Some of my fondest childhood memories are road trips with my parents. Most often we would cross the mountains to visit Aunts, Uncles and Cousins at the Holidays or in the summers. Some years we would drive 2,000 miles to Nebraska or Wisconsin to be with Grandparents. On the long trips, we wouldn’t just cross the Cascades, we would cross the Rockies as well. After moving to the Midwest in my early twenties, I continued driving home to the west coast crossing the Rockies, Cascades and Sierra Nevadas. Driving cars through mountain passes is only demanding when the weather is bad.
Since we got our first RV in 2002, Linda has been reading Full-Timer blogs. In 2012, Linda told me to read the following postings in RV Dreams blog:
Howard was driving down a mountain pass on a 7% grade and shifted down to a lower gear to slow down his truck and 42 foot fifth-wheel. The downshift increased his diesel engine’s RPM above the maximum and the engine blew. Not only did the engine repair/replacement take a long time but it also cost over $10,000.
While serious, RV Dreams engine trouble seemed like the sort of accident that would never happen to us.
Later that fall, our motorhome lost its serpentine belt. While driving on the freeway, I lost engine power, steering and brakes. Fortunately traffic was light. I was able to get off the road safely. From a repair standpoint, this normally isn’t a big deal but it took out the fan which chewed a hole in the radiator and more. CoachNet caught our call and we were towed to Cecil Atkission Ford in Hondo Texas. After 28 days at the dealer and quite a bit of runaround, they said they fixed the problem.
Our plan was to drive from the dealer to Palmetto State Park. We never made it to our destination. The serpentine belt failed spectacularly again within 60 miles. The traffic was heavy when the steering, brakes and engine power failed. I got REALLY lucky and got the motorhome off the road after some harrowing maneuvers. In my next call to CoachNet, I made it abundantly clear that Cecil Adkission Ford would not be working on my anything ever again.
CoachNet agreed to tow the motorhome to Blue Bonnet Ford in New Braunfels. Blue Bonnet has great people and lifts that can lift gas powered Ford chassis motorhomes. The Blue Bonnet mechanic working on our motorhome traced the root cause to a cracked plate on the front of the engine block where components with pulleys are attached. At high RPMs, the crack caused vibrations that made the serpentine belt come off track.
After the good service from Blue Bonnet, I continued using them for all my motorhome chassis maintenance and repair needs.
It is fair to say that for years, every time the motorhome engine RPMs got close to 4,000, we got nervous afraid the engine would explode. Fortunately for us, the fix the good folks at Blue Bonnet made was a permanent one.
I was driving the motorhome eastbound on I-10 on our way to Buckhorn Lake Resort RV Park outside of Kerrville Texas. The approach to the exit and the exit itself are extremely steep. I had the transmission in Tow/Haul mode. I was going too fast as I hit the exit and applied the brakes and the transmission downshifted. Surprised, I let off the brakes and then reapplied them. Again, the transmission downshifted again driving the RPMs into the 5,000 range. Nearly scared me to death. Fortunately, the engine was fine and I was able to stop at Goat Creek Road where I safely and slowly turned left. Thank you Blue Bonnet!
Before the motorhome, We had a Ford diesel truck with Tow/Haul mode. It never downshifted when braking in Tow/Haul mode. The motorhome Tow/Haul behavior definitely took me by surprise.
Another blogger, Great Escape from NJ, had diesel engine trouble similar to RV Dreams. The pictures Escape from NJ provided clearly show damage done when valves don’t close before pistons return to the top of the cylinder. This kind of damage can occur when an engine’s RPMs exceeds redline.
All this got me to thinking that maybe mountain driving RV’s isn’t so simple.
RV Mountain Driving Isn’t Simple
At the end of summer in 2016 we spent a week in Ruidoso New Mexico. Using the Circle B RV Park as our base, we did daily site-seeing trips to find interesting future Lincoln National Forest (and other public lands) campsites in the area. We had heard that Cloudcroft New Mexico (9,000 foot elevation) has great campgrounds providing respite from summer heat. We went to investigate. The campgrounds were good. We decided to take the long route back to the Circle B. From Cloudcroft, we headed west on US-82 toward Alamogordo New Mexico .
