In cruise, rpm 500 jump

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In cruise, rpm 500 jump

September 30, 2012, 09:04:36 pm

When towing a jeep with a 40 ft FT with ISM 450, in cruise at 60 mph, the coach would downshift from 6 gear to 5th gear on some hills. The rpm moved from around 1300 to 1500. But on some occasions it soon then jumped to 2000 rpm and then almost immediately back to 1500 rpm, then it would try the 2000 rpm again. I shut off cruise the moment this began to happen. It only happened in cruise.

I asked some technical folks about this. In time I began to speculate, my latest theory, that perhaps it was downshifting again, from 5th to 4th gear, and that kicked up the rpm. Then (my theory) as quickly as it did, the system said, "nope, I do not need that much rpm, go back to 5th gear', only to reconsider and go again to 2000.

I do not have that theory confirmed but it is about all that makes sense to me. A technician said that he re-set the shift points, then test drove about 40 miles and could not duplicate what I had seen though he was not towing and it was after resetting the shift points.

All that is to say. Anyone think this is what was going on, an un-needed downshift to fourth? Would it be so simple that the shift point was set wrong (I understand these transmissions can "learn") or could it be something other?

P.S. If you are new to this discussion, there are related references you might want to check out.
There are a couple of very good threads about shifting changes at certain speeds and with referral to those threads and Beamalarm, this may help others who have not yet considered shift points, rpm management and transmission/engine interface and not only the role of rpm management but the role of torque management. Search, "shift points", all boards.

There is also the option to place the transmission in the economy mode, which I was not, but that could have related to my question. Brett Wolfe had a good article in FMCA a couple of months ago, check that out.

And then there is the valuable overview of the retarder, if you did not find this already, on Barry's site

For me, though, for now is to better understand that 500 rpm swing, where my shift points are and how they vary with the torque demand and protection of over rev or lugging, that I will operate the system properly.

Mike

Re: In cruise, rpm 500 jump

Reply #1 – September 30, 2012, 09:28:37 pm

One of the big advantages of having a Silverleaf display is that it can show which gear the transmission in actually using. In the display below, when the engine is running the first "?" shows the highest gear selected, while the second "?" shows the gear the transmission is in. This is very handy while going both up or down hills.

By the way, I started using the "mode" switch and I think it has improved my mileage. When we get back to Tucson I'll check the mileage for our summer trip and determine the difference.

Re: In cruise, rpm 500 jump

Reply #2 – September 30, 2012, 09:34:42 pm

George, thanks. I do so want one of those devices.....Dave M. showed me the one he installed, I really like electronic toys and this one seems a necessity! I just need to find a used pc.

I do not think I am seeing the gear selected by the system illustrated on my shift panel. Maybe when i downshifted manually to head up a mountain I got a read out of 5 or 4 gear, but not when it is changing on its own. I think the only thing I ever see there is "6" when I am in drive. I may be wrong, but think not. Wish it would tell me the gear on that panel

Re: In cruise, rpm 500 jump

Reply #3 – September 30, 2012, 09:39:56 pm

There is a shift panel that does show what gear it is actually in, but think that a silverleaf vmspc would be a better investment as there are a lot more things that it shows you

Re: In cruise, rpm 500 jump

Reply #4 – September 30, 2012, 09:47:48 pm

I had a bouncing tach on my 2000. Replaced it and all was fine.

Re: In cruise, rpm 500 jump

Reply #5 – September 30, 2012, 09:57:09 pm

Yup, next time that happens, push the MODE button and see if it reoccurs. Mode (actually Economy Mode) lowers the RPM at downshift.

But, with any transmission/engine, you WILL encounter grades that cause the transmission to HUNT (higher gear can't quite make the grade, but next lower gear is lower than need be). This causes the up/down/up/downshift that hurts any transmission. When (not if) you find yourself in that situation, use the down arrow to select the lower gear which prevents the up/down/up shifting. Hit the up arrow/ or "D" once the grade lessens and you can stay in that higher gear.

