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Ray,IN · 09-18-2020, 09:14 PM

For those with an exhaust brake, there is a very important fact to keep in mind when descending a steep grade.
The engine ECM and transmission TCM "talk to each other" to prevent the engine from over-speeding and destroying itself. When this happens the TCM will command an upshift to a higher gear.This reduces engine RPM to a safe range.

NOW you only have your service brakes to slow downhill speed until you have reduced speed enough for the TCM to allow a downshift.
At the top of the downgrade, reduce speed and engage exhaust brake to maintain control. When speed climbs 5 MPH use the service brakes with a firm pedal to bring speed back to your original safe zone. DO NOT pump air brakes!

This is why an engine compression brake is vastly superior to an exhaust brake.
That is my opinion and true to the best of my knowledge.

kenwyatt · 09-18-2020, 09:18 PM

Did some truck driving myself and trained in the mountains around Salt Lake City. All great comments. One thing my instructor always hammered on us is the bit about feeling pressured by others. Used to say when you are letting others drive your rig for you then YOU are in a WORLD OF XXXX!! Take it easy, slow and follow the tips above. You will get to the point you can enjoy the ride up or down but it has to be at a pace you are comfortable with

iowaone · 09-18-2020, 09:36 PM

What Ray talked about in post 15 above is very important in my book. If the you let the transmission up shift and you aren’t prepared for it, it can be very scary. So learn at what rpm it will upshift going down hill. It’s about 2500 rpm for mine. I always make sure to brake to bring it down to 2000 rpm if I am getting close to 2500. If it up shifts it will give you a thrill. You suddenly pick up speed and have to brake hard to get it back under control. The Allison does a great job controlling speed but you must help it by not letting it up shift when you are trying to control speed.

MN_Traveler · 09-19-2020, 09:23 AM

All great advice ... so I’ll just add three (really reinforce) perspectives.

The first is that I definitely prefer manually managing the transmission - gives more control and “feel” of the situation.

Second - going uphill I never have a target speed. The grade and your rig will define that. I downshift until I can maintain speed without the pedal being totally floored (what others have called 80% throttle).

Third - going downhill I never have a “target speed” (again, grade and your rig will define that). My objective is to settle at a constant speed WITHOUT using the service brakes. Downshift and/or use the engine brake until you hit that equilibrium. If the RPMs are too high to allow you to downshift, then use the service brake in one continuous press to slow you to the point where you can downshift. Repeat until you get to that equilibrium point. Mind what others have said about NOT pumping or overusing the service brakes and for starting out this whole process right at the summit as you start down, and never let your speed increase to a point where you feel you can’t control it using only the transmission and engine brake. Runaway truck ramps exist for a reason, and you NEVER want to create a situation where you need to use one.

Believe it or not, once you find those equilibrium “sweet spots”, both going up and going down can be quite relaxing and not white knuckle at all. I as once going eastbound on I70 down the grade into Denver (long steep grade). I ended up tucked in behind a very fully loaded truck carrying hay bales. I think we both settled in at something like 15-20 mph. Comfy and in control the whole way with our hazard blinkers going beep-beep-beep. Sure, there were cars whizzing by us ... but so what?

richard5933 · 09-19-2020, 09:49 AM

You can go down a mountain pass too slow as many times as you like. You only get to go down too fast once.

There is absolutely nothing wrong with going below the speed limit on the downhill side if needed to maintain control. You'll often see heavy trucks doing that. The general rule is to go down the other side in the gear you used to go up, so if you find that the transmission had to downshift a few times to get up you should keep the same gear going down the other side.

I know that some will say let the transmission determine what gear to be in. The problem is that the transmission can't see the curve coming up ahead that you'll have to safely navigate, so it's up to you to keep your speed down to a safe level.

The engine brake will be more effective towards the upper end of the RPM range as well, so better to be in a lower gear towards the upper end of the range than the next gear up towards the lower end, at least if you want to have the engine brakes help you maintain a safe speed. And you do want that.

J Birder · 09-19-2020, 10:36 AM

The photo below, shows a diagram of a section of U.S. 14A, in Wyoming, between Lovell and Burgess Junction in the Bighorn Mountains. The 10% grade for 10 miles is about as steep as any that you are likely to find. I pull a 27' 5th wheel with my Silverado 2500 HD, with diesel engine, Allison 5 speed tranny, and single stage engine brake. I have towed this stretch of road a few times going up, and many times going down.

Going up is easy. I leave the transmission in "D" and control my speed with the accelerator. If a down shift is needed , I let the TCM take care of that. I never pay any attention to it. I generally do the speed limit with no trouble. If you choose to drive at less than the speed limit, you may get a bunch of traffic lined up behind you. It in nice to pull over and let them go by, but only if you come to a spot where you can do it safely. If not, just ignore them.

