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Mountain Driving

May 09, 2010, 12:35:01 am

We had our first mountain driving experience today in our 2003 U295 36'.  It was a short drive from Phoenix to Payson, AZ up highway 87.  It includes several long grades both up and down.  We have a Silverleaf system and I can see the gear being used, rpm, torque and hp.  Everything went OK, but we do have a number of questions for those with more experience....

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.

2.  Should we shift down when going down hill?  One piece of advice often given is to use the lowest gear going down hill that you used going up hill.  With the Silverleaf, I can see what gear I am in even when I have not shifted down manually.  So if the Allison got down to 3 going up, should I manually use 3 going down?  Or should I just let the transmission pick the gear?

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.

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. 

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?  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 looses altitude rather than gaining it. 

Thanks in advance for the advice.

George and Pat Hatfield

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Re: Mountain Driving

Reply #1 – May 09, 2010, 09:02:42 am

My reply in italics.

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 down hill? 

Your choice as 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 momemtum.

One piece of advice often given is to use the lowest gear going down hill that you used going up hill. 

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 use of the service brakes.  Service brakes should only be used to slow 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 insures 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 RPM's 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 rain storm 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 air flow through the small passages.

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

Consult Cummins for 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 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 looses altitude rather than gaining


Brett

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Re: Mountain Driving

Reply #2 – May 09, 2010, 03:39:46 pm

Our rig is a bit different set up than yours but diesel operation is diesel operation. We have a 1990 Grand Villa with the old Detroit 8.2 and an Allison 4 speed transmission. The Detroit redlines at 2600 RPM and turns way more RPM than the later Cummings. The torque curve max on the Detroit is close to 2250 RPM. Over the years I have found that is I control the shift points and keep the RPM's near the max torque and keep from flooring the throttle peddle when climbing hills the engine temperature will stay around 220 on a hot day. The transmission temperature is more a function of the strain in the torque converter, the fluid coupling between the engine and transmission, than anything else. A normal fluid coupling torque converter will reach near lock up meaning that the engine and input shaft of the transmission are turning at near the same speed. As engine speed drops of with the throttle mashed and the engine pulling for all its worth the fluid coupling will be slipping trying to multiply force. That is when the transmission oil gets beaten around in the converter and heats up. So, the higher the engine speed the less the strain on the converter and the less heat that it develops. Combine that with the increase in the flow in the coolant because the water pump is moving faster, the fan turning faster on a belt driven fan, and a reduced throttle position meaning less fuel being burned and temperatures behave better. On the down hill side the torque converter is being driven by the drive wheels with the engine running at a slower speed trying to hold the rig back. The oil in the converter is again being beaten like in a blender and the temperature goes up. Once running down the highway with the engine load lighter the converter reaches near lock up or on the later units a mechanical clutch in the converter locks it up and the fluid cools down.

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Re: Mountain Driving

Reply #3 – May 09, 2010, 04:42:06 pm

Torque converter slip is NOT an issue in any Allison 3000 or 4000 series 6 speeds.

Their torque converter locks at "higher RPM's" in 2nd gear (feels like a mini shift when it locks up) and is always locked in 3rd, 4th, 5th and 6th.

So the only time transmission heat will occur is in stop and go traffic and when using the retarder.

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Re: Mountain Driving

Reply #4 – May 09, 2010, 11:06:27 pm

Thanks for the input everyone.  What I get from this is....

1.  It is better to manually shift going up hill to keep the rpms around 1500 or so with moderate throttle.  This is the rpm we usually drive at in 6th gear on the flat.  In other words, I will be driving by the tachometer.  I will avoid flooring the throttle (which I rarely do).  If I can't maintain the rpms I need, I will shift down.

2.  Going down hill, I will first get the speed down to where I want it and then start with the lowest gear that I used coming up the hill and set the retarder to maintain a safe speed (usually about 35-40mph).  If the transmission temp starts going up I'll downshift to increase the rpm and back off on the retarder.  I will reserve my brakes and use them only for short pulses to bring the speed down to the desired level.  I will keep the rpms below 1800 or so.

If any of this seems to be in error, please let me know.  I've got a couple more days before we head out again from Payson to Flagstaff!

George

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Re: Mountain Driving

Reply #5 – May 10, 2010, 01:11:30 pm

I have not heard of any cases where a motorhome went out of control going UPHILL. It is the going down part that can be scary. I typically slow to about 55 mph at the crest and switch on my Jake Brake. This downshifts the tranny to 4th and will keep the coach at 55 on every steep grade that I have been on so far. I also put on the four way flashers and keep my eye on the rear view mirror looking for fast approaching trucks. Once on a very steep grade with a hairpin turn and no jake brake, I put the coach in first gear and idled down 3-4 miles at about 10 mph. Never touched the service brakes and I am sure some traffic behind me was not happy but I made it down safely. I was as courteous as possible by moving over as far as possible and waving traffic by me when appropriate.

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Re: Mountain Driving

Reply #6 – May 10, 2010, 10:26:22 pm

Four way flashers aren't necessary unless your driving speed is 15-20MPH below the posted speed limit.  Most steep downhill speed limits for truckers is 35MPH and rarely do I ever see their 4-ways on unless their uphill speed is below 20MPH and thus the same going downhill.  Certainly using 4-ways at 55MPH up or downhill is unnecessary.

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Re: Mountain Driving

Reply #7 – May 10, 2010, 10:50:12 pm

Your safe speed of descent (speed held in check without using service brakes) will be FASTER than loaded 18 wheelers and SLOWER than empty ones.

Strictly a matter of physics.

So you will be less of a hindrance to traffic than the loaded 18 wheelers.

Just drive and enjoy.

Brett

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Re: Mountain Driving

Reply #8 – May 13, 2010, 10:38:14 am

Yesterday we had another dose of mountain driving from Payson, AZ to Lake Havasu, AZ.  The trip up from Payson to the top of the Mogollon plateau is steep, two lane and windy.  I used the recommended manual shifting technique and it worked like a charm.  I keep the RPMs at or above 1500.  At times I was down into third gear, but was able to maintain at least 35mph (except when the curve signs advised 20mph).  The engine and trans temperatures remained around 180 most of the time (outside temps in the 60s).  The only time the transmission temp went up to 220 was on a long down grade which isn't surprising given all the energy it and the retarder were absorbing.  We did not tow the car up the steepest part of the grade and I think it helped. 

The Silverleaf was a great help since I could not only easily monitor the tach, transmission gear and temps, but also engine load.  When we purchased Silverleaf I thought it was a bit of a gimmick, but it has proven to be very useful not only for mountain driving, but also in monitoring the cruise control and following our fuel usage and mileage. 

Thanks again for all the advice.

George

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