Re: In cruise, rpm 500 jump

Reply #6 – September 30, 2012, 10:09:54 pm

The transmission control panel on our MD-3060 R shows two numbers while under way. One shows the highest selected gear. The other shows the current selected gear. Under normal operation the highest gear shows 6 and the current gear varies from 1 through six, depending on speed and conditions.

I couldn't tell from your description whether the rpm change was due to an additional downshift, or some other reason. If my transmission hunts for a gear while on cruise control I change the conditions by changing the set speed, releasing cruise control, choosing econ mode, or selecting a lower gear. The choice depends on conditions.

Re: In cruise, rpm 500 jump

Reply #7 – September 30, 2012, 10:46:48 pm

I agree with Brett to use the MODE ON selection for gear shift points. This will reduce the amount of "hunting" the transmission does in uneven or hilly terrain. Also, the Allison dash display on newer coaches does not display the gear in use, only the top gear selected is displayed.

And, indeed since I started using the MODE ON selection, my fuel consumption has been much reduced. I will post the results once I have a few refills under "Forrest's" belt...

Peter

Re: In cruise, rpm 500 jump

Reply #8 – September 30, 2012, 11:54:11 pm

Wonder if it might have been the torque converter unlocking and locking.

Re: In cruise, rpm 500 jump

Reply #9 – October 01, 2012, 12:54:41 am

J.D. I cannot say if the rpm change was related to a shift to a lower gear, but that is the only thing that I could think of. It was in cruise and in the default Mode. It would change to 2000 rpm for a very very short time, then back to 1500 rpm. In a couple of seconds it would try that again. I would then take it off of cruise and that changing did not occur again unless I put it back in cruise, in some hills.....not mountains, but long grades coming out of NM into Texas.

I wish I knew more to describe this. But when the MOT people reset the shift, then drove 40 miles, they went thru some pretty good hills near Nacogdoches. I have yet to try it myself

I really need to pursue the torque converter question Dave Head raises. Do not know anything about it, but another thing I need to find out about, ask about.

Thanks too to Peter and Bret on the Mode On option. Another thing to try. I do seem, as I recall, to show only the top gear selected, not a gear when it downshifts - but will check that out.

Mike

Re: In cruise, rpm 500 jump

Reply #10 – October 01, 2012, 02:09:39 am

I have only seen one Allison shift selector that showed both gears (highest selected and actual gear the trans is in) and that was on a 2007 SOB coach.

Re: In cruise, rpm 500 jump

Reply #11 – October 01, 2012, 10:16:52 am

Quote from: George Hatfield – October 01, 2012, 02:09:39 am

I have only seen one Allison shift selector that showed both bears (highest selected and actual gear the trans is in) and that was on a 2007 SOB coach.

Our Allison shift selector shows both the highest selected and actual gear the transmission is in. We have a 1993 GV Special Edition.

Roalnd

Re: In cruise, rpm 500 jump

Reply #12 – October 01, 2012, 10:27:43 am

Quote from: George Hatfield – October 01, 2012, 02:09:39 am

I have only seen one Allison shift selector that showed both bears (highest selected and actual gear the trans is in) and that was on a 2007 SOB coach.

I just walked out to the coach to verify that I am not remembering something that doesn't really happen (again!

). Indeed our panel has a display at the top that has two indicators labeled "Select" and "Monitor." "Select" indicates the gear that has been selected by the operator. "Monitor" indicates the gear chosen by the transmission ECU. I turned on the key and they both indicated "N" for neutral.

Re: In cruise, rpm 500 jump

Reply #13 – October 01, 2012, 11:30:26 am

Well JD, I guess FT "improved" it for 2001. I only have one indicator panel.....wish I had both as you do. (I went to look, worried I just forgot what I see every time we drive it even for hours and hours!) Mike

Re: In cruise, rpm 500 jump

Reply #14 – October 01, 2012, 12:44:01 pm

Here is a picture to demonstrate the Allison display that includes the "Monitored" gear, and the selected gear.

