Signs of a problem with the shifter linkage include the gear shift feeling less rigid than normal or not aligning properly when shifting gears. Another symptom is when it's no longer possible to change gears. Unusual noises, such as clunking, grinding, humming and whining, are also another indication. Also check for leaking automatic transmission fluid on the spot where you parked your car.
A gear shift that does not feel as stiff as before or has difficulty shifting gears is an indication of a stretched gear cable. In this case, the cable may need to be replaced.
If the transmission refuses to go into gear, possible causes include depleted transmission fluid, use of incorrect fluid type and the need to adjust clutch linkage and shift cables. Other causes include a malfunction in the vehicle's computer system. This can be reset by detaching the battery for at least 30 minutes. When you reattach the battery, the computer system will reset itself, which may take about thirty minutes.
A clunking, grinding, humming or whining sound when you shift gears almost always mean a problem with the shifter linkage. Have this type of problem inspected and serviced immediately.
Leaking automatic transmission fluid is a relatively minor problem that only requires plugging the leak, but if the fluid is dark and smells burnt, some repairs may need to be done in the shifter linkage assembly.