Foretravel Motorhome Forums => Foretravel Tech Talk => Topic started by: Tim on December 15, 2017, 10:59:09 pm
Title: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Tim on December 15, 2017, 10:59:09 pm
The transmission or drive train sometimes thunks / bangs / lurches when downshifting from 3rd to 2nd gear when coasting to a stop. Other times, it is pretty smooth when coasting to a stop from 6th gear to 1st gear. The gas pedal and brakes are not engaged. Anyone know if this is normal and/or what the problem / solution is.
It accelerates and shifts gears smoothly.
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Old phart phred on December 15, 2017, 11:11:57 pm
When is last time the U-joints were greased? Do you have a grease gun to grease them?
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Tim on December 15, 2017, 11:17:29 pm
2200 miles ago, with red heavy-duty grease.
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: jcus on December 15, 2017, 11:25:46 pm
The transmission or drive train sometimes thunks / bangs / lurches when downshifting from 3rd to 2nd gear when coasting to a stop. Other times, it is pretty smooth when coasting to a stop from 6th gear to 1st gear. The gas pedal and brakes are not engaged. Anyone know if this is normal and/or what the problem / solution is.
It accelerates and shifts gears smoothly.
Have had two 3000 series and two 4000 series transmissions do the same thing. Was worried on the first one, but now think maybe ecm is just confused. As it only happened now and then, and no codes thrown, I did not worry too much, because it was not constant, I ignored it. So far so good.
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Barry & Cindy on December 15, 2017, 11:49:57 pm
Allison ECM 'learns' driver throttle style of driving to control shift points. Sounds like you would benefit from an ECM reset so it can start over learning how to shift:
"Re-Learn is required when replacing the transmission, working on the transmission, installing a Shift Kit or adding horsepower.
The computer that controls the Allison transmission has adaptive strategy that constantly adjusts shift clutch pressures to match engine torque and vehicle load. It stores a database and records the relationships between rate of acceleration and throttle opening. It also calculates engine torque output based on inputs such as fuel consumption, boost pressure, air density, temperature, throttle position and many other factors.
To insure an ideal shift with maximum speed and minimal strain on the drive train the computer must 're-learn' when replacing the transmission, working on the transmission, installing a Shift Kit or adding horsepower.
Start the re-learn by making at least six sets of light throttle upshifts through all the gears, then make another six sets at 1/3, 1/2 and 3/4 throttle. Downshift the same way by starting at light throttle and working up to full throttle.
When the shifts are quick and smooth press the 'tow haul; button and repeat the previous re-learn process in 'tow haul' mode. Re-learn normally takes up to two hours (this time can be significantly reduced by using Tech2 or PC based Allison software to clear the memory and place the TCM in the 'fast learn' mode).
During the relearn there will probably be some clunks, bumps and/or short flares, especially during the 3-4 shift. This is normal."
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: jcus on December 15, 2017, 11:54:22 pm
Allison ECM 'learns' driver throttle style of driving to control shift points. Sounds like you would benefit from an ECM reset so it can start over learning how to shift:
"Re-Learn is required when replacing the transmission, working on the transmission, installing a Shift Kit or adding horsepower.
The computer that controls the Allison transmission has adaptive strategy that constantly adjusts shift clutch pressures to match engine torque and vehicle load. It stores a database and records the relationships between rate of acceleration and throttle opening. It also calculates engine torque output based on inputs such as fuel consumption, boost pressure, air density, temperature, throttle position and many other factors.
To insure an ideal shift with maximum speed and minimal strain on the drive train the computer must 're-learn' when replacing the transmission, working on the transmission, installing a Shift Kit or adding horsepower.
Start the re-learn by making at least six sets of light throttle upshifts through all the gears, then make another six sets at 1/3, 1/2 and 3/4 throttle. Downshift the same way by starting at light throttle and working up to full throttle.
When the shifts are quick and smooth press the 'tow haul; button and repeat the previous re-learn process in 'tow haul' mode. Re-learn normally takes up to two hours (this time can be significantly reduced by using Tech2 or PC based Allison software to clear the memory and place the TCM in the 'fast learn' mode).
During the relearn there will probably be some clunks, bumps and/or short flares, especially during the 3-4 shift. This is normal."
Where is the tow-haul button? My duramax had that, but not my Foretravel.
