Diagnostic Hints
Checking fluid level and condition will often reveal enough information to determine the necessary repair without going any further in the diagnostic procedure. The following conditions suggest a transmission replacement will be necessary:
- Fluid smells burned (like a rotten egg or burned electrical coil) and has taken on a very dark color.
- Fluid is contaminated with particles of friction material.
- Fluid is discolored a milky pink. This is an indication of coolant leakage into the ATF; repair coolant leak and flush cooler lines before installation of replacement transmission.
If in doubt about fluid condition, the oil pan should be removed for further evaluation. It is normal to see a slight build-up of material on the pan magnets and a dark film on the bottom of the pan. Excessive deposits will be obvious and require transmission replacement. A quick check of peripheral systems (i.e. kick-down cable adjustment on hydraulic transmissions or EGS faults memory on Electronic/Hydraulic versions) must be made to ensure that the primary cause of failure is found and corrected to prevent a reoccurence.
As two of the more time consuming checks are pressure testing and pin by pin testing, their value in performing an accurate diagnosis and repair must be carefully considered before investing the time and effort to carry out such testing.
If the transmission is to be exchanged, the additional information gained from pressure testing is usually not needed. Pressure testing is most valuable when making a diagnosis in order to effect a transmission repair. Pressure testing is helpful in the diagnosis of problems which are unresolved after the basic checks have found no faults or resulted in the correction of faults but the problem still exists. Once the transmission is out of the car and apart on the bench, this valuable clue to the condition of the transmission's hydraulic "pulse" is lost and the chance of finding and repairing the fault the first time are reduced.
Pressure testing is used to check the following hydraulic circuits:
- Pump
- Throttle valve (H versions)
- Throttle pressure regulating solenoid/force motor (EH versions)
- Governor (H versions)
- Forward clutch (models with forward clutch pressure tap)
- Converter clutch piston/seals (models with converter pressure tap)
A mainline pressure check will not reveal the condition of servos or clutches as these are typically applied through metered orifices. A mainline pressure reading in this case will only reveal what is happening up to the orifice.
Monitoring mainline pressure while extending the transmission throttle cable (kickdown cable) or activating the force motor will verify the function of both the pump and throttle pressure circuit. Low main pressure readings may indicate a problem in one or more of the following areas; clogged filter, loose valve body bolts, leaking governor sealing rings, stuck governor valve, or a worn or damaged pump. A sticking main pressure valve or modulation valve, defective force motor, or incorrectly adjusted transmission throttle cable are some possible areas which could result in having either a high or low main pressure.
Monitoring torque converter pressure will be useful in the diagnosis of shudder complaints upon lock-up clutch engagement. If the pressure behind the lock-up clutch piston exceeds 0.7 bar, the clutch will not engage fully allowing slippage of the torque converter clutch. This could indicate either a defective torque converter or scored hook ring lands on the stator support. A quick check of the transmission serial number against that given in the Diagnostic Hints Chart for "Stator support scored by sealing ring" will tell you which is the most likely cause.
While not as valuable as a main pressure check, checking the forward clutch apply pressure will confirm or refute a questionable diagnosis of forward clutch failure before removing the transmission. Dont expect forward clutch pressure to be within specifications if main line pressure was not. The validity of the pressure testing depends upon the thoroughness of the check up to this point. A misadjusted transmission throttle cable or incorrect fluid level will affect the pressure readings.
EGS pin by pin testing is intended to complement the diagnostics of the 2013 Diagnostic Module. The majority of the checks can be made quickly and easily with the 2013 or MoDiC without having to make or break any connections other than the connection of the 2013 or MoDiC to the diagnostic plug. Pin by pin testing may be necessary after the 2013 specifies an area to check. It is for these instances that we have developed the helper cards with pin identification and nominal values. The first version electronic transmission, E7 control system, does not have on board diagnostics and therefore pin by pin testing will take on more significance when diagnosing this early system.
Faults which are a product of electrical malfunctions are rarely, if ever, eliminated by transmission replacement. The transmission fault indicator (warning light or matrix display), when illuminated, is an alert that the problem is electrical and not mechanical. The EGS system must be diagnosed and repaired using the BMW Diagnostic Module, ETM Diagnostic Procedures, and the "Understanding AEGS" guide.
To make EGS diagnosis simpler, technicians who are familiar with the "Understanding AEGS" guide need only perform the electrical checks using the correct version helper card. The electrical check on the E34 and E36 THM-R1 helper cards are not covered in the "Understanding AEGS" guide, but technicians experienced in pin by pin testing will find the checks quick and easy. An electrical problem has been detected when the test results differ from the nominal values listed in the helper card. At that point, the technician should refer to the "Understanding AEGS" guide, appropriate ETM, or Diagnostic Hints chart for further troubleshooting tips.
Based on our analyses, we have produced the attached Diagnostic Hints chart showing the most common failures including symptoms and recommended repairs. As certain problems are typically confined to specific production ranges and/or transmission types, this information is included where applicable.
Please bear in mind that this chart is not intended to cover all failure causes. Its intent is to share with the technicians the experience and knowledge gained from studying a large population of failed transmissions. If the vehicle being diagnosed does not exhibit one of the listed symptoms, Group 24 of the appropriate repair manual contains more exhaustive troubleshooting charts.