January 2000 Issue

Transmission Tech/Talk
January 2000 Issue
Volume 17, No. 1

In This Issue
GM Diesels: What Is A Code 88?
GM 4L80-E: Stuck In First Gear, No Vehicle-Speed Signal
THM 4T80-E: Malfunction Indicator Lamp Illuminated And DTC P0503 Stored
Chrysler 42RE: Buzzing Noise In Reverse
Mazda/Ford GF4A-EL: Premature Coast-Clutch Failure
GF4A-EL: Codes 6, 25 and 55
Mazda/Ford GF4A-EL: Diagnostic-Connector Locations And Code Retrieval

Issue Summary:

After TCM replacement on GM diesels, no 3-4 shift or TCC occurs and a code 88 is stored. We explain what caused the code to be set.
GM has made some changes in the PCM on 4L80-E transmissions to eliminate one of the causes of no upshift from first gear.
The malfunction indicator lamp illuminated and DTC P0503 stored on a 4T80-E transaxle may be caused by movement of the final-drive internal ring gear relative to the transaxle case.
In a Chrysler 42RE, a buzzing noise in Reverse may be caused by either a partially clogged main filter or the pressure-regulator valve.
A cracked forward piston or reverse/forward and coast-clutch drum can cause premature coast-clutch failure on GF4A-EL units.
GF4A-EL transaxles with codes 6, 25 and 55 that place the unit in failsafe may require replacement of the PCM.
We also have diagnostic-connector locations and code-retrieval information on GF4A-EL units.

After TCM replacement, no 3-4 shift and no TCC occur, and code 88 is stored.

Code 88 indicates that top dead center is offset by plus or minus 2°. This can be caused by a pump-priming problem or the TCM having lost its memory (for example, a disconnected battery). When a TCM has been replaced or has lost its memory, it will need to be flashed with the required TDC offset parameters to eliminate code 88. Once new parameters are set, code 88 will be erased and over-drive and TCC will be restored.

Have the TCM flashed with new TDC offset parameters.

Note:

If codes 91-98 are present, they too will need to be addressed. These are cylinder-balance fault codes. An injector-balance test can be performed with a factory scan tool for each cylinder. The injection pump also should be checked for timing.

GM 4L80-E
Stuck In First Gear, No Vehicle-Speed Signal

The transmission is stuck in first gear. When you scan for codes on 1991-93 trucks and vans, no codes are stored. On 1994 trucks and vans, a code 74 is stored for “no input-speed-sensor signal.”

When data is displayed on the scan tool, in addition to no input rpm, no output rpm is seen, and with the vehicle moving forward, the scan-tool data display indicates vehicle speed is 0 mph.

The 4L80-E transmission on 1991-94 trucks and vans has computer strategy that will not allow the PCM to look at output rpm if there is no input rpm.

Without input rpm the PCM ignores output rpm, causing the PCM not to see any vehicle-speed signal when the vehicle moves forward; therefore, the PCM has no reason to shift the transmission out of first gear.

It wasn’t until the 1994 model year that the PCM acquired the ability to store a code 74 when there was no input-speed-sensor signal.

On 1995 and later trucks and vans, the computer strategy was changed to eliminate this problem. On those trucks and vans there will be output-rpm and vehicle-speed signals even though there is no input-speed signal.

When a no-upshift condition exists, do not assume there is a problem with the vehicle-speed system. On 1991-93 trucks and vans, scan for data and make certain you have an input-rpm signal. On 1994 vehicles, a code 74 stored with the preceding symptoms is a definite indicator that the input-speed sensor (See Figure 1) is causing the problem.

THM 4T80-E
Malfunction Indicator Lamp Illuminated And DTC P0503 Stored

Some owners of 1998-99 Cadillac or 1998-99 Auroras equipped with the 4T80-E transaxle, built before Jan. 21, 1999 [Build Year 9, Julian Date 021](See Figure 2), may comment on one of the following conditions: Speedometer fluctuates or is inoperative; cruise control is inoperative; decreased fuel economy; malfunction indicator lamp (MIL) is illuminated and diagnostic trouble code (DTC) P0503 is stored in PCM memory. This DTC is vehicle-speed sensor intermittent.

The cause may be an air-gap variation between the output-speed sensor and the differential carrier created by movement of the final-drive internal ring gear relative to the transaxle case. This also causes wear of the bushing in the final-drive housing.

