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Unread Yesterday, 01:33 PM   #1
perpster
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8th Gear: When, or Why Not? Parameters?

I notice that in automatic shift mode on interstates, my 2015 JGC Overland likes to cruise in 7th gear instead of getting into 8th gear. IIRC, on flat ground it would get into 8th, but on non-level (but not very steep) interstates (I-81 and I-380 in PA) it liked to stay in 7th while cruising (not passing, not accelerating, not using cruise control. No cargo, no passengers). Paddle shifting to 8th didn't seem to be too much for the engine (and improved the instant mpg by about 1 mph).

Does anyone know what the shift parameters are for automatic shifting into 8th gear? Is this normal behavior?

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Unread Yesterday, 01:55 PM   #2
lurtonk
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I don't think there is anything wrong.
Both 7 and 8 are overdrive.
I noticed on a recent trip that I got better fuel economy at 77mph than I did at 78mph and it might have been a gear shift.
I read somewhere that Chrysler has 40 different control maps for the 8-speed depending on vehicle, engine, final drive ratio, 4wd/2wd, QT1/QT2/QD, etc.
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Unread Yesterday, 01:56 PM   #3
hemispin
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Mine shifts into 8th at about 50 mph on level ground.
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Unread Yesterday, 02:01 PM   #4
perpster
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Quote:
Originally Posted by lurtonk View Post
I don't think there is anything wrong.
Both 7 and 8 are overdrive.
I noticed on a recent trip that I got better fuel economy at 77mph than I did at 78mph and it might have been a gear shift.
I read somewhere that Chrysler has 40 different control maps for the 8-speed depending on vehicle, engine, final drive ratio, 4wd/2wd, QT1/QT2/QD, etc.
Quote:
Originally Posted by hemispin View Post
Mine shifts into 8th at about 50 mph on level ground.
Interesting. I did not know 7th was also an overdrive gear.

I will check to see at what mph on level ground mine goes into 8th. 50 sounds about right. I was just very surprised to see it in 7th gear in the 70's on not-so-steep interstates.
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Unread Yesterday, 03:36 PM   #5
David1
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Depends on the load. Crusing at 80 into a headwind it will pick 7th. At 100 it is still in 7th unless I manually put it in 8th or no headwind and level ground. I have hit my top speed limiter in both 7th and 8th gear.
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Unread Yesterday, 04:12 PM   #6
perpster
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Quote:
Originally Posted by David1 View Post
I have hit my top speed limiter in both 7th and 8th gear.
"top speed limiter"?
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Unread Yesterday, 05:16 PM   #7
2014RedHemiGC
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Quote:
Originally Posted by perpster View Post
"top speed limiter"?
Yes, most vehicles have an electronically controlled speed limiter which is usually based on the tires' speed rating. I've never tested it in the Jeep.
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Unread Today, 12:42 AM   #8
dsyzdek
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From http://www.wk2jeeps.com/wk2_transm.htm

8HP45 Gear Ratios:

1ST 4.69:1
2ND 3.13:1
3RD 2.10:1
4TH 1.67:1
5TH 1.28:1
6TH 1.00:1
7TH 0.84:1
8TH 0.67:1
REVERSE 3.30:1
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Unread Today, 01:03 AM   #9
perpster
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Quote:
Originally Posted by dsyzdek View Post
From http://www.wk2jeeps.com/wk2_transm.htm

8HP45 Gear Ratios:

1ST 4.69:1
2ND 3.13:1
3RD 2.10:1
4TH 1.67:1
5TH 1.28:1
6TH 1.00:1
7TH 0.84:1
8TH 0.67:1
REVERSE 3.30:1
Thanks dsyzdek! Very interesting. FWIW, here's the rest of the link you posted:

Chrysler Group LLC is the first domestic automaker to feature the state-of-the-art ZF eight-speed automatic transmission in its vehicles. The new 8HP45 electronic eight-speed automatic transmission made its debut in V6 models of the 2012 Chrysler 300 and 2012 Dodge Charger. Next year (1st quarter 2013) the new eight-speed will be offered in several more Chrysler vehicles, including the 2013 Dodge Ram and all trim levels of the refreshed 2014 Jeep Grand Cherokee.

Chrysler Group LLC will initially receive the new 8-speed transmissions from the ZF Getriebe GmbH plant in Saarbrücken, Germany. In parallel with Chrysler Group, ZF is setting up a new transmission production plant in North America, where the 8-speed transmissions will be produced. Chrysler Group is licensed to produce the new 8HP's at the company's Indiana Transmission Plant I and the Kokomo casting plant, starting in 2013.

