Allison Automatic Transmission Controls

[jayprosser@gmail.com]

My 2004 Fleetwood, Providence has the Allison Auto Transmission. On the selector there is a "MODE" button. What does that do? When I push it, it lights up but I am afraid to do anything with it. Thanks

[wolfe10]

My 2004 Fleetwood, Providence has the Allison Auto Transmission. On the selector there is a "MODE" button. What does that do? When I push it, it lights up but I am afraid to do anything with it. Thanks

There are TWO TOTALLY DIFFERENT "LET THE TRANSMISSION CHOOSE THE CORRECT GEAR" MODES/PROGRAMS in the Allison ECU which is the "electronic brain" controlling shifting and other functions.

In ECONOMY MODE, the transmission will not downshift even at WOT (Wide Open Throttle) until the engine pulls down to peak torque RPM in some application and 200 RPM lower that "regular mode" in others.

In REGULAR MODE, the transmission WILL downshift much earlier (to maintain higher engine RPM).

ONLY at higher throttle positions is there any difference, so on flat ground you will NOT notice any difference (except accelerating from a stop IF you are at or close to wide open throttle).

It can make a BIG difference in rolling hills. If you are in rolling hills and regular mode (particularly with the cruise control on), it is common for the transmission to shift down to 5th on the uphill and back to 6th on the downhill. Repeat this process hundreds of times. In economy mode, you will stay in 6th gear unless the hill is so steep or so long that the engine can not pull it without dropping below peak torque RPM. If you can pull a hill in a higher gear (lower engine RPM) AND the engine does not overheat, THAT IS WHAT CATERPILLAR RECOMMENDATIONS FOR THE MOST ECONOMICAL WAY TO CLIMB A HILL with a modern turbo, after-cooled diesel engine.

If you know you will need a lower gear because of the steepness of the grade and/or are engine temperature is rising higher than thermostatically controlled temperature, if driving in economy mode, use the down arrow to drop a gear (this is what I do) or switch out of economy mode. Be sure to switch back into economy mode when past the steep section, or agree to pump extra fuel in the tank.

IF your engine begins to overheat, your HP/weight ratio is low OR if it irritates you to loose a few mph on a hill in the name of saving fuel, in the hills, by all means drive in regular mode.

It confuses me to hear people advocate driving in economy mode only on flat ground, as there is not 1% difference in shift RPM's between regular and economy mode on flat ground, excepting accelerating from a stop if you use WOT.

Every time you start the coach, the transmission is in regular mode. This is the default setting. IF you push the mode button, it goes to "economy mode" AND the light illuminates.

There is no "absolute" on how much difference in fuel economy driving in economy mode will have. On flat ground where you will be in 6th gear irrespective of what mode you are in, there will be ZERO difference. The MOST difference in mileage will be in rolling hills, where in regular mode, particularly if on cruise control you will start up a hill in 6th gear, go to WOT in 6th gear, downshift of 5th gear still at WOT (WHERE IT IS USING A LOT MORE FUEL). After the hill is crested, the transmission will up-shift to 6th, then likely coast a little in 6th gear (unless you are driving with the exhaust brake on-- if you are it then applies the exhaust brake AND downshifts TOWARD the pre-select gear which is generally either 2nd or 4th).And so on 6-5-6-5-6-5-6-5.......

A modern turbo inter-cooled diesel is much more efficient at low RPM high throttle settings. This is NOT my opinion. It is stated in just those words in the Cat Owner's Manual.

Note: In either mode, you are free (and welcome) to use the up and down arrows to PRO-ACTIVELY choose the correct gear. You can not screw anything up-- even if you down-arrow to 1st gear at 70mph, the transmission understands that you meant "please downshift to the next lower gear as soon as the engine RPM will not exceed the pre-set amount. Then downshift again when safe....."

By the same token, you can shift between regular and economy mode as often as you want with the transmission in any gear when you make the change.

OPINION: I drive in economy mode 99% of the time, including in REAL mountains, but use the up and down arrows to choose the proper gear. I use regular mode ONLY when I am willing to say, "I am willing to throw a lot of fuel away to gain a little performance." When passing on 2 lane roads, THIS IS the case.

Brett Wolfe

[Guest Wayne77590]

It is my understanding also, that when you use the down arrow to shift to a lower gear, say 6 to 5, then 5 becomes the highest gear it will use until you push the arrow to 6, or exceed the RPM's for that gear, or shut the engine off and restart. So, if you need 4th gear and down shift with two clicks of the arrow, when the engine RPM's dictate, you will be in 4th gear, and it will not come out until you use the arrow to shift up, or exceed the RPM for that gear. I use it sometimes going down grades to keep off the brakes and let the engine do the slowing. So far so good.

Brett, if I'm not correct in this, please post here. I'm always open to good savvy information. Your explanation above is great.

[wolfe10]

It is my understanding also, that when you use the down arrow to shift to a lower gear, say 6 to 5, then 5 becomes the highest gear it will use until you push the arrow to 6, or exceed the RPM's for that gear, or shut the engine off and restart. So, if you need 4th gear and down shift with two clicks of the arrow, when the engine RPM's dictate, you will be in 4th gear, and it will not come out until you use the arrow to shift up, or exceed the RPM for that gear. I use it sometimes going down grades to keep off the brakes and let the engine do the slowing. So far so good.

Brett, if I'm not correct in this, please post here. I'm always open to good savvy information. Your explanation above is great.

