For 1990-2009 cars

# PCM CODE P028: LOSS OF DRIVE INTERMITTENT DOWNSHIFT TO 2ND

#### SUBJECT: PCM Code P028; Intermittent Loss of Drive; or Intermittent Downshifts to 2nd Gear (Replace Transaxle Pressure Switch)

VEHICLE APPLICATIONS: 1993 CADILLAC ALLANTE (V-Cars)

TRANSMISSION APPLICATIONS: HYDRA-MATIC 4T80-E (MH1)

TRANSAXLE MODELS: 1993 2AJN

#### CONDITION:

---------- Some 1993 model year Allantes with the HYDRA-MATIC 4T80-E transaxle may experience a PCM diagnostic trouble code P028 caused by an erratic signal from the transaxle pressure switch. The transaxle pressure switch may also cause the PCM to:

1. Intermittently default the transaxle to 2nd gear, with or without a code setting. This may be described as an "intermittent loss of drive" or "shift busy-ness."

2. At high speeds (out of second gear availability) the roller clutch will overrun and the transaxle will seem as if it is neutral until Code 28 sets.

3. At speeds in which second gear is available but a low throttle is commanded, the driver will sense the vehicle slowing and will tend to increase throttle angle until second gear is achieved.

4. If, at any time, the PRNDL flutters between a valid selection and an illegal combination, the transaxle will "hunt" between gears and the timer will reset on every change causing the code to take much longer to set.

#### DIAGNOSIS:

---------- To diagnose this condition, perform the following procedure:

1. Refer to PCM Diagnostic Trouble Code Chart P028 on page 6E-A-88 of the 1993 Allante Preliminary Service Manual.

2. Follow the code P028 diagnostic chart to determine the cause of the condition. This condition may be caused by a wiring problem, so following the code P028 diagnostic chart is important to assure proper repair.

3. If the code P028 diagnostic chart indicates transaxle pressure switch replacement or an intermittent condition, install service transaxle pressure switch. Refer to Parts Information in this bulletin.

#### SERVICE PROCEDURE:

To replace the transaxle pressure switch, perform the following procedure:

1. Raise vehicle and support.

2. Remove sixteen bottom pan bolts (62), bottom pan (59) and gasket (61). Allow transaxle fluid to drain and discard gasket (see Figure 1).

3. Remove left and right scavenger screens (51, 52) and seals (53) and discard. Do not score or damage case when removing seals (see Figure 1).

4. Disconnect shift solenoids A, B, and transaxle pressure switch connectors (see Figure 2).

5. Disconnect manual valve linkage clip (915) (see Figure 2).

6. Remove nine oil transfer plate bolts and oil transfer plate (956) (see Figure 3).

7. Remove ten lower channel plate bolts (534), two nuts (955), and lower channel plate, control valve, and accumulator assembly from transaxle. DO NOT REMOVE indicated bolts in lower channel plate (see Figure 3).

8. Remove six bolts (941) and transaxle pressure switch (936) (see Figure 4).

9. Install new transaxle pressure switch (936) and six bolts (941). Torque bolts to 12 N.m (106 in. lbs.).

10. Centre the low reverse band position and low reverse band using a 30 cm (12 inch) length of .5 mm (.020 inch) shim stock. A mispositioned low reverse band during reassembly will result in a no first gear/no engine braking/no reverse condition (see Figure 5).

11. Place lower channel plate, control valve and accumulator assembly in transaxle using the forward support studs to locate.

12. Hand start ten bolts and two nuts into channel plate. Bolts without flanges are located next to oil transfer plate. Torque bolts and nuts to 12 N.m (106 in. lbs.). Remove shim stock (see Figure 6).

13. Install oil transfer plate and hand start nine bolts. Torque bolts to 12 N.m (106 in. lbs.) (see Figure 3).

14. Connect manual valve (915) to detent lever (17) (see Figure 2).

15. Route wiring harness under spacer plate rib and detent lever. Connect wiring harness to shift solenoids A, B (909) and transaxle pressure switch (936) (see Figure 2).

16. Install left and right scavenger screens (51, 52) and seals (53) (see Figure 1).

17. Remove any residual transaxle fluid from the gasket sealing surface of the case, the outside surface of the case, case bolt holes, bottom pan and bolts. Use a rubber tipped air hose on the case bolt holes to remove fluid, if necessary. All of the bottom pan bolts MUST be completely dry before installation.

18. Position the new gasket and bottom pan on the transaxle. Hand start all of the bottom pan bolts, then follow the torque sequence listed below (see Figure 7):

* Torque all bolts first to 3 N.m (27 in. lbs.) in the sequence shown in Figure 8, then to 6 N.m (53 in. lbs.) and finally to 12 N.m (106 in. lbs.).

#### IMPORTANT: The final torque specification of 12 N.m (106 in. lbs.) must not be exceeded. If this specification is exceeded, the bottom pan gasket and case could be damaged and a transaxle fluid leak may result.

19. Add four (4) litres of transmission fluid and top off fluid level as necessary. See fluid level checking procedure. After filling the transaxle with fluid, get into the on-vehicle diagnostic system and access PCM "data" parameter PD71 PSM 2YX and verify the following gear state combinations:

Park/Neutral . . . . . . . . . . . . . . . . . 101 Reverse . . . . . . . . . . . . . . . . . . . 100 D4 . . . . . . . . . . . . . . . . . . . . . . 001 D3 . . . . . . . . . . . . . . . . . . . . . . 011 D2 . . . . . . . . . . . . . . . . . . . . . . 111 D1 . . . . . . . . . . . . . . . . . . . . . . 110

If combinations 000 and 010 are displayed, there may be a problem with the PSM's related connectors.

#### FLUID LEVEL CHECKING PROCEDURE:

To accurately check fluid level of the HYDRA-MATIC 4T80-E transaxle, the unit must be at normal operating temperature (82 degrees C - 93 degrees C / 180 degrees F - 200 degrees F).

#### NOTE: PRESENTLY, IF THE 4T80-E IS CHECKED COLD, an incorrect fluid level will be shown on the indicator.

1. Park vehicle on a level surface (with transaxle temperature between 82 degrees C and 93 degrees C {180 degrees F and 200 degrees F}).

2. With foot on brake, move shift lever through each range, pausing for about 3 seconds in each range ending in park.

3. Apply parking brake.

4. Let engine idle for 3 minutes.

5. Remove transaxle fluid level indicator, wipe clean and reinsert, fully twisting closed.

6. Wait 3 seconds and remove indicator.

7. Read both sides of indicator and, using lowest level, fluid must be within cross-hatched area.

#### NOTE: If the brake pedal is applied during this procedure, it will affect the fluid level reading.

#### SERVICE PARTS INFORMATION:

Description Part Number ----------- ----------- Transaxle Pressure Switch 8683880 Bottom Pan Gasket Kit (includes 8684953 gasket, left and right scavenger screens and seals)

For vehicles repaired under warranty use:

Labour Time: 0.8 hour.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.