Compact and lightweight, the new transmission required no structural changes to either vehicle and despite the addition of more gears, the transmission actually weighs just 3kg more than the outgoing five speed automatic transmission. The addition of three more gear ratios in the proven ZF eight-speed automatic transmission design help to reduce the gaps normally associated with upshifting and downshifting. Passengers will experience world-class levels of refinement due to small, evenly spaced ratios that provide quicker acceleration and smoother shifts.

Fully electronic and aided with software that provides an adaptive shifting strategy, the 845RE has been designed to provide optimal shifting characteristics consistent with the Chrysler and Dodge brands. Adaptive shifting takes into account a number of variables including engine torque gradients, kick down, longitudinal and lateral acceleration, hill detection, friction detection and downshift detection in determining shift patterns. Additional parameters include vehicle speed control, electronic stability control activation and high temperatures that can affect transmission shifting.

High efforts in shifting also have been eliminated with gear selection controlled by a shift-by-wire system. The position selected by the driver (P, R, N, D) is transmitted electronically with no mechanical linkage from the shifter to the outside of the transmission. Elimination of this linkage removes any shift effort from the driver's gear selection. Shift-by-wire also allows improved calibration and smoothness of garage and parking lot shifts.

All shifts, whether single, double or multiple are performed as a direct result of the pedal position. Downshifts are made with very short response times and lead directly to the requested gear. Multiple upshifts can be decoupled temporarily if the torque request changes during the upshift. This helps eliminate the shift searching of the transmission.

Quicker shifts are possible as a result of a "Sport" shift mode that requires simple driver input. When the driver chooses to let the transmission shift in "Sport" mode, gear changes are quicker and revs are held higher for even more performance-oriented acceleration and higher shift dynamics. For even more control, the driver can also use a pair of die-cast steering-wheel mounted paddle controls and view gear selection through an all-new full-color EVIC centered in instrument cluster.

Additionally, lower viscosity transmission fluid is used resulting in improved transmission efficiency and fuel economy improvements especially during cold environments or vehicle start-up. The 845RE also is "fill for life" not requiring oil changes during normal driving conditions.

The 8HP45 Transmission Control Module Assembly (TCMA), which is integrated into the valve body, provides fully synchronized clutch-to-clutch shifting through four planetary gear sets. The TCMA includes a mounting plate that holds the Transmission Control Module (TCM) and a molded wiring harness for connection to various transmission sensors and solenoids. The valve body assembly contains all the sensors and solenoids required for operation, completely inside the transmission. Eight speeds allow the engine to maintain its optimal rpm range, increasing fuel economy and performance.

Transmission control is performed by the TCM based on hard-wired and CAN bus signals from sensors and modules. The TCM receives driveability data from the Powertrain Control Module (PCM) and other modules over the CAN-C bus. It also receives shift lever position information from the electronic shifter over a dedicated transmission CAN bus. The TCM processes this input data and controls operation of the torque converter clutch, park lock system, solenoid valves, and pressure regulating valve.

The input and output speed sensors are Hall-effect sensors that measure shaft rotational speed. The input speed sensor is located at the top, near the center, of the of the TCMA and reads input shaft speed from the magnetic ring on the P2 carrier. The output speed sensor is located at the back of the TCMA and reads output shaft speed from the P4 carrier.
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Unread Today, 07:55 AM   #10
GreenGreySS
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Quote:
Originally Posted by David1 View Post
Depends on the load. Crusing at 80 into a headwind it will pick 7th. At 100 it is still in 7th unless I manually put it in 8th or no headwind and level ground. I have hit my top speed limiter in both 7th and 8th gear.
Now I'm curious. What was your top speed?
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Unread Today, 08:27 AM   #11
David1
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Quote:
Originally Posted by GreenGreySS View Post
Now I'm curious. What was your top speed?
119mph even going down hill. The vehical will actually apply the brakes so you can't even coast faster
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Unread Today, 08:40 AM   #12
lurtonk
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2015 JGC 3.6v6, Eco on, D mode, full auto, temperature around 50.
I watched my gearing this morning driving to work.
The slowest speed I saw 7th was about 48mph but then it would immediately shift to 8th.
Driving 65-75 it would be in 8th most of the time.
Small hills would cause a downshift to 7 but then back to 8 pretty soon.
Gentle acceleration would cause a downshift to 6 but when the speed came back up, back to 8.
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Unread Today, 08:46 AM   #13
lurtonk
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My Corvettes all had speed limiters at 190+ but I never got close to that (165).

