How to check the transmission

Automatic transmission test drive

Use more caution when test-driving someone else's vehicle. Get used to the brake pedal feel, adjust the mirrors, driver's seat and get familiar with all the controls. Proceed to drive only when you are comfortable. One of the indications of a transmission problem is the delayed engagement, when there is a long delay between the moment you shift the shifter into Drive ("D") or Reverse ("R") and the moment the transmission actually engages.
It's easier to note delayed engagement after a car was sitting for a while: With the transmission in Park position, start the engine and wait until the engine rpm has reduced to normal level (650 - 850 rpm).
With your foot holding down the brake pedal, shift to the Drive ("D") position. Almost immediately the transmission should engage; it feels like the car wants to creep forward. This should happen very smoothly, without a strong jerk or clunk.
Shift to "N" (Neutral) and the transmission should disengage. Now, still holding the brakes, shift into Reverse ("R"). Again, the transmission should kick in almost immediately; you should feel the car wants to creep backward. This also should be very smooth, without a jerk or clunk.
Now, still holding the brake pedal down, try to shift from D to R and back to D; there should be no strong jerks or clunks.
If there is a notable long delay (more than 1 seconds) between the moment you shift and the moment the transmission kicks in, such a transmission might be either too worn or has some problem, avoid this car. It's possible that it could be some minor problem with the transmission electronics, but it's better be more cautious than pay for expensive repairs later; you always can find another car.
Similarly, if you feel a strong jerk or clunk while shifting, the car may have a transmission problem, avoid such a car.

Now it's time to test drive the car.
Accelerate gradually in Drive mode to feel the gear shifting; all shifts should be smooth, without hesitations, jerks or slipping.
You should be able to feel when the transmission shifts by the slight change in the engine tone and RPMs. If the transmission shifts with a strong jerk or jolt, shudder or a delay, it probably has a problem and it's better to avoid buying this vehicle.
Driving at low to medium speed, if you accelerate, the transmission should downshift (kickdown) into lower gear.
The next step: if the car has the Overdrive button, check the Overdrive function.
While driving steady at medium speed, turn the Overdrive OFF; you should feel the transmission downshifting into the lower gear. Turn the Overdrive back ON and you should feel the upshifting; normally the Overdrive should be ON.
Another thing that may indicate the transmission problem is the slipping. If the transmission is slipping - that is when you accelerate, the engine rpm increase but the speed remains the same - it means that the transmission is worn or has some electronic or mechanical problem.
Test-drive for as long as possible; sometimes the transmission may work well when it's cold but would start acting up when warmed up or visa versa.
If during a test drive, you feel anything that doesn't feel right with the transmission, maybe it's better to avoid this car. Examples of transmission problem symptoms include slipping, shifting too harsh, jerks or shudder during shifts, delayed shifting, clunks and other noises.
If the "Check Engine" or any other warning light comes on while driving, have the problem assessed with your mechanic before buying a car.

Manual transmission test drive:

Now, (with caution - safety first!), shift the transmission lever into neutral. Apply the parking brake. With the engine idling, press the clutch pedal all the way, hold it down and listen for noises. Then release the pedal and listen for noises again. There should be no loud noises in either positions.
The next step is the test drive.
Try to drive the vehicle at different speeds in all gears, one by one. Every gear should shift smoothly and easily without any noises or jerks. While driving in the second or third gear, try to accelerate sharply; the clutch should not slip.
If you feel the clutch is slipping (the engine rpm increase but the vehicle speed remains the same), the clutch will most likely have to be replaced, and it's not cheap.
Try to accelerate and decelerate; there should be no grinding, whining or humming noises under any condition. The transmission should shift easy into any gear. Make sure to try the Reverse gear; the transmission should shift easy into reverse; without grinding or extra effort.
Before making your final decision, have the car inspected by a mechanic of your choice. There are many possible issues that only an experienced mechanic will be able to detect during an inspection.
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