25/09/2016
MECHANICAL PROBLEM SOLVING AND DIAGNOSIS
INTRODUCTION
An automatic transmission is a rather complicated device, and there are many possible reasons for complete or partial failure. When experienced technicians are given a problem, they can often go directly to the solution by basing their diagnosis on past experience. If they are not absolute sure of the case, they will perform the necessary tests to locate the specific cause of the problem.
A sound knowledge of how an automatic transmission operates helps greatly when diagnosing transmission failures. An example is the transmission that does not move on drive but starts in manual-1 and then, if shifted back to drive, operates normally. The power flow is the same in drive-1 and manual-1 with one exception. In drive one, a one-way clutch is the reaction member, in manual-1, the one-way clutch is assisted by a band. In this case, the problem is caused by a faulty one-way clutch. Another example is a transmission that moves in drive but does not up-shift. Knowledge of the transmission yells us that the intermediate band is not applying, but why? In this case, there are several possible faults besides the band, for example, no governor pressure, a modulator or throttle valve fault, or a stuck 1-2 shift valve.
The following is the series of tests and checks that can be used to help determine the exact cause of automatic transmission problems:
§ VISUAL INSPECTION: Involves a quick, preliminary overall check of external factors that affect operation.
§ Fluid check: Inspects the fluid to ensure correct and good condition.
§ Road test: Gives the technician the opportunity to check the actual operation of the transmission, confirm the original concern, check shift timing and quality under different throttle openings, monitor torque converter clutch operation, and listen for unusual noises and vibrations.
§ Electrical system check: Ensures that electronic controls are operating correctly.
§ Torque converter clutch (TCC) test: Determines the TCC is operating properly and isolates the cause of the problem.
§ Hydraulic pressure test: Checks the hydraulic system pressure, which gives a good indication of the condition of the pump, pressure control and shift valves, various seals and gaskets.
§ MODULATOR CHECK: Checks the operation of the modulator and vacuum throttle valve system.
§ Stall test: Loads the clutches and bands to check for slippage; also checks the torque converter stator one-way clutch.
§ LEAK CHECK: Locates the source of a fluid leak.
§ Oil pan debris check: Inspects the oil pan to determine if an abnormal amount of debris is present and, if so, the nature of the debris.
§ Air test: Determines if the seals, sealing rings, and gaskets in the hydraulic circuits are operating correctly.
§ Wet air test: A more precise air test locates the exact location or severity of a leak.
§ Noise, vibration, harshness (NVH) check: Checks to locate the cause of noise or vibration.
When diagnosing a transmission problem, a technician will perform only the check appropriate for the problem. The technician’s experience help’s determine the test that are needed, and the checks are made in the most logical order for the particular problem.
visit.www.usmandikko.blogspot.com