Local Conditions has the following to say about this stretch of highway…
US-82 between Cloudcroft and Alamogordo descends 5,000 feet in 16 miles! STEEP GRADES, NUMEROUS TIGHT TURNS AND ONE TUNNEL. Trucks without retarder brakes and/or longer than 65 feet in length are PROHIBITED on this section of US-82. Consider using an alternate route.
As we drove downhill in our Jeep Wranger Rubicon, we tried to imagine what we would need to do differently if we were driving the motorhome while towing the Jeep. Loaded for travel, the motorhome weighed around 20,000 pounds and was at the top end of its gross vehicle weight rating. It always felt overloaded. The thought-exercise yielded the following guidance:
- Keep the speed low. How low depends on how steep the descent is. The rule of thumb appears to be to go no faster than one can reasonably drive up the hill while keeping the engine in its most efficient horsepower range. Also it is bad form to overheat engines so pushing it at the top end of the power curve may not be advisable. On this particular mountain, I think I might be able to do it at 30 MPH. Probably second or third gear.
- Gear down and let the engine do the braking. How this is done depends on the vehicle. On the motorhome, making sure I wasn’t going to fast already, I would shift down to 3rd gear as the starting point. I would slow down (using brakes) and push into 2nd gear if I kept gaining speed. If 2nd gear RPM got too high, I would brake again and get into 1st gear. I’m guessing that the best RPM range is where the engine has the most fuel efficient horsepower.
- Don’t let the engine RPM redline unless the brakes have failed and death and dismemberment are imminent. Otherwise, save the engine by using the brakes. My first diesel redlined before 4,000 RPM. I’m not sure yet where the new one redlines. The V10 gas motorhome engine redlines (I think) around 5,000 RPM. Maybe higher.
- Ditch extra weight before starting the descent. Empty RV tanks. Our toad (Jeep) weighs in around 4,500 pounds when loaded for travel. We can save that weight just by unhitching and driving it separately down the hill.
- Research mountain routes before driving them. Tools like the Mountain Directory identify and describe particularly steep sections of popular highways. Know before you go.
I might add the following thought-exercise item for the next RV shopping trip:
- Get a truck rated much higher than the intended load. Bigger brakes provide a larger safety margin when driving in the mountains.
To get home from Ruidoso, we drove toward Roswell New Mexico on US-70 then continuing on US-380. This is a mountain highway and Roswell is 3,500 feet downhill from Ruidoso. I practiced what we discussed on the Cloudcroft to Alamogordo highway, keeping the motorhome under 45 MPH in third gear. I chose that speed because I was able to maintain this speed driving up hill in the opposite direction. RPMs were in the good range. Some of the turns recommended slower speeds and I used the brakes sparingly, mindful that overuse would cause wear and possibly failure.
The route from Ruidoso to Roswell is easy compared to Cloudcroft to Alamogordo. I’m pretty sure I would need more practice on simpler routes before moving up to harder routes like Cloudcroft to Alamogordo. I would definitely need to gain a deeper understanding on the capabilities and limitations of the motorhome before tackling a ‘hard’ route.
This year (2017), we sold our motorhome and bought a new full size diesel pickup truck and slide in truck camper. Hoping to avoid more surprises (e.g. Tow/Haul mode) I need to better understand the controls and capabilities of my new truck. My little inner voice keeps saying, “So how is that working for you so far?” Learning the new truck is a work in progress.
I’ve read and reread and reread and reread the owner’s manual. The new truck has wonderful and complex features (magical compared to 20 years ago) to help manage mountain pass descent speeds. The manual has been my starting point to understanding the tools Ford endowed my truck with. But I still don’t know exactly how to correctly apply the tools described in the manual to my situation:
- truck (F450 versus F350 versus F250),
- load (2,500 lbs truck camper, 1,000 lbs incidentals and 4,500 lbs toad) and
- road conditions (4 to 20% grades).