Going down is more complicated, but not really much more difficult. I pull into the Brake Check Turnout, make sure that the engine brake is on, set the transmission is in Manual Mode, and set it to 3rd gear. Shortly after the first Runaway Truck Ramp, I find that I'm beginning to pick up speed, so I shift down to 2nd gear. The rest of the way down, I alternate between 2nd and 3rd gears. There is one section where I will actually need to use 1st gear.

I may occasionally hit the brake pedal fairly hard for about 1 second. So I don't need the Brake Cooling Turnout, because I have used them so little, that I know they aren't hot. When the road levels out, I put the transmission back into "D," and I.m on my way. I have never had traffic backed up behind me on the way down.

Joel

wolfe10 · 09-19-2020, 10:45 AM

Quote:

Originally Posted by xc-mark

Everything I have read for post above is all good advice and there is a few other points I would like to add.

#1 the Allison can be programed to have two distinct shift modes. It can hold a gear to long if the factory didn't consider the HP of the motor and weight of the coach . If you fine your running below 1400~1500 RPM up hill and it will not downshift you should consider going to a Allison dealer to change the TCM shift patterns to better suit your motor. It also can be programed for ECO mode and Performance mode . just by hitting the mode button on the fly. Some engine platforms can be programed to run the cooling fan with the Jake brake to add even more engine braking for down hill use.

Perhaps a little clarification needed.

ALL Allison 3000 and 4000 transmissions have two distinct shift patterns in their ECU-- POWER (most are programmed to use this as the default every time the engine is started) and ECONOMY (need to push the MODE button to select).

You can change between the two as often as you like-- either stopped or moving.

Same for using down and up arrows-- they are idiot-proof.

Can the Allison be left in Drive-- sure.

Can a smart driver, seeing what is FRONT of him do a better job-- sure.

Jon_C · 09-19-2020, 03:08 PM

Also let your brakes cool before engaging the parking brake. The drums expand when hot and if you engage the brakes when hot It can cause stress cracks as drums cool and contract.

Jzack · 09-19-2020, 03:38 PM

Is there a secret to using the up-down buttons on the Ariens touch-pad? I have the MH2500 Allison and those buttons have never worked. I always just assumed the computer thought it knew better than the dummy driver and was over-riding me.

Also, I think downshifting a gas rig to get the RPMs up before a hill may be appropriate since the torque peak is at a high 4,000 RPM for a V-10, but diesel rigs are different. My Cummins torque peak is at 1,500 RPM, the lowest end of the operating range, so it is beneficial to let the engine lug down to the min. RPM before grabbing the next lower gear. This is what all the big rigs do when negotiating the hills.

Ray,IN · 09-19-2020, 11:07 PM

Quote:

Originally Posted by Jzack

Is there a secret to using the up-down buttons on the Ariens touch-pad? I have the MH2500 Allison and those buttons have never worked. I always just assumed the computer thought it knew better than the dummy driver and was over-riding me.

Also, I think downshifting a gas rig to get the RPMs up before a hill may be appropriate since the torque peak is at a high 4,000 RPM for a V-10, but diesel rigs are different. My Cummins torque peak is at 1,500 RPM, the lowest end of the operating range, so it is beneficial to let the engine lug down to the min. RPM before grabbing the next lower gear. This is what all the big rigs do when negotiating the hills.

Gasoline and diesel engine have one thing in common. The water pump speed is directly connected to engine speed. When it is turning slower it pumps less coolant that at higher RPM.
This is why keeping rpms higher on a long steep grand is best for avoiding overheating the engine, either fuel type.

Winemaker2 · 09-20-2020, 07:00 AM

Excellent comments above.
Only a couple minot points I'd add.
If you are not used to manually shifting your Allison Trans practice ahead of time on some moderate inclines. With a little practice you will gain confidence. Just watch RPM and anticipate where the Allison might shift down and do it manually to maintain RPM at torgue peak.
I'm surprised I didn't see it above but a rough guide for descent (of same grade as ascent) you will want to use the same gear required to climb the same grade.

MN_Traveler · 09-20-2020, 07:28 AM

JBIRDER - a question: how did you manage to **differing** number of times towing up to the pass versus down from the pass? [emoji4]

Old Scout · 09-20-2020, 07:40 AM

….just "experienced" 14 and 14a in the Bighorns this summer--but in a towed not the RV....we used 16 to get from Cody to Buffalo with the RV--steep but not as steep as 14/14a....great scenery in the Bighorns, and we also visited the "Medicine Wheel"......

J Birder · 09-20-2020, 08:03 AM

Quote:

Originally Posted by MN_Traveler

JBIRDER - a question: how did you manage to **differing** number of times towing up to the pass versus down from the pass? [emoji4]

Easy. There are 3-1/2 routes across the Bighorn Mountains. Sometimes, I take one route eastbound, and a different route west bound.

One of the routes involves about 40 miles of unpaved road. It used to be fun in my class B. but I won't tow my 5er on it, so I only count it as 1/2 of a route.

Joel