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Jerry Whiteaker on December 16, 2017, 12:08:01 am
Yes, my coach does that too sometimes. Do not know anything about a tow-haul button. Always thought the memory was lost when the chassis batteries were disconnected, but maybe I am wrong. Somebody educate me.
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: craneman on December 16, 2017, 12:25:19 am
Maybe mode button?
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: amos.harrison on December 16, 2017, 08:55:36 am
It's normal. I just live with it.
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Chuck & Jeannie on December 16, 2017, 09:09:33 am
Agree with above - happens sometimes, but no worries. I notice it seems to occur when I am confused about whether I want to go or stop, like in busy rapidly-changing traffic situations. When my throttle input becomes jerky, the transmission can get caught off-guard, which results in a harsh downshift. Another good reason to try to drive smoothly...
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: bbeane on December 16, 2017, 09:11:06 am
Mine does it, the ones we have in the trucks where I used to work do it.
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Old Hippie on December 16, 2017, 09:21:17 am
I was told it was the torque converter working on the down shift. Mine sometimes does it in stop and go traffic.
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Jim Frerichs on December 16, 2017, 09:53:06 am
I'm probably stirring the pot here... but what happens in the poor, old ECU when our spouse drives the coach?? Mileage probably gets better, huh? Tee Hee. Jim
2002 U320 42'
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Pierce & Gaylie Stewart on December 16, 2017, 11:24:37 am
Ours, like some above have posted, occasionally jerks/clunks. Have driven FD trucks equipped with Allisons that did the same thing.
Pierce
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: wolfe10 on December 16, 2017, 12:54:47 pm
Normal.
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Pamela & Mike on December 16, 2017, 01:21:50 pm
Not be argumentative against the majority but this problem ( thunks / bangs / lurches when downshifting from 3rd to 2nd from OP) does have some trouble shooting and some fixes for the 3000 & 4000 series. The thing is this problem may not drop a flag in the memory of the ECM. This is a starting place to keep you from just throwing parts at it.
The #1 cause is "Speed Sensor" signal quality caused by wiring issues. Like bad connections, chaffed wires, etc. Loose output nut on 4000 series Damaged tone ring on 3000 series (you may just want to live with this as it is a high $$$ fix) even a slight nick or ding can cause a spike in the signal. If it passes this spot at the exact right timing then this will cause a spike in the signal. Speed sensors themselves failing. Best way to check is watch the signals in a graph form, they should be smooth up & down. If spiking and erratic then they are the issue.
Pamela & Mike
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: gracerace on December 16, 2017, 05:51:08 pm
Normal. Mine does it, has no codes.Trans gets caught between gears with it's pants down!
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Chuck & Jeannie on December 16, 2017, 09:11:41 pm
Inside the Allison Brain: "OK, time to downshift to 2nd...........OH LOOK, A CHICKEN!"
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Tim on December 18, 2017, 01:41:47 pm
When an automatic transmission shifts, are the little clutches gradually engaged or are they suddenly engaged. Like a manual transmission, rapid clutch engagment causes banging and complaints from my passengers.
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Mike Leary (RIP) on December 18, 2017, 02:26:24 pm
My 3060 Allison took some getting used to, having a "mind of it's own", but works fine in town. ^.^d
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Balcanthez on December 18, 2017, 04:57:36 pm
Mine does the "clunk" going back into 1st when cold. It goes away after it's warmed up, but really surprises you at the first stop. I found it reduces when I manually shift down until it's at operating temp.
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Bill Willett on December 18, 2017, 05:05:24 pm
Take the coach to an Allison dealer and have them hookup there computer, have it checked for wide open throttle and closed throttle, they can also check to see if someone has reprogrammed it.
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Hammer2100 on December 18, 2017, 07:22:33 pm
Throttle position sensor may be sticking, out of adjustment, or just not telling the transmission you are at idle. I relocated my cable and made a world of difference.
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Hans&Marjet on December 18, 2017, 07:39:59 pm
Throttle position sensor may be sticking, out of adjustment, or just not telling the transmission you are at idle. I relocated my cable and made a world of difference.
Relocated the cable ..where/how...??pics please . :) :) :)
Title: Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Post by: Chuck Pearson on December 18, 2017, 08:13:41 pm
Noticed some abrupt downshifts in city traffic. Try turning off the retarder, worked for me.