During diagnosis, always verify the customer complaint. If the verification includes a test drive, be sure to check the level of ATF first. This will ensure that a low level is not contributing to the problem and give you an idea as to the condition and service that the vehicle has seen. Although preliminary checks suggest making adjustments, drive the vehicle before any adjustments in order to experience the same condition as the customer. If you are unable to verify the problem, ask the customer to accompany you on the test drive and point-out when the condition occurs.
When test driving a vehicle, have a plan and record your findings. The chart that follows is quite thorough and provides room for comments. Rather than trying to remember the results of a specific test, simply refer to the diagnostic form. Not only do you want to find out what has failed, but also what is functioning properly. Armed with this information, you will save time in your diagnosis and be more thorough.
Road Test - Automatic Transmission
For example, if the transmission does not slip while accelerating from a stop with wide open throttle, line pressure is sufficient. If shift points occur at the proper speeds, throttle pressure and governor pressure are sufficient. Or for ECT transmissions, throttle sensor and speed sensor inputs are being received by the ECU and the circuit and solenoids are working properly.
Upshift quality is important to consider during the road test because it is an indicator of proper line pressure and accumulator operation. If all upshifts are harsh, it indicates a common problem such as line pressure and should be verified with a pressure test. If a harsh upshift is evident in a specific gear, check the accumulator which is associated with the holding device for that specific gear.
Following the road test, compare your findings with the troubleshooting matrix chart in the repair manual. (An example can be found on page 15.) The matrix chart will assist you in identifying components or circuits which can be repaired while the transmission is mounted in the vehicle. Or identify the components which should be inspected with the transmission on the bench.
Based on your diagnosis, if the transmission can be repaired with an on vehicle repair, the off-vehicle repair should be attempted first. Should the transmission require removal from the vehicle, a remanufactured transmission should be evaluated against the cost of an in-house overhaul.
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