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42RE Shifting SolutionsPosted 7/5/1998By Greg Montero
Jeep Grand Cherokee started using the 42RE transmission in 1993 1/2 models with the 4.0L engine. The "4" stands for 4-speed, "2" is for torque capacity, "R" is for rear-wheel drive, and "E" is for electronically controlled. This transmission is electronically controlled using a governor pressure solenoid to vary the governor pressure. The shifts are controlled by the valve body by conventional shift valves for 1-2 and 2-3 upshifts. The 3-4 upshift and 4-3 downshifts are controlled by a solenoid. Although the shifts are controlled by shift valves for the lower gears, the higher gear is electronically controlled because the governor pressure acting against it is controlled by the transmission control module (TCM). The throttle pressure is controlled by a standard cable and throttle valve. The TCM inputs for this system include engine rpm, throttle position sensor, vehicle speed sensor, transmission output speed sensor, governor pressure sensor, transmission fluid temperature sensor and overdrive "off" switch. The TCM outputs are the 3-4 shift solenoid, the governor pressure solenoid and the torque converter clutch solenoid. Here's how this system operates: As the output shaft begins to move, a 2-wire AC generator (transmission output speed sensor) begins to signal the TCM of the rpm. As the shaft speed increases, the TCM controls the governor pressure solenoid accordingly by a pulse-width modulated signal. Feedback to the TCM is provided by a governor pressure sensor to verify that the pressure actually changes. The pressure sensor is a 5-volt reference sensor. When the wheels are not rotating, voltage around 0.6 V is normal. As the wheels begin to speed up, the voltage should increase proportionately. If the voltage does increase, then the pressure is increasing as it should - this should be verified with a pressure gauge. This variance of pressure acts upon the shift valves in the valve body along with the throttle pressure to provide the shifts. As speed warrants, the TCM will apply the 3-4 overdrive solenoid, which is the only shift solenoid used on this system. The OD inputs are overdrive off switch, TPS, VSS, transmission temp sensor, output speed sensor and engine rpm. The torque converter clutch (TCC) solenoid will normally operate in overdrive, but the TCM can operate the TCC in third gear if the overdrive off switch is selected. Inputs for TCC include OD off switch, TPS, engine rpm, VSS, output speed and transmission temperature sensor. The transmission temperature sensor is mounted on the overdrive/torque converter clutch solenoid assembly and provides input to the TCM. When the temperature sensor reports 30 degrees F or colder, there will be no overdrive, and below 50 degrees F, there will be no torque converter clutch. If the transmission fluid temperature sensor reports an overheating condition (over 260 degrees F) and the transmission is currently in overdrive, the TCM will downshift to third gear. The TCM will also light the LED in the dash-mounted OD off switch until the temperature drops below 230 degrees F. However, the torque converter clutch may still be operational. This system has self-diagnostics. Fault codes can be read with a scan tool or by cycling the ignition key three times, then counting the flashes of the LED in the OD off switch. The diagnostic connector is located inside the vehicle to the left of the center console. The default mode of the TCM is third gear. Manual shifting of reverse and second gear is possible when in default mode. Default mode happens after a fault code is set. I hope this helps if you get one of these Jeeps in your shop!
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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