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Old 05-13-2019, 09:33 AM   #1
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Mountain Driving

I have 03 Allegro 36 ft DP bus. It has 330 cat engine and allison tranny. Push button shifting. When driving in mountains when do you down shift? My tranny is 6 speed automatic. Can I just start down shifting when I start over the top and do one gear at a time or go for the lowest gear which is number 1? The start shifting up when I reach the bottom. First time driving DP in the mountains so any help you can give me. Thanks
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Old 05-13-2019, 10:20 AM   #2
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Quote:
Originally Posted by jastinjr View Post
I have 03 Allegro 36 ft DP bus. It has 330 cat engine and allison tranny. Push button shifting. When driving in mountains when do you down shift? My tranny is 6 speed automatic. Can I just start down shifting when I start over the top and do one gear at a time or go for the lowest gear which is number 1? The start shifting up when I reach the bottom. First time driving DP in the mountains so any help you can give me. Thanks
As a general rule, you will go down the mountain the same gear you came up given the slope is about the same but you may have to downshift more if necessary. I have never found it necessary to go all the way to 1st gear. I generally will manually downshift as I'm going up to keep the RPMs between 1800-2000 then stay in the same gear going down. I do have a two stage compression brake that will automatically downshift as the speed drops so I monitor the rpms and speed. You should have an exhaust or compression brake that will help slow coming down but be SURE you don't ride the brakes. Use stab braking to keep the speed down by applying the brakes fairly hard for a few seconds then get off of them. Be aware that if your speed builds up too high and the RPMs go above maximum the transmission will upshift to protect the engine. That can spook you the first time it happens so just be aware that you need to stab brake to slow down.
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Old 05-13-2019, 10:27 AM   #3
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Does it have an exhaust brake? Turn it on and leave it on. In many instances thats all you will need plus a couple of light taps on the brakes. DO NOT ride the brakes down hill. Rule of thumb is get on the brakes, brake hard to below a comfortable speed, get off the brakes let the cool. Brake hard, reduce speed off brakes to cool. What ever spped trucks are going down, match it or slower.
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Old 05-14-2019, 10:20 AM   #4
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You can't hurry a diesel up a mountain with more throttle. Meaning it is only going to go up hill so fast for the HP and torque the load will allow the engine to make. If you are pedal to the metal for 2000 rpm, 35 mph in 3rd and that is all the Cummins will make, and you are watching the temp rise; gently, slowly and smoothly back of the pedal. Just slightly to see if the 2000 rpm will maintain with less throttle and less fuel being pumped into the engine making just more heat. If not, it is time to back off till you get slow enough for the next lower gear. I don't hold wide open as the MPH/RPM are dropping, I find it does no good. Turn off the dash air as you approach the grade if your AC condenser is in line with your radiator and trans cooler. I drive mountains by temp gauge and RPM, (have a cheat sheet with max rmp for each gear) checking the temp gauge near constantly. I keep my engine temp below 210. A engine rebuilt or replacement is a lot $$$$$ for a few mph. Tr. My alarm has never gone off as I do not let it get hotter than 210. Just my opinion, but works on my 400 Cummins.
I just slow down, live will it and enjoy the slower ride up hil


When you are going down an long hill/mountain you should continue to use your service brakes to help control your speed. Do not drag your brakes, as they will overheat very quickly, do not pump your brakes as they are air operated and you could pump yourself out of air quicker than the engine driven air pump can keep up. When you need the brakes to help the engine brake control your speed, give the pedal a firm steady push to make a noticeable reduction in your speed and then get off the pedal. Repeat as necessary to control the speed. If you only use the exhaust brake, when the revs start to exceed the no load rpm, the Allison will upshift to reduce the engine rpms. When that happens you will find yourself going down hill, in a higher gear than you want to be, and gaining speed very fast. Guess how I learned this? :scared Richard
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Old 06-10-2019, 09:44 AM   #5
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Could you post your cheat sheet for me? Thanks
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Old 06-10-2019, 10:51 PM   #6
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We are at Laguna Seca campground this week. There is an 18% grade hill up to it about 1/2 a mile long from the highway. Yes 18%. Half way up I was wishing I had disconnected the grand Cherokee at the bottom. Full tank of fuel and water also. Canít drink the water there. Made it though.
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Old 06-10-2019, 11:44 PM   #7
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Could you post your cheat sheet for me? Thanks
From his post it appears that Richard has a Cummins engine in his coach , that in addition to possible different tire size and differential ratio could make any speed to RPM in a given gear; info he could provide; of no use to anyone else, unless they have the exact same set up .

