UNDER THE HOOD

When to flush your transmission fluid

Q:How can you tell when it is time change transmission fluid?

We have a 2010 Ford Escape with 62,500 miles. We’re the original owners and have operated under normal driving conditions— no towing.

I took the car into the dealer for an oil change. At pickup, the service center employee went over the mechanic’s report with us. It said the transmission fluid was dirty and needed to be flushed.

I came home, looked in the manual that came with the car and found that Ford says the 150,000-mile mark is the right time to change the fluid.

Two weeks later, I took the car to the dealer again, this time for a brake job. I stopped by the service manager’s office to the conflicting information from the mechanic and manual. He looked it up and agreed that we could wait until 150,000 miles, noting that the Mercon V fluid used in the truck is “naturally dark” and that there was “no need to change it.”

But again, the mechanic who did the actual work suggested a transmission flush.

So, who’s right? And how can you really tell when transmission fluid needs to be changed?

—Allen, Morgan Hill,

Calif. A: It's tough to say without being able to observe the fluid's color and odor.

Stained brown fluid with a burnt odor indicates trouble is looming. However, some darkening is normal.

To sort things out, use a paper towel to compare a dipstick drop with a drip of Mercon V fluid fresh out of the bottle. Also take note of the odors.

If it were my car, I'd take advantage of the transmission's convenient drain plug, replenishing the 3 quarts or so that will come out. The total capacity is 9 quarts.

Do it again after a good drive, and perhaps one more time later.

This will have the nearly the same effect as a professional flush without the big price tag.

The internal filter can't be renewed without splitting the transmission case sections. I'd renew the fluid at 75,000 miles regardless of Chevy's longer specified interval. Same goes for long-life spark plugs, fuel filters and other components—replace them before it's absolutely necessary. It's cheap insurance.

Q: Brad, I need your help identifying what’s causing the clunking noise in my 1987 Mercedes 300D. It has an automatic transmission and more than 217,000 miles on it.

When shifting, it makes the noise. It’s worse at slower speeds.

I had it in a shop, and they replaced a bad left engine mount and a center driveshaft support bearing. They said the front and rear flex discs were good. I need a suggestion on what to have checked next. Thanks for your help.

—Al, Morgan Hill, Calif. A: Two letters with the same name and town on the same day!

Do you hear the clunk when engaging in drive, or perhaps in reverse?

I'd start with a close look at the rear transmission mount. Use a bar or jack to stress the rubber block. Carefully listen—perhaps using a stethoscope—to the driveline parts with the vehicle elevated as a helper accelerates and slows the driveline between zero and 35mph.

I'd also want to look at, feel and stethoscope all driveline parts while my assistant shifted between park and reverse and between park and drive. And again as he loaded up the driveline, throttling against the brakes, in gear.

I'd also check the rear suspension bushings as this model squats and lifts a lot with torque changes.

Your flex discs are an oldschool alternative to universal joints. Keeping an eye out for wrap-up while under torque could yield a surprise.