All transmission complaints should be verified by road testing the vehicle and duplicating the customer's complaint. A knowledge of the exact conditions that cause the problem and a thorough understanding of transmissions will allow you to accurately diagnose problems. Many problems that appear to be transmission problems may be caused by problems in the engine, drive shaft, universal (U) or constant-velocity (CV) joints, wheel bearings, wheel/tire imbalance, or other conditions. Make sure these are not the cause of the problem before you begin to diagnose and repair a transmission. Diagnosis becomes easy if you think about what is happening in the transmission when the problem occurs. If there is a shifting problem, think about the parts that are being engaged and what these parts are attempting to do.
Always refer to your service manual to identify the particulars of the transmission you are diagnosing. It is also helpful to check for any Technical Service Bulletins that may be related to the customer's complaint.
Abnormal transmission noises and vibrations can be caused by faulty bearings, damaged gears, worn or damaged clutches and bands, or a bad oil pump, as well as contaminated fluid or improper fluid levels. Vibrations can also be caused by torque converter problems. All noises and vibrations that occur during the road test should be noted, as well as when they occur.
During a road test, the transmission should be operated in all possible modes and its operation noted. Your observations during the road test, your understanding of the operation of a transmission, and the information given in service manuals, will help you identify the cause of any transmission problem. If conducted properly, road testing is diagnosis through a process of elimination.
Before beginning your road test, find and duplicate from a service manual, the chart that shows the band and clutch application for different gear selector positions.
Typical band and clutch application chart. This chart should be referred to during a road test and when determining the cause of any shifting problems. Courtesy of Nissan North America.
Using these charts will greatly simplify your diagnosis of automatic transmission problems. It is also wise to have a notebook or piece of paper to jot down notes about the operation of the transmission. By doing this you won't have to worry about remembering all of the details nor will you have to decide if you remembered them correctly. Some manufacturers recommend the use of a symptom chart and/or checklist that is provided in their service manuals.
Prior to road testing a vehicle, always check the transmission fluid level and condition of oil and correct any engine performance problems. Then, begin the road test with a drive at normal speeds to warm the engine and transmission. If a problem appears only when starting and/or when the engine and transmission are cold, record this symptom on the chart or in your notebook. After the engine and transmission are warmed up, place the shift selector into the DRIVE position and allow the transmission to shift through all of its normal shifts. Any questionable transmission behavior should be noted.
During a road test, check for proper gear engagement as you move the selector lever to each gear position, including park. There should be no hesitation or roughness as the gears are engaging. Check for proper operation in all forward ranges, especially the 1-2, 2-3, 3-4 upshifts and converter lockup during light throttle operation. These shifts should be smooth and positive and occur at the correct speeds. These same shifts should feel firmer under medium to heavy throttle pressures. Transmissions equipped with lockup torque converters should be brought to the specified lockup speed and their engagement noted. Again, record the operation of the transmission in these different modes in your notebook or on the diagnostic chart.
Force the transmission to "kickdown" and record the quality of this shift and the speed at which it downshifts. These will be compared later to the specifications. Manual downshifts should also be made at a variety of speeds. The reaction of the transmission should be noted as should all abnormal noises, and the gears and speeds at which they occur.
After the road test, check the transmission for signs of leakage. Any new leaks and their probable cause should be noted. Then compare your written notes from the road test to the information given in the service manual to identify the cause of the malfunction. Typically, the service manual has a diagnostic chart to aid you in this process.