As promised in last month’s Powertrain Pro article, we will take a brief look at the CVT-7 transmission in this month’s article (figure 1). If you recall, this little CVT can be referred to as the JF015E in Dodge, the RE0F11A in Nissan and the FICJB in Mitsubishi applications. As you can see in figure 1, torque input is delivered to the transmission through a conventional torque converter.
There are 3 very interesting aspects to this CVT transmission. The first being a full complete apply of the converter clutch taking place at approximately 11 mph (10 km/h).
The second being the use of a low brake, high clutch and reverse brake located between the pulley assembly and the differential (figure 2). Similar to the way Honda places their Start Clutch as discussed in July’s Powertrain Pro article.
The third is the elimination of a stepper motor and ratio control valve to ratio the pulleys. This stepper motor and ratio control valve have been replaced with the use of Line Pressure Solenoid (A) and Primary Pressure Solenoid (B) as identified in figure 3.
It is a nicely compact little CVT and very pleasant to work on. If CVT transmissions have not been a transmission you would ever consider working on, this one might change your mind. It all comes apart and goes back together again very easily, including the Primary and Secondary Pulleys.
Having a low brake, high clutch and reverse brake located between the pulley assembly and the differential similar to the way Honda places their Start Clutch, allows for a variety of strategies and features. Smaller pulleys while achieving a 7.3 total ratio spread would be one advantage to this type of design. The placements of the pulleys are closer to being horizontally mounted rather than being closer to a vertical mount with the secondary pulley higher than the primary pulley. This provides a compact little CVT.
The location of the clutch and brake assemblies is referred to as the “Auxiliary Gearbox” section of the transmission which a ravigneaux planetary gear is a part of. The carrier is driven by the secondary pulley assembly through the carrier’s short shaft that spline into the pulley assembly. The High Clutch Drum contains the ring gear which locks to the carrier when the High Clutch is applied. The Reverse Brake spline to the High Clutch Drum and is used to hold the ring gear stationary. There is a sun gear and shell which the Low Brake is spline to holding the sun gear stationary when applied.
In addition to providing both forward and reverse direction of the vehicle, it also provides Idle Neutral Control, coordinated and non-coordinated shifting.
Nissan describes the Idle Neutral Control as follows:
If a driver has no intention of starting (moving) the vehicle in D position, TCM operates the low brake solenoid valve and controls the oil pressure of the low brake to be low pressure. Therefore, the low brake is in the release (slip) status and the power transmission route of transaxle is the same status as the N position. In this way, the transaxle is in idling status and load to the engine can be reduced to improve fuel economy.
Idle neutral control is started when all of the following conditions are fulfilled. However, during idle neutral control, idle neutral control is stopped when any of the following conditions is not met.
Driving environment: Flat road or road with mild grade
Selector lever position: *D* position
Vehicle speed: 0 km/h (0 MPH)
Accelerator pedal position: 0.0/8
Brake pedal: Depressed
Engine speed: Idle
Turn signal lamp/hazard signal lamp: Not activated
Idle neutral control stops or is prohibited when the TCM and ECM detect that the vehicle is in one of the following conditions.
- Engine coolant temperature and CVT fluid temperature are the specified temperature or
more, or the specified temperature or less.
- When a transaxle malfunction occurs.
- When the vehicle detects DTC and is in the fail-safe mode.
- When idle speed increases due to heavy electric load*.
*: When any one of rear window defogger switch, A/C switch, headlamp, fog lamp is
turned ON or when the steering wheel is operated.
Idle Neutral Control Resume Condition
When the idle neutral control finishes, if the vehicle is driven at more than the specified speed and the idle neutral control start conditions are satisfied, the idle neutral control starts again. If the vehicle has a malfunction, the idle neutral control does not start.
System Shift Control
The ravigneaux planetary assembly provides a 1.714 reverse gear ratio but it also provides a low gear 1.821 ratio and a high gear 1.000 ratio. In other words, the pulley ratio from a low ratio wrap to a higher ratio wrap can be driven through the ravigneaux planetary gear set while in reduction when the low brake is applied. An actual shift can be made from the low brake to the high clutch taking the ravigneaux gear set out of reduction into a direct drive rotation.
