But cooler is better.

5.  We did tow our Honda CRV today (3500 lbs). When these kinds of grades are anticipated, would it be better to unhook and drive the car separately?

Only if the grades are so steep/temperatures so high that you are stressing the engine or retarder.

We got 6.1 mpg for the 100 or so mile trip to Payson. From Tucson to Phoenix we got 9.8 mpg. In that trip, one loses altitude rather than gaining. Brett Wolfe