Wayne,

You are correct. Just as in your car, or other vehicle, if you downshift the Allison (using the down arrow), it will not shift above the gear you select until you up-arrow OR if you want all 6 speeds available, by pushing "D". The exception is if you try to take speed in that gear to above design perimeters either with the throttle or steepness of the grade. Said another way, the transmission will not allow the engine speed to go high enough to harm the engine-- it will up-shift.

And you are correct in how you descend a grade. With any heavy vehicle, the service brakes (brake pedal) is NOT to be used to maintain a safe speed of descent. Using the exhaust or engine brake in combination with the transmission down arrow to choose a condition where your speed of descent is in EQUILIBRIUM. The equilibrium speed is one where you are neither accelerating nor decelerating. It varies with slope of the grade and your total weight. (GCW). As a guide, physics dictates that your equilibrium in a motorhome will be faster than a loaded 18 wheeler and slower than an empty one.

If you do have to use the service brakes, do so in one reasonable application (no pumping), slowing down enough to down-shift to the next lower gear. Repeat until you find a gear that keeps speed in equilibrium. At first, it may seem strange that your speed of descent is much more related to this equilibrium speed than to road conditions. Said another way, on a given grade, your safe speed is the same on a straight road and a curvy one (assuming curves can safely be driven at or above your equilibrium speed).

We were on a 10+% grade coming out of the Green Mountains in VT in 2nd gear with the exhaust brake on-- no use of service brakes at all. Dianne was concerned that someone would run over us (we were going about 15 MPH). We came around a corner, and a loaded logging truck was descending at around 5 MPH-- his equilibrium speed.

Brett Wolfe

[cbayman@embarqmail.com]

Brett,

Thank you for your imput, as once again, i learned some more about my Allison Transmission from this forum. I like Jay had

the same questions a few years ago. The dealers do not give a buyer to much information when you make your big purchase.

Once again thanks for sharing your knowlege, with everyone on the different forums that you are on, it is appreciated.

Happy Holidays.

[johntelling]

Brett,

Thanks for all that information about Allison transmissions and Cat engines. It would appear that it is impossible to either over rev or lug these things - mine is a C7 350. I normally drive with the retarder turned on so I won't forget it, which causes the transmission to lurch to a lower gear, and the engine revs to go up pretty dramatically.

My guess is that Allison and Cat got together and figured out how to avoid blowing these things up. In the good old days we were taught to avoid that lurching thing since it might be harmful to the engine.

My question is - am I doing any harm by leaving the retarder on all the time?

JT

[wolfe10]

Brett,

My question is - am I doing any harm by leaving the retarder on all the time?

JT

The simple answer is "not enough harm to be concerned about it".

Certainly, repeated high RPM downshifts will shorten engine, transmission and drive shift life. BUT, all these components are well over-designed. Also, be aware that you are going to burn more fuel doing this, as exhaust brakes on, then throttle to re-accelerate does take extra fuel, particularly since this eliminates coasting (my favorite "gear").

Personally, it would drive me crazy to have the transmission downshift, engine RPM's go way up and rapid deceleration every time the throttle is closed.

And, with your Caterpillar C7, you have other options that can be programmed by a Caterpillar dealer including "Latch Mode" in which the exhaust brake is applied when the service brake is applied and stays on until you go back on the throttle.

Were I to be inclined to drive with the exhaust brake on, I would pay a Cat dealer to reprogram to this options. Latch Mode allows full use of the exhaust brake in emergency stops (applies instantly when you step on the brakes) and allows the exhaust brake to stay on for long descents without coming on every time you let off the throttle.

Brett Wolfe

[johntelling]

The simple answer is "not enough harm to be concerned about it".

Certainly, repeated high RPM downshifts will shorten engine, transmission and drive shift life. BUT, all these components are well over-designed. Also, be aware that you are going to burn more fuel doing this, as exhaust brakes on, then throttle to re-accelerate does take extra fuel, particularly since this eliminates coasting (my favorite "gear").

Personally, it would drive me crazy to have the transmission downshift, engine RPM's go way up and rapid deceleration every time the throttle is closed.

And, with your Caterpillar C7, you have other options that can be programmed by a Caterpillar dealer including "Latch Mode" in which the exhaust brake is applied when the service brake is applied and stays on until you go back on the throttle.

Were I to be inclined to drive with the exhaust brake on, I would pay a Cat dealer to reprogram to this options. Latch Mode allows full use of the exhaust brake in emergency stops (applies instantly when you step on the brakes) and allows the exhaust brake to stay on for long descents without coming on every time you let off the throttle.

Brett Wolfe

Brett,

Thanks very much for that information! It does drive me nuts letting the thing do that. I wonder why they don't program the 'latch mode' into it in the first place?

By the way, I can't tell you enough how great I find the FMCA forums!

JT

[wolfe10]

Brett,

Thanks very much for that information! It does drive me nuts letting the thing do that. I wonder why they don't program the 'latch mode' into it in the first place?

By the way, I can't tell you enough how great I find the FMCA forums!

JT

JT,

Thanks.

And the chassis maker is the one who tells the engine manufacturer how they want the engine programmed-- perhaps with some input from coach builders and/or owners. I really don't know the "decision process".

But I do know that for the cost of reprogramming the engine's ECM at a Caterpillar dealer, you can "have it your way".

Brett Wolfe