What can go wrong with my Automatic Transmission

Transmission Fluid

Transmission fluid is one of the most important components of any automatic transmission.  An automatic transmission creates a lot of internal heat through friction. The friction of the fluid churning inside the torque converter, friction created when the clutch plates engage, and the normal friction created by gears and bearings carrying their loads. This in turn will reduce the effectiveness of the transmission fluid and will start to reduce the serviceability of the transmission.

Torque Converter

All modern vehicles with automatic transmissions are fitted with “lockup torque converters”.  The purpose for this type of torque converter is to reduce fuel consumption.  However, lockups do have their own problems, with symptoms of failure including noticeable gear slippage and a pronounced shudder when shifting.

Oil leaks

Automatic transmission fluid has a very low viscosity. This makes automatics very susceptible to leaking. There can be a number of reasons for transmissions to leak fluid. Common places for transmissions to leak are

  • Transmission sump / gasket
  • Extension housing seal / bush
  • Torque converter / seal / bush
  • Selector shaft / seal
  • Oil pump

In some automatic transmissions there are up to 20 different places that a leak can develop from.  If a transmission has an oil leak, it can cause slippage on the clutch application which will cause the transmission to become hot and burn out.  The transmission fluid then becomes overheated, which can cause electrical components to fail.

Electrical Components

All modern automatic transmissions are electronically controlled. Transmissions use a number of different components in the operation. Shift solenoids are used to energize the clutches and bands.  Pressure solenoids are used to control pressure to the clutches and bands.  Some pressure solenoids become contaminated and start to stick which causes high pressure and very firm shifts

These components can fail, in two different ways.

  • Electrically :-   They short out
  • Mechanically:- They block up with contamination from the fluid.

There are a number of different electrical components that have a controlling factor in the operation of the transmission (TPS switch, Inhibiter switch, speed sensors, pulse generators, knock sensors, temperature sensors etc.).  All these components have to work in unison so that that the transmission can operate at its best performance and durability.  

ECU (Electronic Control Unit) i.e computer.

The ECU controls the whole gear shifting operation of the transmission and can fail in a number of ways.

  • Blown power fuse to the ECU
  • Shorted circuit board in the ECU (due to jump starting vehicle)
  • Uncalebrated ECU (due to replacement of battery) etc …

Mechanical Components

In an Automatic Transmission there are numerous amounts of mechanical components. The common one that most people have heard of are the clutch’s & bands. There are many, many more, from accumulators to actuators, and oil pumps. Some transmissions have over 800 parts in them which makes them a very complex piece of machinery.   


 The radiator has a separate section for the transmission oil to be cooled.  It is very important that the flow is at full capacity.  If there is a slow flow due to the cooler failing, it will not allow the various transmission parts, such as clutches, bands and bushes, to be lubricated.  This will cause the transmission to burn out. Water can also leak into the cooling side of the transmission which is caused by not changing or flushing out the radiator on a regular basis.  Oil mixed with water will cause the oil to discolour. A transmission which is contaminated with water will not work well.

Filter blockage

If a transmission has not been serviced for a long time, the filter can clog up with material from normal wear and tear which will cause the transmission to be starved of oil.  This will result in the loss of "drive" and "reverse".

These are only a few examples of what can go wrong with your transmission.