Note: Automatic transmission malfunctions may be caused by five general conditions: poor engine performance, improper adjustments, hydraulic malfunctions, mechanical malfunctions or malfunctions in the Powertrain Control Module or its signal network. Diagnosis of these problems should always begin with a check of the easily repaired items: fluid level and condition (see Tune-up and routine maintenance), and shift cable adjustment (see Shift cable — replacement and adjustment). Next, perform a road test to determine if the problem has been corrected or if more diagnosis is necessary. Because the transmission relies on many sensors in the engine control system, and since the transmission shift points are controlled by the Powertrain Control Module, you’ll also want to check to see if any trouble codes have been stored in the PCM (see CEmissions and engine control systems for a list of trouble codes and how to extract them). If the problem persists after the preliminary tests and corrections are completed, additional diagnosis should be done by a dealer service department or transmission repair shop. Refer to the Troubleshooting Section at the front of this manual for transmission problem diagnosis.

Preliminary checks

1. Drive the vehicle to warm the transmission to normal operating temperature.

2. Check the fluid level as described in Chapter Tune-up and routine maintenance:

a) If the fluid level is unusually low, add enough fluid to bring the level within the designated area of the dipstick, then check for external leaks.

b) If the fluid level is abnormally high, drain off the excess, then check the drained fluid for contamination by coolant. The presence of engine coolant in the automatic transmission fluid indicates that a failure has occurred in the internal radiator walls that separate the coolant from the transmission fluid (see Cooling, heating and air conditioning systems).

c) If the fluid is foaming, drain it and refill the transmission, then check for coolant in the fluid or a high fluid level.

3. Check the engine idle speed.

Note If the engine is malfunctioning, do not proceed with the preliminary checks until it has been repaired and runs normally.

4. Inspect the shift cable (see Shift cable — replacement and adjustment). Make sure that it’s properly adjusted and that it operates smoothly.

5. Check the Transmission Range (TR) switch adjustment (see Emissions and engine control systems).

Fluid leak diagnosis

6. Most fluid leaks are easy to locate visually. Repair usually consists of replacing a seal or gasket. If a leak is difficult to find, the following procedure may help.

7. Identify the fluid. Make sure it’s trans- mission fluid and not engine oil or brake fluid (automatic transmission fluid is a deep red color).

8. Try to pinpoint the source of the leak. Drive the vehicle several miles, then park it over a large sheet of cardboard. After a minute or two, you should be able to locate the leak by determining the source of the fluid dripping onto the cardboard.

9. Make a careful visual inspection of the suspected component and the area immediately around it. Pay particular attention to gasket mating surfaces. A mirror is often helpful for finding leaks in areas that are hard to see.

10. If the leak still cannot be found, clean the suspected area thoroughly with a degreaser or solvent, then dry it.

11. Drive the vehicle for several miles at normal operating temperature and varying speeds. After driving the vehicle, visually inspect the suspected component again.

12. Once the leak has been located, the cause must be determined before it can be properly repaired. If a gasket is replaced but the sealing flange is bent, the new gasket will not stop the leak. The bent flange must be straightened.

13. Before attempting to repair a leak, check to make sure that the following conditions are corrected or they may cause another leak. Note: Some of the following conditions cannot be fixed without highly specialized tools and expertise. Such problems must be referred to a transmission shop or a dealer service department.

Gasket leaks

14. Check the pan periodically. Make sure the bolts are tight, no bolts are missing, the gasket is in good condition and the pan is flat (dents in the pan may indicate damage to the valve body inside).

15. If the pan gasket is leaking, the fluid level or the fluid pressure may be too high, the vent may be plugged, the pan bolts may be too tight, the pan sealing flange may be warped, the sealing surface of the transmission housing may be damaged, the gasket may be damaged or the transmission casting may be cracked or porous. If sealant instead of gasket material has been used to form a seal between the pan and the transmission housing, it may be the wrong sealant.

Seal leaks

16. If a transmission seal is leaking, the fluid level or pressure may be too high, the vent may be plugged, the seal bore may be damaged, the seal itself may be damaged or improperly installed, the surface of the shaft protruding through the seal may be damaged or a loose bearing may be causing excessive shaft movement.

17. Make sure the dipstick tube seal is in good condition and the tube is properly seated. Periodically check the area around the speedometer gear or vehicle speed sensor for leakage. If transmission fluid is evident, check the 0-ring for damage. Also inspect the driveshaft oil seal for leakage.

Case leaks

18. If the case itself appears to be leaking, the casting is porous and will have to be repaired or replaced.

19. Make sure the oil cooler hose fittings are tight and in good condition. The transmission oil cooler lines on these models are equipped with quick connect fittings — always inspect the 0-rings if a leak is suspected.

Fluid comes out vent pipe or fill tube

20. If this condition occurs, the transmission is overfilled, there is coolant in the fluid, the case is porous, the dipstick is incorrect, the vent is plugged or the drain back holes are plugged.

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