Sorting out confusion on transmission flushes

  • Article by: PAUL BRAND
  • Updated: June 3, 2011 - 4:07 PM

Q I've seen where you recommend a transmission flush every 30,000 to 50,000 miles, and your Alldata automotive database even shows this at 15,000 for some vehicles. How is that reconciled with what the owner's manual says? For my Dodge Caravan, the recommended interval is 60,000 miles if you tow or provide taxi or police service. Otherwise, it says 120,000 miles. For my Ford Escape, the interval is 150,000 miles. Why does the owner's manual say this, the dealer says something else, and other shops have their own opinions? In my case I'm thinking 120,000 and 150,000, respectively, but there must be some middle ground between where I'm paying hundreds of dollars unnecessarily and where I still do enough to keep the vehicles going.

The second question is about the term "flush." My Dodge and Ford dealers both say you need the fluid changed (drop the pan, change the filter, replace the pan, top the fluid) and to never ever flush the transmission or it will cause a failure. I recently had a flush done on my Caravan at 66,000 miles using the transmission pump via the cooler lines to slowly replace the old fluid with new. It seems to me this shouldn't be a problem, but when I mentioned it to the dealer service guy, he berated me for ruining my transmission. What kind of flush do you think is best, and why do the dealers say this is wrong?

A Excellent questions -- thank you. And wouldn't you be surprised to be pulled over by a "soccer mom" squad car?

My thinking on all fluid changes on an automobile is framed by a single question: Who owns, pays for and is responsible for the vehicle, the owner or the carmaker? Secondly, fluid changes and flushes done before significant depletion, deterioration or oxidation of the fluid is cheap insurance against failure. It is for precisely this reason that carmakers recommend more frequent changes under "severe" service conditions. $150 to $200 for a transmission fluid change or flush sounds much better to me than $2500 for a new transmission.

There is evidence of problems caused by automatic transmission fluid changes or flushes at high mileage. Specifically, the detergent action of fresh automatic transmission fluid can dissolve or loosen oxidized deposits in the unit's hydraulic system, blocking the flow of fluid. That's why I suggest the 30,000-50,000-mile fluid flush intervals.

In terms of method, dropping the transmission pan for cleaning and installing a new filter, then topping up the fluid, is not a fluid change. It's a filter change. Exchanging 100 percent of the old fluid for new fluid via the transmission pump and cooler lines is a fluid flush that will remove old fluid and contaminants from the transmission -- as long as it's done regularly.

I need to point out that many modern automatic transmissions are sealed and not user-serviceable. These require specialized equipment at dealerships and professional shops and are obviously more costly.

Q Can you suggest a fix for our "squeal"? We have a '99 Olds Bravada, with the 4.3-liter engine, with a high-pitched squeal in the engine compartment. I have tried two different new serpentine belts, replaced all equipment that is attached to the belt, and the pulleys. The only pulley not replaced is the smooth pulley on the water pump. It's driving my wife nuts!

A Start by identifying whether the squeal is due to belt slippage or a pulley and bearing. While taking care to keep fingers and clothing away from the belt, with the engine running and the "squeal" squealing, spray aerosol brake cleaner directly on the ribbed side of the belt first, then the smooth side right where it contacts the smooth water-pump pulley. Did the squeal stop? If so, it's slippage or misalignment. If not, it's one of the ancillary devices driven by the belt. To identify which, carefully place the probe of a mechanic's stethoscope -- or a long screwdriver or dowel rod -- on the body of each device driven by the belt and listen for the squeal.

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