AUTOINC. ONLINE - Knowing Chrysler RFE Transmission Helps Diagnose Problems

Knowing Chrysler RFE Transmission Helps Diagnose Problems

Posted 8/1/2008
By Arlen Mauland

The Chrysler RFE transmission control series is available in Dodge trucks and Jeeps equipped with the 3.7L or 4.7L engines. These five- and six-speed automatic transmissions are electronically controlled and share many of the same technologies as the Chrysler front-wheel drive overdrive transmission plus a few more clutches, solenoids and pressure switches. The transmission is controlled by the transmission control module (TCM), which is located within the powertrain control module (PCM) on some models. In this article, I will refer to it as the TCM even though it may be located in the PCM.

The TCM pulse-width modulates various hydraulic solenoids that apply a combination of clutches, each producing a different gear ratio. These clutches include the L/R clutch, the 2C clutch, the OD clutch, the 4C clutch and the UD clutch, each controlled by a solenoid. The solenoids are fed B+ from the transmission control relay (TCR). This voltage also goes to the pressure switches.

"These five- and six-speed automatic transmissions are electronically controlled and share many of the same technologies as the Chrysler front-wheel drive overdrive transmission plus a few more clutches, solenoids and pressure switches."

Each of the clutches has a pressure switch that is used by the TCM to monitor the clutch hydraulic pressure. When no hydraulic pressure is applied to the switch, it is open to ground. When the switch is open, current flows through a 300 ohm resistor, through the pressure switch sensor circuit to the TCM. The TCM sees B+ on the circuit and the status of the pressure switch is open. When hydraulic pressure is applied to the pressure switch, the TCM sees zero volts and the status of the pressure switch is closed.

The TCM is able to adapt to the characteristics of the transmission. Clutch volume index (CVI) is a number stored in memory that tells the TCM how much fluid is required to apply each clutch. As the clutch wears, the CVI number increases. As the CVI number increases, the solenoid for that clutch is pulse-width modulated at a faster rate to apply the clutch quicker. The CVI numbers are a very good indicator of the condition of the clutches, as long as the memory in the TCM has not been cleared. The normal range of CVI values is displayed on the scan tool.

The TCM updates the CVI under normal driving. Each clutch CVI is updated during a particular upshift or downshift. Quick-learn should be performed whenever the TCM or transmission is replaced. The scan tool can be used to perform quick-learn, moving the CVI numbers close to where they should be for the transmission so the vehicle can be driven without experiencing adverse shifting characteristics.

The TCM monitors the electrical circuits and the status of the pressure switches to set and store diagnostic codes. At key on, the TCM will perform a diagnostic routine on itself and if OK, will energize the TCR. With the TCR energized, the TCM looks for B+ on all the solenoid and pressure switch circuits. If any circuit is open or grounded, it turns off the TCR and stores a fault code. Solenoid codes are always electrical problems. Pressure switch codes can be electrical or hydraulic problems. If the transmission goes into limp-in at key on, the problem is an electrical circuit problem. However, the TCM continually performs diagnostic checks on the solenoid circuits at all times when the engine is running. Every eight seconds, any solenoid that is not currently energized is pulsed and the induced voltage kick is monitored. If the induced voltage spike is too weak, the TCM pulses it two more times. If the test fails three consecutive times, the TCR is de-energized and a fault code is stored for that solenoid.

The TCM also controls line pressure using a pulse-width modulated line pressure solenoid. A line pressure sensor is used to monitor the actual line pressure so that the TCM can adjust the pulse-width modulation to the line pressure solenoid accordingly.

The TCM monitors the input (or turbine) shaft speed (ISS) and output shaft speed (OSS) sensors. The TCM monitors the OSS sensor to calculate when a shift is needed. Other inputs are throttle position sensor (TPS), manifold air pressure (MAP), engine coolant temperature (ECT) and engine rpm. If the transmission is in gear and the ISS sensor indicates that the input shaft is turning and the OSS sensor indicates zero RPM, the TCM sets a gear ratio error code. This indicates clutch slippage or a hard part failure. The TCM is programmed to know the appropriate gear ratio for each combination of clutches that are applied.

The TCM knows the gear shift lever position from inputs received from the transmission range sensor (TRS). The TCM sends 12 volts out on each circuit to the TRS. The TRS grounds a different combination of these circuits for each gear shift lever position. The TCM also receives a start input. This circuit gets B+ from the ignition switch when in the start position. The purpose of this input is to shut down all TCM diagnostics when the engine is being cranked.

This overview of the RFE transmission controls generally applies to the Chrysler front-wheel-drive, electronically controlled transmission as well. This information should provide the foundation to help diagnose tough transmission problems.

Arlen Mauland is an IDENTIFIX DaimlerChrysler, Mitsubishi and Hyundai specialist. He is ASE master and L1 certified.

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