Transmission Controls With PCM

Diagnostic Chart:

Circuit Schematic:

4TH GEAR STUCK ON
When a gear is commanded on, the vehicle speed to turbine speed ratio is monitored to assure the correct gear ratio. 4th gear stuck on indicates that another gear was commanded but the actual ratio driving the vehicle is equal to 4th gear. This is a calculated value (turbine speed/vehicle speed = gear ratio). 4th gear clutch is turned off by grounding circuit 898 at the PCM.

DTC PARAMETERS
DTC 78 is set when the PCM has detected the following conditions for more than three seconds, twice per ignition cycle.
- 1st gear commanded by PCM.
- Engine under load and load value is greater than 200.
- Speed ratio across the torque converter, engine speed versus turbine speed, less than 180. A speed ratio of 2S5 would be TCC on.
- No noise on turbine or vehicle speed signals.
- Vehicle speed greater than 5 km/h (3 mph).
- Turbine speed greater than 224 rpm.
- Turbine speed to vehicle speed ratio equals 4th gear for more than 200 milliseconds.

TEST DESCRIPTION
Numbered steps below refer to numbered steps on chart.
1. Check for information flags 76, 77, or 99. They must be eliminated first as a possible cause of DTC 75.
2. Check actuator operation. Use actuator quick check. The actuator quick test will PWM the actuators at a set rate. Listen and compare the noise of the 3rd gear actuator to that of the other actuators. The noise should be similar.

NOTE: Prior to performing actuator quick check, the fuel pump probe hole must be connected to 12 volts. See "Actuator Quick Check" procedures. Component Tests and General Diagnostics

3. Problem could be a mechanical failure. This step begins internal diagnosis.
4. Cross leaks to/from 4th clutch oil passages can cause 4th clutch to be on at all times. Possible leak due to seals, gaskets, porosity etc.
5. As air pressure is applied and released, the clutch plates should also apply and release. If the clutch plates are moving normally, the problem is probably in the valvebody. If clutches are not moving correctly, the problem is in the end cover or an internal transaxle problem exists.

DIAGNOSTIC AIDS
When attempting to diagnose an intermittent problem use the Scan tool[1] to review supplemental diagnostic information. The supplemental data can be used to duplicate a problem.
- This code may set or intermittent 4th gear starts may be caused by a poor electrical connection at either the PCM, actuator connector plate or UHJB terminals. Wiggling connectors while performing an actuator quick test may help isolate the problem area.

Check the tightness of the female terminal grip with a spare male terminal.
- This DTC will probably be accompanied by DTC 25 - No TCC, DTC 22 - No 2nd Gear, and DTC 24 - No 3rd Gear.

[1] Select Malfunction History from Scan tool Transaxle Controller (TC) menu.