Re: In cruise, rpm 500 jump

Reply #15 – October 01, 2012, 02:27:24 pm

Yes, Early 3000 and 4000 transmissions used the two window displays.

Then the bean counters changed it to the single window.

Customer complaints lead to a return to the two window display shift pads.

Brett

Re: In cruise, rpm 500 jump

Reply #16 – October 01, 2012, 03:52:39 pm

Hmm... that brings to mind two question... Are the two window displays plug and play replacements and if so, any idea what it would cost?
Don

Quote from: wolfe10 – October 01, 2012, 02:27:24 pm

Yes, Early 3000 and 4000 transmissions used the two window displays.

Then the bean counters changed it to the single window.

Customer complaints lead to a return to the two window display shift pads.

Brett

Re: In cruise, rpm 500 jump

Reply #17 – October 01, 2012, 05:38:40 pm

Quote from: acousticart – October 01, 2012, 03:52:39 pm

Hmm... that brings to mind two question... Are the two window displays plug and play replacements and if so, any idea what it would cost?
Don

Don,

I don't believe so, but you might call Allison with your transmission SN and ask.

Allison help line 800 252 5283

Re: In cruise, rpm 500 jump

Reply #18 – October 01, 2012, 06:31:10 pm

Peter and Brett....and others.....I drove the coach today to remind myself what I forgot or did not know. There is, as I said and had that right, only one display panel. However IF I downshift manually, it shows the gear that I tried to go to. I was in 6th at about 1100 rpm, told it to go to fifth. The display went from a "6" to a "5" though I did not see or feel it shift. The rpm stayed low. So I guess it only showed where I wanted it to go, not the actual what it was.

I imagine if I had been able to travel on a bit longer it would have gone ahead and shifted but I had to let up and turn off.

SO....I have one panel, it will show a change in gear number at least on manual downshift order.

Re: In cruise, rpm 500 jump

Reply #19 – October 01, 2012, 06:46:40 pm

On our 2003 U295 coach, which shows one number for the transmission gear, the number shown is the highest gear one has selected. It does not show the gear the transmission is actually using. So if you selected "5" and the trans is in "4," it will show "5."

Re: In cruise, rpm 500 jump

Reply #20 – October 01, 2012, 07:28:51 pm

As others have indicated, it sure would be nice to know the actual gear in use without having to purchase a VMSpc product.

Re: In cruise, rpm 500 jump

Reply #21 – October 01, 2012, 08:01:00 pm

Mark.... I agree completely. Why did that feature go away in newer coaches? How much was saved? Anyway, IMHO VMSpc is relatively inexpensive as motorhome stuff goes. Knowing the gear you are actually in and the load (also shown) can be very helpful in optimizing fuel consumption and minimizing engine temp while climbing steep grades. And when going down the other side, the retard temp is even more useful. I have watched the temp go close to 300 degrees even when I am trying to keep it down! God only knows what it goes to while "flying blind." We can't do without VMSpc. Once you have tried it, there is no going back for most of us.

Re: In cruise, rpm 500 jump

Reply #22 – October 01, 2012, 08:32:28 pm

Thanks George for confirming - it has been on my list but I'm not sure the best way to visually implement (tablet, netbook, backup monitor,dedicated screen). Anyhow that is off topic for this thread.

Re: In cruise, rpm 500 jump

Reply #23 – October 01, 2012, 08:48:43 pm

I also have a 98 u295 that has a seperat panel which is mounted directly in front of the driver mounted to the bottom of the tv cabinet that shows which gear is selected and also which gear it is actually in. I guess the PO had it installed when he bought it , it also has an hour meter for the aqua hot mounted there. I really like the vmspc that I got for the 03 as I really like to know which gear I am in.

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