68RFE Automatic Transmission

P0736-GEAR RATIO ERROR IN REVERSE

For a complete wiring diagram refer to Diagrams/Electrical. Electrical Diagrams


Theory of Operation


The transmission system uses two speed sensors, one to measure input rpm and one to measure output rpm. These inputs are essential for proper transmission operation. Therefore, the integrity of this data is verified through the following checks:


NOTE: Various Gear Ratio Error DTCs will set only if the transmission slips after first achieving the correct ratio. Therefore, a transmission with no drive (or reverse) may not set any Gear Ratio Error DTC because it never achieves a correct gear ratio.

1) Once in a particular gear, if the gear ratio varies from the correct gear ratio, the corresponding in-gear trouble code is set (DTCs P0729 and P0731-36).


2) An excessive change in input or output speeds indicating signal intermittent will result in the DTCs P0716 and/or P0721 being set.


3) If the common speed sensor ground circuit is lost, both sensor inputs will read the pulses from the input speed sensor when at a stop in neutral. Since the input speed sensor reads 60 teeth from the input clutch hub and the output speed sensor reads 30 teeth from the park gear, the result is an apparent speed ratio of 1:2 and may cause the DTC P1794 to set.


- When Monitored:

Continuously with the ignition on, engine running, with the transmission in gear.

- Set Condition:

If the ratio of the Input rpm to the Output rpm does not match the current gear ratio. This DTC can take up to five minutes of problem identification before illuminating the MIL







Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. 68RFE Pre-Diagnostic Troubleshooting Procedure

Diagnostic Test

1. CHECK FOR RELATED DTCS

With the scan tool, check for other transmission DTCs.

Are there any Transmission; solenoid, line pressure, speed sensor, and/or loss of prime DTCs present?

Yes

- Refer to appropriate symptom in the Transmission category. Perform the test for P0944-LOSS OF PRIME first if it is present. P0944

No

- Go To 2


2. CHECK IF DTC IS CURRENT


NOTE: Make sure to record all DTC EVENT DATA stored in the scan tool for future reference in this test procedure.

With the scan tool, perform the Reverse Gear Clutch test. Follow the instructions on the scan tool.

Increase the throttle angle, TPS degree, to 30�, for no more than a few seconds.


CAUTION: Do not overheat the transmission.

Did the clutch test pass, input speed remain at zero?

Yes

- Go To 3

No

- Go To 4



3. CHECK DTC EVENT DATA

The conditions necessary to set this DTC are not currently present.

With the scan tool, check the DTC EVENT DATA to help identify the conditions in which the DTC was set.

Does the DTC EVENT DATA show an input speed reading of zero?

Yes

- Go To 8

No

- Go To 4



4. POSSIBLE INTERNAL TRANSMISSION CAUSES

Remove the Transmission Oil Pan and Valve Body and check the following:
- Clutch or metal debris in the pan
- LR clutch tower seal on the main valve body or cuts and the LR accumulator piston and seals.
- Air check the Reverse and LR Hydraulic Clutch circuits (in the case) for excessive air leakage.


NOTE: Both Hydraulic Clutch circuits contain a small bleed orifice, a small amount of air leakage is normal. The Reverse hydraulic clutch circuit bleed orifice connects to the OD hydraulic clutch cavity. A small amount of air leakage into the OD hydraulic clutch circuit is considered normal.

Were there any problems found?

Yes

- Repair as necessary. If clutch debris is present, remove the transmission and inspect the Reverse and LR clutches, and inspect the Transmission Oil Pump and Main Regulator Valve for damage or sticking in its bore.
- Perform 68RFE TRANSMISSION VERIFICATION TEST. 68RFE Transmission Verification Test

No

- Go To 5

5. CLUTCH TEST FAILURE

Did the clutch test fail in reverse?

Yes

- Go To 6

No

- Test complete.
- Perform 68RFE TRANSMISSION VERIFICATION TEST. 68RFE Transmission Verification Test

6. CHECK FOR FAILED REVERSE GEARSET SPLINES

Working through the windows in the bottom of the case, check for a failed Reverse Sun or Annulus spline as follows:

1) Hold the Transmission Output Shaft by holding the Transmission Manual Lever in the park position.

2) Apply the 4C clutch using 30-40 psi shop air at the apply port in the center bulkhead.

3) Attempt to rotate the input annulus gear and drive shell (the drum that encloses the two rearmost gearsets).


NOTE: Due to the overrunning clutch, the gear and drive shell normally rotates in one direction only.

Can the drive shell be rotated?

Yes

- Repair internal transmission as necessary. Check the reaction annulus gear weld to the revers carrier, and the reaction sun gear weld to the 4C clutch hub.
- Perform 68RFE TRANSMISSION VERIFICATION TEST. 68RFE Transmission Verification Test

No

- Go To 7


7. CHECK THE DRIVE SHELL WELD

Release the output shaft and the 4C clutch.

Apply the L/R Clutch only using 30-40 psi of shop air.

Attempt to rotate the drive shell as in the previous step.

Can the drive shell be rotated?

Yes

- Repair Transmission as necessary. Check the drive shell weld to input annulus gear.
- Perform 68RFE TRANSMISSION VERIFICATION TEST. 68RFE Transmission Verification Test

No

- Disassemble and inspect the input clutch assembly. Check the Reverse clutch reaction plate snap ring. Check the OD and Reverse clutch seals, discs, and bleed orifice (in the OD/Reverse clutch piston). Check the Reverse hub and shaft. If no problems are found, check the L/R clutch assembly and repair as necessary. If all clutches are OK, replace the Transmission Solenoid/TRS assembly.
- Perform 68RFE TRANSMISSION VERIFICATION TEST. 68RFE Transmission Verification Test

8. CHECK FOR INTERMITTENT OPERATION

Intermittent gear ratio DTCs can be set by problems in the Input and Output Speed Sensor circuits and/or Speed Sensor Ground circuit.

Remove the Ignition Switch Feed fuse from the TIPM.


CAUTION: Removal of the Ignition Switch Feed fuse from the TIPM will prevent the vehicle from being started in gear.


WARNING: The Ignition Switch Feed fuse must be removed from the TIPM. Failure to do so can result in personal injury or death.

Install the Transmission Simulator, Miller tool #8333.

Check the Speed Sensor wiring and connectors for good connection, then perform a wiggle test using the Transmission Simulator, Miller tool #8333.

Were there any problems found?

Yes

- Repair as necessary.
- Perform 68RFE TRANSMISSION VERIFICATION TEST. 68RFE Transmission Verification Test

No

- Test complete.