Some affected transaxles may have a mis-machined final-drive internal ring gear on the outside diameter and a final-drive internal-ring-gear snap ring with a corner break at the thickest side, as shown in Figure 3.

Some vehicles with the 3.11 or the 3.71 final-drive-ratio carrier also may have one or more planet pinions with mismachined gear teeth, as shown in Figure 4.

Inspect and replace as necessary all of the parts and concerns listed, using the part numbers under Service Information. We also have provided a chart with information on final-drive ratios for the various models, as the ratios will not interchange from model to model.

Final-Drive Internal Ring Gear . . . . . . . . . . . . . . . . . . 8677862
Final-Drive Housing (Bushing not serviced) . . . . . . . . . . . . 8683544
Final-Drive Housing Seal . . . . . . . . . . . . . . . . . . . . . 8679017
Final-Drive Ring-Gear Snap Ring . . . . . . . . . . . . . . . . . 8681080
Final-Drive Carrier, 3.11 Ratio (Pinions not serviced) . . . . . . 24205234
Final-Drive Carrier, 3.71 Ratio (Pinions not serviced) . . . . . . 24205233
Bottom Pan Gasket. . . . . . . . . . . . . . . . . . . . . . . . . 24209512

Some vehicles equipped with the 42RE transmission may exhibit a buzzing noise while the manual selector lever is in the Reverse position.

The cause may be either a partially clogged main filter or the pressure-regular valve itself.

First, install a brass screen in place of the felt-media filter in the bottom pan, as shown in Figure 5.

If this does not cure the condition, replace the aluminum PR valve in the valve body with the previous-design steel PR valve (See Figure 6), available under OEM part number 4130169.

Pressure-Regulator Valve (Steel) . . . . . . . . . 4130169
Brass Screen (Aftermarket ATC) . . . . . . . . . . 12010D

Mazda/Ford GF4A-EL
Premature Coast-Clutch Failure

Vehicles equipped with GF4A-EL transaxles may exhibit premature and repeated coast-clutch failure before or after overhaul.

The cause may be a cracked forward piston or a cracked reverse/forward and coast-clutch drum allowing forward-clutch pressure to partially apply the coast clutch in 4th gear, causing a partial bind-up.

Note:

This problem cannot be detected easily when you air-check the clutch packs.

Replace the reverse/forward and coast-clutch drum and/or forward-clutch piston as shown in Figure 7.

Vehicles having a 2.0L engine with a GF4A-EL transaxle may produce a code 6 for the VSS (vehicle-speed sensor), code 25 for the FPCS (fuel-pressure control solenoid) and code 55 for the PSG (pulse-signal generator) all at the same time, placing the unit in failsafe.

It has been our experience that when these three codes have been stored together, the replacement of the powertrain control module has corrected this complaint. Be sure that each circuit has been checked and verified for proper operation before replacing the computer.

The PCM is below the front center of the console (See Figure 8).

From here you can verify all three circuits for proper operation before changing the computer (See Figure 9).

If the checks show the following conditions, change the PCM:

Code 6 VSS – You should see about a 5-volt DC pulse on the green wire with a red tracer going to terminal 1M as the wheels rotate slowly. You’ll observe an average of 2.5 volts during driving conditions.
Code 25 FPCS – Unplug connector 3. With the key on and engine off, you should see system voltage on the green wire going to terminal 3M.
Code 55 PSG – Plug connector 3 back into the PCM. Set meter to AC volts and probe the white wire at terminal 3F with the positive meter lead. Probe the red wire at terminal 3H with the negative lead. You should see AC voltage as soon as the vehicle is started, and voltage should increase proportionally with turbine-shaft rotation.

Mazda/Ford GF4A-EL
Diagnostic-Connector Locations And Code Retrieval

Mazda MX6/626 and Ford Probe ’93-95

The Hold lamp will flash if transmission-related trouble codes are being stored.

Location:

The multiple-pin diagnostic connector is beside the battery. (See Location 1 in Figure 10).

Procedure:

You can retrieve codes by inserting a jumper wire between TEN and GND terminals on 2.0L models, and between TAT and GND terminals on 2.5L models. Codes will flash over the OD light on 2.0L models and over the Hold light on 2.5L models.