The automatic transmission, the most complex mechanical component in your vehicle, is directly connected to the engine, emission and computer control systems. These systems, which combine to form the power train, are so interconnected that a malfunction in the transmission can appear as an engine or electrical problem. Worse yet, a fault in the engine, computer or electrical system could be diagnosed as a transmission problem. To avoid a misdiagnosis, a technician needs to invest quality time in your vehicle. So please be patient. They must do a thorough job so that you can be sure your vehicle has been accurately diagnosed. To understand what your technician is doing, we've outline the basic checks he must perform in order to make an accurate diagnosis.
Identify the Problem:
Diagnosing and repairing your vehicle is the top priority. In order to do that, however, the technicians need to identify the cause of your complaint. That's why their diagnostic procedure includes a few simple questions and a road test. The purpose of the road test is to duplicate and verify your complaint while evaluating the overall condition of the drive train.
Check Transmission Fluid:
The automatic transmission will not operate correctly unless it contains the correct amount of transmission fluid. If the fluid level is low, hydraulic pressures will drop and eventually cause serious damage to internal components. By checking the fluid level the technicians can determine whether your cars' malfunction may be due, in part, to a low fluid level, which is often times caused by a minor leak.
Too much fluid can also cause serious problems. When
the fluid level is high enough to reach the spinning components inside the
transmission, the churning action traps air in the fluid, causing it to foam.
"Foamy" fluid increases oxidation and can reduce pump efficiency which
could reduce internal pressures and cause serious problems like band and clutch
To check the fluid level, the engine must be warmed up
completely and idling in either Park or Neutral, depending on the vehicle. If
you would like to learn how to check the fluid level, ask your technician to
show you the correct procedure.
Check Fluid Condition:
The condition of the fluid can reveal a great deal
about an automatic transmission. When the fluid is clean and red it usually
means that the unit has not sustained any internal damage and that it has not
been exposed to excessive heat.
If the fluid is still translucent but is slightly
brown, it is worn and needs to be replaced. If, however, the fluid is either
brown and smells burnt or is no longer translucent, it indicates exposure to
excessive heat, and a serious problem with either the cooling system or the
When the fluid is pink and has the consistency of a
strawberry milkshake, it means that water has entered into the transmission. In
most cases, water in the unit means that both the transmission and the cooling
system will have to be repaired.
A sure sign of a serious transmission problem is black
transmission fluid. When the fluid is black, it indicates a serious internal
Engine-to-Transmission Communications Link:
For an automatic transmission to operate properly, the engine and transmission must communicate with each other at all times via a communications link and the vehicle's computer. This link, which can vary depending on the vehicle, the manufacturer and the transmission, can be a cable, a linkage, a vacuum line, and electric sensor or a switch. The link tells the transmission what the engine is doing so that it can adjust accordingly. When the engine isn't working hard - under light load or light acceleration - neither should the transmission. Conversely, if the engine is under heavy acceleration or under heavy load, the transmission must respond accordingly.
When a poor communications link exists, it can cause
hard or soft shifts, slipping shifts, early or late shifts, no shift at all or a
torque converter clutch malfunction. And because any of these conditions can
cause a premature transmission failure, a thorough check of the communications
link should never be overlooked.
Check Electronic Data:
Most vehicles on the road today are equipped with a variety of sensors and electronic data gathering devices which include an onboard computer, throttle position sensor, MAP and BARO sensors, oxygen sensor and coolant temperature sensor. All of these electronic controls send and receive information which can affect the performance of both the engine and the transmission. As a result, specialized diagnostic tests and equipment may be needed to check these and other components to determine whether or not they are contributing to the problem.
Part of any complete diagnostic procedure includes documenting the vehicles' performance and condition. An accurate, well documented diagnostic procedure is critical to technicians who may work on any number of different vehicles during the course of the day. Proper documentation is also invaluable when ordering parts, sourcing technical information, and for future reference.