Automatic transmissions and their control are becoming increasingly complex. Accordingly, the diagnosis of a malfunction of the unit is not always easy.

The variety of control systems and the rapid development of new components are putting the skills of a mechanic or diagnostician to the test. Unfortunately, there are no universally applicable and universally applicable criteria for a comprehensive diagnosis. Nevertheless, some principles for the assessment of the state of an automatic transmission can be put together. From our practice we have collected some helpful hints.


Important for a diagnosis are the following facts:
Which type of automatic transmission is installed at all? How many steps does it have or is it even a CVT?
How is the automatic transmission controlled? electronic control unit or even with cable, vacuum diaphragm

A test drive should be done with a lot of (toe-tip) feeling in order to determine or check the driving behavior or the complaint as accurately as possible. Before you go on a test drive, however, check with advantage the following points:

1. Oil Level

If the automatic transmission is equipped with an oil dipstick, with few exceptions, the oil level is measured on a flat surface with the engine running in position P (N). The right level is important. Measure warm and several times:
cold lower mark (area)
warm high mark (range)
Newer machines often no longer have an oil dipstick. An overflow pipe or a level screw are used as the most common alternatives. However, then the vehicle must usually be raised to control the oil level.

Typical problems with incorrect oil level are:
If the oil level is too low: no drive when cold and (or) in inclines and bends.
Oil foams, oil loss in case of ventilation of the housing if the oil level is too high.

2. Oil Quality

The condition of the ATF (automatic oil) can say a lot about the mechanical condition of the machine, therefore always the oil condition assess (possibly larger quantity).
Clean red oil = Automat is usually mechanically healthy, depending on the brand, the oil can also be golden yellow. The smell is slightly sweetish, not unpleasant.
Used, old vending oil is brown to dark brown, smells old, but does not stink. The oil has aged through many heating cycles.


Note: Perform the service before the machine malfunctions

Burnt ATF smells pungent bitter, with heavy damage of the machine, the oil stinks unpleasant and can be interspersed with metal abrasion. Important elements such as brake bands, couplings or individual mechanical components are destroyed here with certainty. The diagnosis is simple: the unit must be removed and revised or replaced. A service is useless and only a waste of time and money.

Rule of thumb: Always inform the customer that a vending machine service is pointless at this point in time and may possibly aggravate the machine's behavior, as the air trapped by the oil change is churning up system deposits.

3. With cable pull control

The cable control system pressure, the throttle pressure and often also the kickdown downshift.

Rule of thumb: Cable must synchronously with the throttle mechanism resp. be operated with the throttle cable. At idle throttle cable is pulled slightly, not sagging. At full throttle, there should still be slight play on the throttle cable.

An excessively high flow of the throttle cable pull = hard and late switching
Trailing of throttle cable pull = system pressure too low, circuits too early and dragging.

Typical cable control errors:
Cable is kinked, frayed or corroded, the machine switches late and hard, even during downshifting because the cable pull does not return to normal position.
Cable cable torn or latching defective, automatic machine switches.

4. With negative pressure control

Intake manifold vacuum is applied to the vacuum diaphragm (modulator).
When the engine is idling = high intake manifold vacuum (-550-600mBar) = low system resp. throttle pressure
At full throttle position = no (0) vacuum in the intake manifold = max. System pressure resp. Throttle pressure. (equivalent to withdrawn vacuum line)
Too low vacuum = too high throttle pressure, late hard shifting.

Typical problems with the vacuum control:
Vacuum line is damaged and leaking, the vacuum at the vacuum box is insufficient. Automatic switches late and hard (marten bite, cracked tube transitions).
Oil in the vacuum line = the machine turns spongy and imprecise

5. With electronic control

Most systems are self-diagnosed or have a diagnostic connector. Error codes stored in the system are valuable information for finding malfunctions, therefore always read out and delete the stored error codes before the test drive if possible. For many vehicles it is sufficient to disconnect the battery for a few seconds, for other vehicles, the error codes must be actively deleted (from year 2001, the OBD connector is mandatory).
Of course, a defective gear, just because it grinds or because somewhere the pressure is lost and error codes, therefore, here is the motto:


Each system has its own characteristics and yet there are certain similarities. Switching operations are controlled today by solenoid valves.  
There are switching valves, pressure control valves, push and control valves, etc. Each gearbox has its switching logic.

Here's an example:

MagnerVentil / Solenoid   A   B
D1 =   1   0
D2 =   1   1
D3 =   0   0
D4 =   0   1

It can also be seen from this table that each gearbox has an "emergency program". In the event of problems with the machine or in the electronics, the central unit switches the switching valves to 0, in our example this would then be 3rd gear.

Switching commands are issued by a control unit, which evaluates many information from sensors and switches of the entire vehicle.

The most commonly used sensors are: VSS Speed ​​Sensor, TSS Turbine Wheel Sensor, MAF / MAP Air Mass Air Pressure Gauge, FTS Temperature Sensor, TPS Throttle Potentiometer, MLPS Selector Lever Position Sensor etc. In case of misbehavior of one or more of these sensors, the transmission can fall into the "emergency circuit", the pressure is then set to "high", an automatic circuit is usually no longer. Visually, this is partly indicated by a flashing indicator light.

You do not always have time to call? We will call you back as well.