604 transmission with a 740 code led to many different approaches and attempts to troubleshoot the underlying problem. As the possible causes were narrowed down, the focus began to turn toward the converter. Replacing the converter with a factory unit eliminated the 740 code, and since the code would show up on any road test, this seemed like the perfect vehicle to identify the root cause.
Time after time, the faulty converter was removed from the vehicle and one change was made or a part replaced before the converter was placed back into the vehicle. After many days and many more R&Rs, the problem was found to be poor converter flow, caused by a solid fiber thrust washer between the cover and turbine hub. Finding the root cause was time consuming, but the information discovered will prevent the same mistake from being made repeatedly in the future. Some other information was also discovered during the process of inspecting the insides of many low-mileage converters in a short period of time. One was the uneven wear pattern on the new friction material. The 740 codes were proof that a TCC slip had occurred, but the technicians expected to see an even wear pattern down the center of the friction material. However, none of the converters had this wear pattern. Most converters had a dark wear pattern at either the inside or outside edge of the friction material (see Figures 1 and 2). The mystery of why the 740 code happened may have been solved, but several new questions had come up.
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Article No.: TCTIP-01-06
Author: Ed Lee
Total Pages: 3
©2006 Sonnax Industries, Inc.