The lowest overall ratio when the low brake is applied and the pulleys are in the lowest ratio wrap is 4.0 to 1. Likewise, the highest overall ratio when the high clutch is applied and the pulleys are in the highest ratio wrap is 0.55 to 1. When 0.55 is divided into 4.0, you discover that this CVT has a 7.3 (7.273) ratio spread for improved efficiency.
System shift control is to provide smooth acceleration free from shift shock. Engine speed and torque is suppressed while the auxiliary transmission shifts (when the low brake is released and the high clutch is applied).
The auxiliary transmission shifts from first to second (the low brake to the high clutch) near the highest shift ratio of the pulley-belt system so as to minimize torque fluctuation when the transmission shifts.
System shift control will also allow the auxiliary transmission to shift to second while the pulley-belt system is on the low ratio side. This makes possible more frequent use of second gear to provide good torque transfer during light throttle conditions.
The system shift control strategy requires various system inputs to properly control the auxiliary transmission. One important input is a G sensor. Nissan has used this sensor in other vehicles for years in conjunction with their AWD systems. The G-Sensor is a lateral accelerator sensor converting front and rear accelerations (g-force) and inclination applied to the vehicle to voltage signal. The TCM evaluates front and rear G and inclination angle of the vehicle from the voltage signal. The TCM then stores calibration data (inherent characteristic values) of the G sensor to provide accurate transmission shifting control. Therefore, it is required to perform calibration of the G sensor after removal, installation or replacement of the G sensor or the replacement of the TCM.
With the simplicity of using only seven valves and five solenoids to control the pulleys, brake and clutch apply as well as converter clutch apply, it is without question, clever engineering.
One strategy that Honda uses with their start clutch is to release it during rapid breaking from high speed to low speed. This allows for the pulleys to remain in rotation so they can quickly adjust for a lower ratio take off. Although nothing I have read indicates the CVT-7 will do the same, it has the capability to employ such a strategy.
With most other CVT transmissions having the planetary system between the input shaft and primary pulley, the stationary half of the secondary pulley sits inside the pulley cover. This allows a tool to be mounted on the secondary moveable pulley half to open it and release the tension of the belt between the two pulleys for service.
Due to the design of this CVT-7 transmission, the stationary half of the secondary pulley faces the ravigneaux planetary gear so it can drive the carrier. This means the moveable half of the secondary pulley sits inside the pulley cover. Both the primary and secondary pulley assemblies are bolted into the cover. There is quite a bit of tension on the belt with no area to mount a tool to release this pressure. Using a vacuum pump does not open these pulleys either.
To remove the pulleys, place tie straps on the belt to prevent separation of the bands and elements that make up the belt. This can be a real mess if it all separates resulting in the need to but a new push belt. Once the tie straps are on, simply unbolt the primary and secondary pulley retaining bolts from the cover and remove the assembly as carefully as possible. What Yoram Levy from Edmond Garage in Israel does to re-assemble the pulleys, he uses a tool to open the secondary pulley allowing him to assemble the belt and pulleys into the main case, then release the tool and install the cover onto the pulleys, very easy and simple to do (figure 5).
When removing the pulleys from the cover (figure 6), a self inflicted injury can easily occur if you are not careful. There is a flimsy exciter ring next to the cover side support bearing on the primary pulley assembly (figure 7). It does not take very much to bend the exciter ring tabs when handling the pulley. If this goes un-noticed during assembly, the compromised air gap for the input speed sensor will certainly cause and erratic input speed signal. This is trouble you do not need. Once the pulleys are mounted into the case, before you put the cover on, carefully re-align any of the exciter ring tabs that have been bent (figure 8).
Speaking about having some trouble, there are several protection control strategies to be aware of.
Should wheel spin be detected, the engine output and gear ratio become limited and line pressure is increased. Low brake pressure is also increased.
When the CVT fluid temperature is high, the gear shift permission maximum revolution and the maximum torque are reduced than usual to prevent increase of the oil temperature. This means power performance may be lowered compared to normal control.
Engine output is controlled according to vehicle speed while reversing the vehicle lowering power performance. Torque returns to normal by placing the selector lever in a range other than reverse.
If the selector lever is placed into reverse while moving forward, reverse will be prohibited.
