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Old 04-25-2016, 09:01 PM   #1
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Cat 3126 with Allison odd downshift in cruise control

Briefly and slows down. I suppose I should just take the cruise off. But after a day of racing, and headed for home tired, it's nice to let it control the speed. Getting on to mild grades....with the race trailer in tow. It's a 2000 year with a 3126 and a six speed Allison. It shifts down briefly, and slows, then shifts up again. Annoying as heck. Is there something I should be addressing?
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Old 04-26-2016, 08:22 AM   #2
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Do you have the economy mode turned on? If not turn in on.
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Old 04-27-2016, 05:37 AM   #3
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It is not in econ mode. Is there any drawback to using it on the freeway with 10k lbs towed behind?
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Old 04-27-2016, 06:35 AM   #4
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Quote:
Originally Posted by Bucky1320 View Post
It is not in econ mode. Is there any drawback to using it on the freeway with 10k lbs towed behind?
No there is no problem.
What the econ mode does is allow the transmission to up shift sooner and delay the down shift.
Two things to pay attention to is obtaining a high enough speed so the transmission shifts into 6th once there you can reduce speed a little and it will stay in 6th, second when climbing grades if the rpm drops below 1500 turn the econ mode off until you are up the grade then turn it back on.
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Old 04-27-2016, 08:27 AM   #5
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Quote:
Originally Posted by wb7auk View Post
No there is no problem.
What the econ mode does is allow the transmission to up shift sooner and delay the down shift.
Two things to pay attention to is obtaining a high enough speed so the transmission shifts into 6th once there you can reduce speed a little and it will stay in 6th, second when climbing grades if the rpm drops below 1500 turn the econ mode off until you are up the grade then turn it back on.
Perfect! Thanks!
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Old 04-29-2016, 06:30 PM   #6
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In our coach, engaging Economy Mode when using the cruise control makes absolutely no difference. When accelerating with the RESUME function she still winds out to max governed RPM before upshifting.

If I am not using cruise, then I do see a difference. At WOT the upshifts hit much sooner and at a lower RPM.
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Old 04-29-2016, 06:44 PM   #7
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I beleive the OPs' problem is that while using the cruise control, when the rig exceeds the set point, at "Closed throttle position" the engine brake comes on and at the same time the Allison downshifts to the target gear(per it program, can be changed) to try to get back to the set point of the cruise control.....and when it slows to the set point, the engine brake kicks off and the Allison shifts back to 6th.
My Coach also did this, I had my Allison re-programed to stay in 6th for a target gear. Now, when my engine brake comes on to maintain the cruise control set point, no downshift/upshift, the engine brake comes on and goes off when it slows to the set point. I did not like the auto downshift in the "Default" TCM program.
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Old 04-29-2016, 07:09 PM   #8
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With cruise engaged and brake turned on, when ours starts coasting down a hill, it will cancel the cruise and downshift at about the same time. Don't like it much, but it's something I've learned to live with. Light pressure on the throttle, canceling the cruise, or turning off the brake, will prevent this. If in just lightly rolling hills, I'll turn off the brake. In bigger hills, I'll turn off the cruise.
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Old 04-29-2016, 10:03 PM   #9
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This is just when going up hills. Not down.
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Old 04-30-2016, 06:16 AM   #10
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Originally Posted by Bucky1320 View Post
This is just when going up hills. Not down.
The economy button should delay the downshifts when pulling hills, mine does. It is suppose to change the downshift program, if it is working.
Per Allison:
In “economy” mode, the transmission
shifts at lower engine speed to provide
added fuel savings during operation.
In “performance” mode, the
transmission upshifts at
higher engine speed for
quicker acceleration.

Here is a great write up from Brett Wolfe over on the FMCA forum:

ALLISON MODE BUTTON
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, CUMMINS, DD, ETC RECOMMENDATION 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 most modern diesel's 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.
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Old 04-30-2016, 08:04 PM   #11
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Cat 3126 with Allison odd downshift in cruise control

The way our coach is programmed, Economy Mode is not functional at all with the cruise engaged. On rolling hills using cruise and Economy Mode, the coach will downshift and upshift the same way as it does with Economy Mode off. Absolutely no difference. Consequently I rarely use it. The fuel mileage difference between on and off is very small.

When rolling down a grade with cruise engaged, the Jake Brake does not come on until the vehicle speed exceeds the set speed by 10 mph. When that happens, the Jake will engage and slow the coach to the set speed. When the vehicle slows to the set speed and the cruise control once again opens the throttle, the Jake releases and we're back in normal cruise mode.
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Old 05-01-2016, 08:54 PM   #12
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I will give it a try and see. Will be a while until we have a race a distance away.
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Old 10-03-2016, 03:59 PM   #13
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Just a follow up. Econ mode made it work perfectly! Thank you!
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