I’ve looked for other sources to help me understand:
- redline (maximum RPM),
- power curve (most efficient RPM versus horsepower range),
- transmission shifting ranges (where the computer is programmed to change gears) and
- efficient engine braking.
The truck has an automatic transmission with 6 forward gears. The shift positions are are as expected with the exception of Manual.
When in Manual shift mode, you can use the +/- buttons on the shift lever to shift up and down. Pushing Tow/Haul button will activate the Tow/Haul mode.
In Manual shift mode, both shifting up and downshifting may not be immediate. While trying it out, I noticed a slight lag between when I ‘requested’ a downshift and when the shift occurred. According to the manual, the truck won’t shift down if the downshift might damage the engine.
When activated, there is a Tow/Haul indicator on the dashboard.
PRS (Progressive Range Selection) is a way to remove high gears from use while the gear selector is in Drive. The +/- buttons on the gear shift either adds the next higher gear into available Drive gears or removes the current highest gear from available Drive gears. You can see that the sixth gear is removed from the list of years in the photo below.
For example, put the truck in Drive. Starting out all, gears 1 through 6 are available for use. Press the ‘-‘ (minus) button and now only gears 1 though 5 are available. Pressing the ‘-‘ button again makes 5th gear unavailable so that only gears 1 through 4 are available. Pressing the ‘+’ (plus) button puts 5th gear back in the list so the truck, when appropriate, can automatically shift into 5th gear.
One use of Tow/Haul mode is to manage downhill speeds:
- activates engine braking but there is no dashboard indicator
- when foot is off the accelerator and brakes, may shift down to slow or maintain current speed
- using the brakes (with the foot off the accelerator) will increase the amount of downshift braking (every press on the brake pedal may nor may not result in a downshift)
Engine braking can also be activated manually by pressing the right button (see below) on the center dashboard console. One press switches on manual engine braking. Two presses switches on automatic engine braking.
I was having trouble remembering the symbol for engine braking until someone on an Internet forum said to just look for the fart icon. For some reason, I just can’t get that word/icon association out of my mind.
Manual engine braking provides immediate full engine braking whenever the gas pedal is released. Automatic engine braking maintains (when possible) the current downhill speed when the foot is off the gas.
Both automatic and manual engine braking can be used with Drive, Tow/Haul mode, PRS (Progressive Range Selection), and Manual transmission settings.
With Tow/Haul and manual or automatic engine braking enabled, both are indicated on the dashboard as shown below.
The Power Curve, Gears and RPM
The goal is to gain enough knowledge to determine how to maintain safe descent speeds on mountain passes and other steep grades.
In my mind, a safe descent speed is within some speed range that enables a driver to maintain safe vehicle control while minimizing brake wear for a given load range while traveling downhill on a given highway.
It might be because I don’t know what to search for or it might be that the information just isn’t published. I can’t find tables that show what speed (MPH) or RPM gives the greatest engine braking power for each gear. I think this type of information would be necessary to determine safe descent speeds for different load conditions.
Wow! My inner engineer just reared its ugly head. This is complicated stuff. Sounds like this will be a bit of a treasure hunt.
I did find analysis on my engine’s performance on the Internet – 6.7 Liter Power Stroke Diesel Specifications and Information. Included is a power and torque graph. I’ve modified the graph so you can see what I saw when I looked at the graph.
The engine delivers the most power between 1600 and 2600 RPMs.
When I first found this graph, I didn’t really know what torque meant. A search quickly returned “Horsepower vs. Torque: What’s the Difference?” from the Car and Driver blog.
Mathematically, horsepower equals torque multiplied by rpm.
This reinforces my belief the engine RPM range for effective diesel engine braking to pay attention to is the one marked on the graph above.