Cummins 5.9 and 6.7 engines rev higher than our Cats and the larger Cummins 8.3, 8.9 lower .

I crest most summits at 45 mph ( remember it's MUCH easier to speed up than slow down) . Exhaust brake on , the exhaust brake program can be changed and you need to know if yours has , when you take your foot fully off the throttle on a downhill with the exhaust brake on , the shift pad can show 2/4/as the target gear or6 . If it shows 4 or 6, then it has been changed at an Allison dealer , if it shows 4 , forth is the lowest gear the trans will automatically downshift to, if it shows 6 the program has been changed to require the driver to manually down shift using the pad. ( providing the exhaust brake is working).
The trans will down shift to keep engine RPM above 2000, at 45 MPH that's forth in my coach, if the RPM drops below 2000 the trans will drop another gear. As mentioned allowing the RPM to climb too high the trans will up shift to avoid engine damage . For my Cat 3126B that's at 2650 RPM; so when the tach hits 2500 I'm on the service brakes to slow RPM to 2000. If you find your having to apply the service brakes very often , then It's time to get on the brakes hard to drop the RPM below 2000 and have the trans down shift to a lower gear. Only on two hills have I had to get the trans down to third to help control speed.
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Old 06-11-2019, 08:17 AM   #8
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Originally Posted by Skip426 View Post
From his post it appears that Richard has a Cummins engine in his coach , that in addition to possible different tire size and differential ratio could make any speed to RPM in a given gear; info he could provide; of no use to anyone else, unless they have the exact same set up .

Cummins 5.9 and 6.7 engines rev higher than our Cats and the larger Cummins 8.3, 8.9 lower .

I crest most summits at 45 mph ( remember it's MUCH easier to speed up than slow down) . Exhaust brake on , the exhaust brake program can be changed and you need to know if yours has , when you take your foot fully off the throttle on a downhill with the exhaust brake on , the shift pad can show 2/4/as the target gear or6 . If it shows 4 or 6, then it has been changed at an Allison dealer , if it shows 4 , forth is the lowest gear the trans will automatically downshift to, if it shows 6 the program has been changed to require the driver to manually down shift using the pad. ( providing the exhaust brake is working).
The trans will down shift to keep engine RPM above 2000, at 45 MPH that's forth in my coach, if the RPM drops below 2000 the trans will drop another gear. As mentioned allowing the RPM to climb too high the trans will up shift to avoid engine damage . For my Cat 3126B that's at 2650 RPM; so when the tach hits 2500 I'm on the service brakes to slow RPM to 2000. If you find your having to apply the service brakes very often , then It's time to get on the brakes hard to drop the RPM below 2000 and have the trans down shift to a lower gear. Only on two hills have I had to get the trans down to third to help control speed.
Skip on reading your post (great info) I found that my 3126E automatically downshifts to 2 going downhill. Do you think this is normal or has it been changed? I have not had any work do on it, maybe the previous owner did.
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Old 06-11-2019, 09:06 AM   #9
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Skip on reading your post (great info) I found that my 3126E automatically downshifts to 2 going downhill. Do you think this is normal or has it been changed? I have not had any work do on it, maybe the previous owner did.
I'm reasonably sure that the program that makes 2nd the target gear ( lowest attainable) is the original program , and that 4th and, operator manual down shift, are the changes that can be made with the proper shop tools.
As mentioned the program will not downshift , if the lower gear will result in the engine RPM going too high.
The one time I was rolling into a rest area with the exhaust brake active and the trans did shift to second , I was glad we always wear our seat belts.
I now know that there are times when the exhaust brake will provide too much braking and realize why some would opt to have the program changed.
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