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Diagnose Automatic Transmission Problems Part 3

What you can do to diagnose an automatic transmission problem.
» Engine/Transmission Relations
» Transmission or Transaxle?
» Transmission Testing
» Transmission Leak Tests
» Diagnosis By Symptom

Air Pressure Tests

To test the bands and servos inside the transmission, air pressure can be substituted for fluid pressure. When compressed air is applied to certain ports, it will apply the particular band or servo. When a band or clutch engages a dull thud will be heard. There should be no hissing heard while band or servo is engaged. This can only be done by a repair shop.

Line Pressure Test

Transmissions need to operate with the proper fluid pressure. If the fluid pressure is too low or too high, shifting can be too soft or too hard. Different parts of the transmission operate with different fluid pressures and a transmission pressure gauge can be attached to a transmission to check these pressures. These readings can be compared to a chart that will pinpoint the most likely cause of a problem.

Shift Point Road Tests

Each shift in a transmission is supposed to occur at a specific time under certain throttle conditions. For example under light throttle a 1-2 shift will take place sooner then a 1-2 shift under heavy throttle. By know when the shifts take place and when they are supposed to take place you can figure out what is wrong with it.

Stall Tests

This test should be performed by a shop, but if you can locate the stall speed specification, you can do it yourself. Stall speed is when the engine rpm gets as high as the transmission will let it. The stall test checks converter one-way clutch operation and installation, the holding ability of the forward clutch, reverse clutch, the low and reverse bands, the planetary one-way clutch, and engine performance. Perform test with engine coolant and transmission fluid at operating temperature. To perform a stall test firmly apply service and parking brakes; with transmission in ranges D, 2, 1, and R, press the accelerator to the floor and hold it just long enough to let the engine reach full RPM. NOTE: Do not hold throttle open for more than five seconds at a time or transmission damage may result. After each range, move selector lever to N and run engine at 1,000 RPM for approximately 15 seconds to cool the converter before making the next test.

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Additional Information provided courtesy of and Warranty Direct
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