I'm so confused. My 2005 Sienna manual recommends automatic transmission fluid replacement every 72 months or 96,000 km and timing belt replacement every 72 months or 144,000 km. My van has only 82,000 km but the dealer where I have done all my scheduled maintenance recommends that I get both done now because of the age of the car. Are these services that I can put off until I get close to the listed kilometres? You have always stressed that car owners follow the schedule recommended in the manual, but it seems there is so much difference between the month and km schedule. – Susan
Generally these recommendations are based on "whichever comes first."
The two issues at play here are age and wear. The transmission fluid acts as not only a lubricant, but a coolant and cleanser. Over a period of time, it may start to lose some of these properties.
The shorter mileage given for this service is due to the fact there are several issues that can contribute to the fluid losing its effectiveness. If your driving habits have been tough on the transmission – especially situations that generate excessive or too little heat – it should probably be replaced soon.
Excessive heat can be caused by towing or continually climbing long hills. Too little heat, from travelling for very short distances, does not allow the fluid to reach sufficient temperature for harmful chemicals and fluids evaporate.
The timing belt is subject to wear and stretching over time, which would cause improper operation of the valves and potentially catastrophic damage. Here, mileage is the most important factor – so this one you could probably leave for a while.
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