Automatic transmission failures can be caused by five main causes: engine inefficiency, misadjustments, hydraulic problems, mechanical failures, or microprocessor or signal network failures. Diagnosing these faults should always begin by checking the most easily correctable causes: transmission fluid level and condition (Chapter Car maintenance), adjustment of selector rods and throttle actuator. Then a road check should be made to determine changes in the operation of the AT after the corrections made and the need for further research. If the problem persists, further diagnosis should be carried out by a BMW dealer or a specialist workshop. For information on possible AT malfunctions, refer to Section Diagnostics of malfunctions of automatic transmission at the end of this Chapter.
Preliminary checks
1. Take the car for a short trip to warm up the AT to normal operating temperature.
2. Check the transmission fluid level as described in Chapter Car maintenance:
- If the fluid level is too low, top up to the level indicated by the marks on the dipstick, then visually inspect the transmission for signs of leaks.
- If the fluid level is unusually high, drain the excess, then check the drained fluid for coolant. The presence of engine coolant in the transmission fluid indicates a violation of the tightness of the inner walls of the radiator separating the coolant from the transmission fluid (see chapter Cooling system).
- If the fluid is foamy, empty the transmission and fill it with fresh fluid, then check for coolant in the drained fluid.
- Check the condition of the fluid on the dipstick. The normal color of transmission fluid is dark red. If she is brown, or has «burnt» smell, drain it and fill the transmission with fresh fluid. If the fluid is in poor condition, this may indicate either a lack of regular maintenance or an internal malfunction in the transmission.
3. Check up turns of idling.
If it is obvious that the engine is defective, do not perform preliminary checks on the operation of the transmission until the problem is corrected.
4. Check for free play of the throttle cable in kickdown mode. Adjust it if necessary (see Section Selector lever drive adjustment).
The throttle cable may function normally with the engine off and cold, but cause problems after the engine has warmed up. Therefore, cable checks should be carried out both with a cold and a warm engine.
5. Check the condition of the selector control rod (see Section Checking the operation and replacing the start permission switch / reversing light switch). Make sure it is properly adjusted and runs smoothly.
Fluid Leak Diagnosis
In most cases, fluid leaks are easy to detect by visual inspection. Repair usually consists of replacing the seal or gasket. If a leak cannot be detected on visual inspection, the following steps may help.
1. Determine the type of liquid. Make sure it is transmission fluid that is leaking, not engine oil or brake fluid (transmission fluid is dark red).
2. Try to identify the source of the leak. Drive the car for a short drive, then park it over a large piece of blank cardboard or newspaper. After a couple of minutes, you can identify the leak by identifying the place on the cardboard where the liquid drips.
3. Perform a thorough visual inspection of the transmission area suspected of leaking and the area around it. Pay special attention to the joints of the mating surfaces of the gaskets. It is useful to use a mirror to inspect hard-to-reach areas.
4. If the source of the leak is still not found, thoroughly flush the suspected area with solvent, then dry it.
5. While the vehicle is in motion, the airflow may blow leaking fluid or oil onto other components, making it difficult to determine the source of the leak. After cleaning the suspected area, either drive the car at low speed (to minimize airflow), or let it warm up to operating temperature without moving (in this case, fully apply the parking brake). Run the engine at different speeds and move the transmission selector through all positions several times. After that, inspect the suspect component again.
6. After identifying the source of the leak, for its competent elimination, it is necessary to determine the cause of the leak. If you replace the gasket while leaving the seat flange bent, the leakage will not stop. The bent flange must be straightened.
7. Before attempting to repair a leak, make sure the following conditions are met, otherwise it may recur.
Some of the conditions listed below cannot be met without the use of highly specialized tools and the proper qualifications of the performers. Such problems should be left to BMW dealers or transmission specialists from a car workshop.
Leaks through the gasket
Regularly check the condition of the transmission oil pan. Check the presence of all bolts and the reliability of their tightening, the condition of the pallet itself (dents in the sump can damage the valve body inside).
If the sump gasket is leaking, the causes may be too high a fluid level, a clogged breather, overtightening the sump bolts, a bent sump flange, damage to the sealing surface of the transmission case, or damage to the gasket itself. If a sealant is used to seal between the sump and the transmission case, it is possible that the wrong grade of sealant was used. Leakage can even occur due to the presence of cracks or porosity in the crankcase casting.
Leakage through stuffing box
Causes of stuffing box leaks can be too high fluid level or pressure, a blocked vent breather, damaged stuffing box walls, damaged or improperly installed stuffing box, damage to the surface of the shaft through the stuffing box, or loose bearing mounting causing excessive shaft freedom.
Make sure the dipstick tube nut is tight. Regularly check the area around the speedometer gear or speed sensor for leaks. If transmission fluid is found, check the O-ring for damage.
Crankcase leaks
If leaks are found in the crankcase itself, this indicates the presence of porosity in its casting, which should be repaired or the crankcase replaced.
Check up reliability of fastening and a condition of union connections of hoses of a cooler.
Fluid is leaking out of the vent or fill tubes
This is a sign that the transmission is overfilled, coolant has entered the transmission, the dipstick is incorrectly installed, or the breather or drain holes are clogged.