I did have a Mazdaspeed Miata with 4.10 gears that would hit the rev limiter at around 136 in 6th gear, which was problematic during track days.
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Unread Today, 09:13 AM   #14
JMono
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Quote:
Originally Posted by perpster View Post
Thanks dsyzdek! Very interesting. FWIW, here's the rest of the link you posted:

Chrysler Group LLC is the first domestic automaker to feature the state-of-the-art ZF eight-speed automatic transmission in its vehicles. The new 8HP45 electronic eight-speed automatic transmission made its debut in V6 models of the 2012 Chrysler 300 and 2012 Dodge Charger. Next year (1st quarter 2013) the new eight-speed will be offered in several more Chrysler vehicles, including the 2013 Dodge Ram and all trim levels of the refreshed 2014 Jeep Grand Cherokee.

Chrysler Group LLC will initially receive the new 8-speed transmissions from the ZF Getriebe GmbH plant in Saarbrücken, Germany. In parallel with Chrysler Group, ZF is setting up a new transmission production plant in North America, where the 8-speed transmissions will be produced. Chrysler Group is licensed to produce the new 8HP's at the company's Indiana Transmission Plant I and the Kokomo casting plant, starting in 2013.

Compact and lightweight, the new transmission required no structural changes to either vehicle and despite the addition of more gears, the transmission actually weighs just 3kg more than the outgoing five speed automatic transmission. The addition of three more gear ratios in the proven ZF eight-speed automatic transmission design help to reduce the gaps normally associated with upshifting and downshifting. Passengers will experience world-class levels of refinement due to small, evenly spaced ratios that provide quicker acceleration and smoother shifts.

Fully electronic and aided with software that provides an adaptive shifting strategy, the 845RE has been designed to provide optimal shifting characteristics consistent with the Chrysler and Dodge brands. Adaptive shifting takes into account a number of variables including engine torque gradients, kick down, longitudinal and lateral acceleration, hill detection, friction detection and downshift detection in determining shift patterns. Additional parameters include vehicle speed control, electronic stability control activation and high temperatures that can affect transmission shifting.

High efforts in shifting also have been eliminated with gear selection controlled by a shift-by-wire system. The position selected by the driver (P, R, N, D) is transmitted electronically with no mechanical linkage from the shifter to the outside of the transmission. Elimination of this linkage removes any shift effort from the driver's gear selection. Shift-by-wire also allows improved calibration and smoothness of garage and parking lot shifts.

All shifts, whether single, double or multiple are performed as a direct result of the pedal position. Downshifts are made with very short response times and lead directly to the requested gear. Multiple upshifts can be decoupled temporarily if the torque request changes during the upshift. This helps eliminate the shift searching of the transmission.

Quicker shifts are possible as a result of a "Sport" shift mode that requires simple driver input. When the driver chooses to let the transmission shift in "Sport" mode, gear changes are quicker and revs are held higher for even more performance-oriented acceleration and higher shift dynamics. For even more control, the driver can also use a pair of die-cast steering-wheel mounted paddle controls and view gear selection through an all-new full-color EVIC centered in instrument cluster.

Additionally, lower viscosity transmission fluid is used resulting in improved transmission efficiency and fuel economy improvements especially during cold environments or vehicle start-up. The 845RE also is "fill for life" not requiring oil changes during normal driving conditions.

The 8HP45 Transmission Control Module Assembly (TCMA), which is integrated into the valve body, provides fully synchronized clutch-to-clutch shifting through four planetary gear sets. The TCMA includes a mounting plate that holds the Transmission Control Module (TCM) and a molded wiring harness for connection to various transmission sensors and solenoids. The valve body assembly contains all the sensors and solenoids required for operation, completely inside the transmission. Eight speeds allow the engine to maintain its optimal rpm range, increasing fuel economy and performance.

Transmission control is performed by the TCM based on hard-wired and CAN bus signals from sensors and modules. The TCM receives driveability data from the Powertrain Control Module (PCM) and other modules over the CAN-C bus. It also receives shift lever position information from the electronic shifter over a dedicated transmission CAN bus. The TCM processes this input data and controls operation of the torque converter clutch, park lock system, solenoid valves, and pressure regulating valve.

The input and output speed sensors are Hall-effect sensors that measure shaft rotational speed. The input speed sensor is located at the top, near the center, of the of the TCMA and reads input shaft speed from the magnetic ring on the P2 carrier. The output speed sensor is located at the back of the TCMA and reads output shaft speed from the P4 carrier.
I think just the V6 transmissions are produced in the US.
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Unread Today, 09:38 AM   #15
perpster
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Quote:
Originally Posted by 2014RedHemiGC View Post
Yes, most vehicles have an electronically controlled speed limiter which is usually based on the tires' speed rating. I've never tested it in the Jeep.
News to me, Governor!
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