Car and Driver’s blog also says
The amount of torque produced is directly proportional to the air flowing through the engine.
I’ve heard before that engines are big air pumps. The maximum airflow through the engine is in the highlighted RPM range. Ford’s diesel engine brakes work by blocking airflow through the exhaust part of the turbocharger. The air being swallowed by the big engine on the air intake side has to work really hard to get past the turbocharger on the exhaust side. Air friction slows the vehicle down. This is diesel engine braking the 2017 Ford way.
The truck’s owners manual states diesel engine braking only works above 1500 RPM. We want to stay away from redlining to protect our engine. Just to be safe, I’m limiting my top engine speed to 2600 RPM. Call me crazy!
Will shifting the automatic transmission down redline (blow up) the engine? Answer from the Ford manual:
Although SelectShift will make some downshifts for you, it will still allow you to downshift at any time as long as the SelectShift determines that damage will not be caused to the engine from over-revving.
Will the automatic transmission shift up automatically to prevent an engine redline (failure)? Answer from the Ford manual:
[In Manual mode or PRS (Progressive Range Selection), the] SelectShift will not automatically upshift, even if the engine is approaching the RPM limit. It must be shifted manually by pressing the + button [or by using the shift lever to move to either Drive or a higher gear].
Simplifying and summarizing the top level strategy:
- Enable the diesel engine brake.
- Keep engine speed between 1600 and 2600 RPM regardless of the gear.
- Sparingly use regular brakes to keep within the RPM range.
- When speed keeps increasing (as in the diesel engine brakes are not providing enough braking), slow down, shift down and maintain a slower speed in the lower gear.
- Use regular brakes as needed to be safe knowing that overuse can cause brake failure.
Descent Speed Management Methods
By this point in writing this post, I’ve figured out that Tow/Haul, Manual Shift and PRS (Progressive Range Selection) aren’t enough by themselves to control descent speeds. Each method is only effective when coupled with engine braking.
Potential descent speed control approaches are:
- Tow/Haul with Manual Engine Braking
- Tow/Haul with Automatic Engine Braking
- Manual Shift with Manual Engine Braking
- Manual Shift with Automatic Engine Braking
- PRS (Progressive Range Selection) with Manual Engine Braking
- PRS (Progressive Range Selection) with Automatic Engine Braking
Whether or not Tow/Haul also enables engine braking, activate Manual Engine Braking or Automatic Engine Braking by hitting the engine braking button once or twice. Rely on the dashboard to show that both Tow/Haul and engine braking are enabled. There are two versions of the engine braking indicator – one for manual and the other for automatic.
The Ford manual is unclear on whether or not PRS (Progressive Range Selection) and Tow/Haul can be used together. However, in testing, PRS appears to work just fine with Tow/Haul.
I haven’t tried any of the decent control methods in the real world so I don’t really know what subtle nuances I’m missing. My best guess at this point is, it doesn’t matter. Each of the six descent speed control approaches has advantages and disadvantages. In my view, the differences are only material in extreme situations if at all. My everyday choice will be a matter of driving preference.
My preference is to use Tow/Haul mode with PRS (Progressive Range Selection) and manual diesel engine braking enabled.
My preference is to use Tow/Haul mode with PRS (Progressive Range Selection) and Manual diesel engine braking enabled. I came to this by remembering mountain pass descents. Most mountain pass descents are not consistently downhill. Sometimes you go uphill for short periods before resuming going downhill. Typically sharp curves require slowing down below the posted limit. Basically, descent speeds go up and down.
PRS lets us keep the transmission in an appropriate descent gear range. In Tow/Haul, the truck can automatically shift down when the brakes are applied. With Manual engine braking, descent speed is managed by foot on the gas. Foot off gas slows the vehicle. Foot on gas disables engine brake similar to how gas engines with manual transmissions work. This seems very natural to me with the exception of shifting down when braking.
I guess I’ll have to let you know when I find out.