General information about troubleshooting (BMW 5 Series E28)

Automatic transmission failures can be caused by five main reasons: poor engine performance, out of adjustment, hydraulic problems, mechanical problems, or failures of the microprocessor or signal network. Diagnosis of these problems should always begin with checking the most easily correctable causes: the level and condition of the transmission fluid (Chapter Routine car maintenance), adjusting the selector rods and throttle actuator. Then, a road test should be carried out to determine changes in the operation of the AT after the corrections made and the need for further research. If the problem is not resolved, further diagnostics should be carried out by a BMW dealer service department or a specialized auto service workshop. For information on possible AT malfunctions, refer to Section Automatic Transmission Fault Diagnosis at the end of this Chapter.

Preliminary checks

1. Take the vehicle for a short drive to warm up the automatic transmission to normal operating temperature.

2. Check the transmission fluid level as described in Chapter Routine car maintenance:

• If the fluid level is too low, add fluid to the level indicated by the marks on the dipstick, then inspect the transmission externally 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 leak in the inner walls of the radiator that separate the coolant from the transmission fluid (see chapter Cooling system).
• If the fluid is foamy, drain the transmission and refill with fresh fluid, then check the drained fluid for coolant.
• Check the condition of the fluid on the dipstick. Normal transmission fluid is dark red. If it is brown or has a "burnt" odor, drain it and refill the transmission with fresh fluid. If the fluid is in poor condition, it may indicate either a lack of regular maintenance or internal problems with the transmission.

3. Check the idle speed.

If the engine is obviously faulty, do not perform preliminary tests on the transmission until the fault has been corrected.

4. Check the throttle cable for free movement in kick-down mode. Adjust it if necessary (see section Adjusting the selector lever drive).

The throttle cable may function normally when the engine is off and cold, but may cause problems after the engine has warmed up. Therefore, check the cable both when the engine is cold and when it is warmed up.

5. Check the condition of the selector control rod (see section Checking the operation and replacing the start enable switch/reverse light switch). Make sure it is adjusted properly and operates smoothly.

Fluid Leak Diagnosis

In most cases, fluid leaks are easy to detect by visual inspection. Repairs usually involve replacing the seal or gasket. If a leak cannot be detected by visual inspection, the following steps may help.

1. Determine the type of fluid. Make sure that it is transmission fluid that is leaking, not engine oil or brake fluid (the transmission fluid is dark red).

2. Try to locate the source of the leak. Take the car for a short drive, then park it over a large sheet of clean cardboard or newspaper. After a couple of minutes, you should be able to locate the leak by identifying the spot on the cardboard where the fluid is dripping.

3. Perform a thorough visual inspection of the suspected leaking transmission section and the area around it. Pay particular 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 still cannot be found, thoroughly flush the suspected area with solvent, then dry it.

5. When the vehicle is moving, air flow can 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 vehicle at low speed (to minimize air flow), or let it warm up to operating temperature without moving it (in this case, fully engage the parking brake). Run the engine at various speeds and cycle the transmission selector through all positions several times. Then inspect the suspect component again.

6. After identifying the source of the leak, in order to properly eliminate it, you should determine the cause of the leak. If you replace the gasket, leaving the mounting flange bent, the leak will not stop. The bent flange must be straightened.

7. Before attempting to repair a leak, make sure the following conditions are met, otherwise the leak may recur.

Some of the conditions listed below cannot be met without highly specialized tools and skilled workers. Such problems should be left to the BMW dealer or transmission specialist at your auto repair shop.

Leaks through the gasket

Regularly check the condition of the transmission oil pan. Check that all bolts are present and securely tightened, and the condition of the oil pan itself (dents in the pan can cause damage to the valve body inside).

If the oil pan gasket is leaking, the causes may be too high a fluid level, a clogged breather, overtightened oil pan bolts, a bent oil pan 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 oil pan and transmission case, it is possible that the wrong type of sealant was used. A leak may even be due to cracks or porosity in the casting of the case.

Leaks through the seal

Seal leaks can be caused by fluid level or pressure that is too high, a clogged breather, damage to the seal seat walls, damage to or improper installation of the seal itself, damage to the surface of the shaft section passing through the seal, or a loose bearing mount causing excessive shaft play.

Make sure that the probe tube nut is firmly tightened. Regularly check the area around the speedometer pinion or speed sensor for leaks. If transmission fluid is detected, check that the O-ring is not damaged.

Crankcase leaks

If leaks are found in the crankcase itself, this indicates the presence of porosity in its casting, which should be eliminated or the crankcase replaced.

Check the reliability of the fastening and the condition of the fittings of the cooler hoses.

Liquid leaks out through the vent or filler tubes

This is a sign of overfilling the transmission, coolant getting into the transmission, improperly installed dipstick, clogged breather or drain holes.

Information obtained from this online resource «bmwman»