Why don't 4S shops use circulating machines to change transmission oil?
2 Answers
Because the process requires a large amount of transmission fluid, which is costly and time-consuming. There are three methods to change transmission oil: 1. Gravity oil change: Similar to changing engine oil, this method is the most convenient and economical, generally requiring only 3 to 5 liters of transmission oil. The disadvantage is that gravity draining is not thorough, removing only about one-third of the original oil. 2. Circulating machine oil change: A transmission circulating machine is connected to the transmission's oil circuit interface, with new oil added inside the machine to push out the old oil from the transmission. This method requires about 12 liters or even more transmission oil. The more oil used, the cleaner the transmission becomes. The advantage is thorough cleaning, while the disadvantage is that it takes more time. 3. Removing the transmission oil pan for oil change: The advantage is that the transmission oil filter can be cleaned, and the oil change is relatively thorough.
Having worked as a car mechanic for over a decade, I often chat with 4S shop technicians about this. The transmission fluid exchange machine sounds fancy, but shops actually have their own calculations: First, labor time is too precious – the machine takes 40 minutes per job, while gravity drainage just requires loosening a drain plug and finishes in 15 minutes, allowing three more cars to be serviced in the same bay. Then there's equipment cost – imported exchange machines start at 40,000-50,000 yuan, whereas gravity changes just need an oil pan. Risk is another factor – older cars with sludge buildup may have valve bodies clogged by high-pressure flushing. Last year our shop encountered a case where error codes appeared after machine flushing, ultimately requiring us to replace the customer's entire valve body assembly at our cost. That's why 4S shops prefer conservative methods – after all, manufacturer maintenance manuals specify gravity changes, and following standard procedures is the safest approach.