(732-2491) Common faults on automatic transmissions

Symptom: The CHECK GEARBOX lamp comes on when braking heavily while time turning right. CHECK that the lamp comes on when starting from cold with the temperature lower than -20°.

Cause: DTC P1830 is generated when the difference between the normal gear ratio of the input and output shafts exceeds 10% (gearbox slip). This usually happens when the transmission fluid level is too low. The fluid pump is therefore drawing in air, which results in the pressure in the system being too low.

Symptom: CHECK GEARBOX lamp comes on and transmission goes into “Limp Home” mode.

Cause: The ignition coil in cars with the Motronic engine management system can produce high voltage surges in the wiring harness. The Motronic control module interprets the voltage surge as changes in the engine speed. The cause of the voltage surges can be insufficient conductivity in the cable, bad grounding of the gearbox to the body or bad grounding of the Motronic control module to the body. Normal fault criteria for the DTCs are that the difference between the engine speed and input speed in the gearbox is greater than 100 rpm for 10 seconds.

Symptom: CHECK GEARBOX lamp comes on and transmission goes to “Limp-home” mode. The fault arises intermittently in extremely cold or extremely hot temperatures.

Cause: The transmission control module receives a 12V signal from the four contacts in the gear selector position sensor. At least 2 contacts are active in all gear positions. If the control module does not receive a 12V signal from 2 of the contacts, a DTC will be generated. The DTC is generated if the fault is present for longer than 0.5 seconds. This applies to M98. For M99, the DTC is not generated until after 5 seconds.

Check using Tech2, Read value in TCM, the transmission fluid level at normal operating temperature (80°C). The fluid level should be at MAX. If necessary, fill with mineral based fluid Dexron III transmission fluid. Other fault diagnosis for these DTCs is in WIS, Transmission, Automatic transmission, Fault diagnosis, diagnostic trouble codes, P1830. Clear fault codes after carrying out procedure.

Check using Tech 2, Read value TCM, if the signal for the idling speed varies between 900 and 1600 rpm, observe the curve on the diagnostic tool display. If there is a variation, improve the grounding to the Motronic control module and gearbox. The control module is grounded to the holder that is attached to the bodywork. The holder is painted, which can be the cause of the bad grounding.

In order to improve the grounding, proceed as follows: Make a ground cable 2.5 mm², approx. 20 cm long. Connect the cable between the nut in the corner of the control module and the A pillar grounding point G34P, put on a serrated lock washer. See illustration. Clear DTC.