How to Diagnose a Ford Expedition Transmission Problem

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If your 2000 Ford Expedition Eddie Bauer edition slips when pulling away from a stop, chatters when you put it in reverse, or dies when you pull up to a stop light or sign, you may have a problem with your automatic transmission. Most manuals like Chilton won't help you with a diagnosis. Instead, you'll need to check out the Motors Manual Transmission book from the library for in-depth information.

Electronic vs. Mechanical

If you have recently flushed your transmission and replaced it with a clean filter and fluid, and if the fuses and relays under the dash and in the engine compartment test out okay, it's likely that your transmission problem is electronic and not mechanical.

Electronic transmissions are controlled by a computer, otherwise known as the powertrain control module (PCM). Troubleshooting the PCM is a complex job that requires some special test equipment that the average do-it-yourselfer does not have—nor is cost effective for them to purchase. However, it is possible to figure out what is wrong, so that you have a good idea of what to tell your mechanic when you bring your Expedition in for service. 

Electronic System Description

The Powertrain Control Module (PCM) and its input/output network control the following transmission operations:

  • Shift timing
  • Line pressure (shift feel)
  • Torque converter clutch operation. The transmission control is separate from the engine control strategy in the powertrain control module, although some of the input signals are shared. When determining the best-operating strategy for transmission operation, the powertrain control module uses input information from certain engine-related and driver-demand related sensors and switches.

Using all of these inputs signals, the powertrain control module can determine when the time and conditions are right for a shift, or when to apply or release the torque converter clutch. It will also determine the best line pressure needed to optimize shift feel. To accomplish this, the powertrain control module uses six output solenoids to control transmission operation.

Bad Solenoid

When your Ford Expedition stalls at a stop, just as it would if it had a manual transmission and you neglected to push in the clutch, that's a sign that one of the solenoids is malfunctioning or has gone out. Usually, the culprit is a bad torque converter control (TCC) solenoid. 

Here is brief description of each of the sensors and actuators used by the PCM for transmission operation:

  • The mass air flow (MAF) sensor measures the mass of air flowing into the engine. The MAF sensor output signal is used by the powertrain control module to calculate injector pulse width. For transmission strategies, the MAF sensor is used to regulate Electronic Pressure Control (EPC), shift and torque converter clutch scheduling.
  • The throttle position (TP) sensor is a potentiometer mounted on the throttle body. The TP sensor detects the position of the throttle plate and sends this information to the powertrain control module. The TP sensor is used for shift scheduling, electronic pressure control, and the TCC.
  • The IAT sensor is installed in the air cleaner outlet tube. The IAT sensor is also used in determining Electronic Pressure Control (EPC) pressures.
  • The operation of the transmission is controlled by the powertrain control module. Many input sensors provide information to the powertrain control module. The powertrain control module then controls actuators which determine transmission operation.
  • The transmission control switch (TCS) is a momentary contact switch. When the switch is pressed, a signal is sent to the powertrain control module to allow automatic shifts from first through fourth gears or first through third gears only. The powertrain control module energizes the transmission control indicator lamp (TCIL) when the switch is off. The TCIL indicates overdrive cancel mode activated (lamp on) and electronic pressure control (EPC) circuit shorted (lamp flashing) or monitored sensor failure.
  • The programmable speedometer/odometer module (PSOM) receives input from the rear brake anti-lock sensor. After processing the signal, the PSOM relays it to the PCM and the speed control module.
  • The turbine shaft speed (TSS) sensors a magnetic pickup that sends the PCM information on the rotation speed of the coast clutch cylinder assembly The TSS sensor is mounted externally on the top of the transmission case. The PCM uses TSS sensor signals to help determine EPC pressure, shift scheduling the torque converter clutch (TCC) operation.
  • The output shaft speed (OSS) sensor is a magnetic pickup that provides transmission output shaft rotation speed information to the powertrain control module. The OSS sensor is mounted externally on the top of the transmission extension housing. The YCM uses the OSS sensor signal to help determine EPC pressure, shift scheduling and torque converter clutch (TCC) operation.
  • The powertrain control module controls the transmission operation through three on/off shift solenoids, one pulse width modulated (PWM) shift solenoid, and one variable force shift solenoid. These solenoids and transmission fluid temperature sensor are housed in the transmission solenoid body assembly. All are part of the transmission solenoid body and are not replaced individually.
  • The transmission fluid temperature (TFT) sensor is located on the solenoid body assembly in the transmission sump. It is a temperature-sensitive device called a thermistor. The resistance value of the transmission liquid temperature sensor will vary with temperatures change. The powertrain control module monitors the voltage across the transmission fluid temperature sensor to determine the temperature of the transmission fluid. The powertrain control module uses this signal to determine whether a cold start shift schedule is necessary. The cold start shift schedule lowers shift speeds to allow for better cold engine operation. The powertrain control module also uses the transmission fluid temperature sensor input to adjust electronic pressure control pressure for temperature effects and to inhibit torque converter clutch operation during the warm-up period.
  • The coast clutch solenoid provides coast clutch control by shifting the coast clutch shift valve. The solenoid is activated by pressing the transmission control switch or by selecting the 1 or 2 range with the transmission range selector lever. In MANUAL 1 and 2, the coast clutch is controlled by the solenoid and also hydraulically as a fail-safe to ensure engine braking. In reverse, the coast clutch is controlled hydraulically, and the solenoid is not on.
  • The torque converter clutch (TCC) solenoid provides torque converter clutch control by shifting the converter clutch control valve to apply or release the torque converter clutch.
  • CAUTION: The electronic pressure control (EPC) solenoid pressure output from the variable force solenoid is NOT adjustable. Any modification to the electronic pressure control solenoid may void the transmission warranty. The EPC is a variable force solenoid. The variable-force type solenoid is an electro-hydraulic actuator combining a solenoid and a regulating valve. It supplies electronic pressure control that regulates transmission line pressure and line modulator pressure. This is done by producing resisting forces to the main regulator and the line modulator circuits. These two pressures control clutch application pressures.
  • Shift solenoids SSA and SSB provide gear selection of first through fourth gears by controlling the pressure to the three shift valves.
  • The digital transmission range sensor is located on the outside of the transmission at the manual lever. The sensor completes the start circuit in Park and Neutral, the backup lamp circuit in Reverse and a neutral sense circuit for GEM control of 4 x 4 low engagement. The sensor also opens/closes a set of four switches that are monitored by the PCM to determine the position of the manual lever (P, R, N, (D), 2, 1).
  • The 4x4 low (4x4L) range switch is located on the transfer case cover. It provides an indication of when the 4x4 transfer case gear system is in the low range. The powertrain control module then modifies shift schedule for 4x4L operation.
  • The brake pedal position switch (BPP) tells the powertrain control module when the brakes are applied. The torque converter clutch disengages when the brakes are applied. The BPP switch closes when the brakes are applied and opens when they are released.
  • The electronic ignition consists of a crankshaft position sensor, two four tower ignition coils, and the powertrain control module. The ignition control module operates by sending crankshaft position information from the crankshaft position sensor to the ignition control module. The ignition control module generates a profile ignition pickup (PIP) signal (engine rpm) and sends it to the PCM. The PIP is one of the inputs that the PCM uses to determine transmission strategy, Wide-Open Throttle (WOT) shift control, torque converter clutch control, and EPC pressure.
  • The distributor ignition (DI) system sensor sends a signal to the powertrain control module indicating the engine rpm and the crankshaft position.
  • An electromagnetic clutch is energized when the clutch cycling pressure switch closes. The switch is located on the suction accumulator/drier. The closing of the switch completes the circuit to the clutch and draws it into engagement with the compressor drive shaft. When the A/C clutch is engaged, the EPC is adjusted by the PCM to compensate for additional load on the engine.
  • The manifold absolute pressure (MAP) sensor senses atmospheric pressure to produce an electrical signal. The frequency of this signal varies with intake manifold pressure. The powertrain control module monitors this signal to determine altitude. The powertrain control module then adjusts the 4R100 shift schedule and EPC pressure for altitude. On diesel engines, the manifold absolute pressure sensor measures boost pressure. The powertrain control module monitors this signal and adjusts EPC pressure.