Dear Car Talk:
I have a 2019 Honda Odyssey. At somewhere around 30,000 miles, my owner’s manual says I will need to change the transmission fluid. The Honda dealership says that they only change a portion of the fluid, for whatever reason
That makes no sense to me. A partial change is like doing a half oil change on your engine. I asked about the shops that have a machine that hooks into the transmission line and pumps all the old fluid out and new fluid in. The dealership said that was not a good idea because junk in the transmission could clog the lines.
That much junk, after 30,000 miles? I will probably go to a shop that uses the “full change” method, but am curious what you think. -- Allan
Well, for most transmission fluid changes, the machine works great. You hook it up to the transmission cooler lines -- one going in and one going out -- you add the correct amount of new fluid to the machine’s reservoir, then you start the engine and the machine pulls out all the old fluid and replaces every bit of it with new fluid.
And, generally speaking, you’re right, Allan. It’s better to change all of the fluid than some of the fluid.
Imagine we were talking about your coffee cup at work. If you found it half full of cold coffee on a Monday morning, would you just add half a cup of new coffee? Or would you pour it out and add all new fluid.
I rest my case, even though I just disgusted myself.
But actually -- and unusually -- on this car, I would use both methods. Here’s why: Honda is unique in that they recommend a transmission fluid change at only 30,000 miles. They also recommend that at each fluid change, you clean off a magnet that’s attached to the drain plug. And that’s probably the most important part of this required maintenance.
The magnet at the bottom of the transmission collects all the metal shavings that are worn off the gears, clutches and bearings. The magnet takes hold of those shavings and prevents them from circulating, and doing damage to other internal components, or clogging the small internal fluid passages in the valve body.
And here’s the problem. If you just use the machine, or use it first, you could force those shavings off of the magnet and recirculate them, either clogging those passages, or leaving the shavings suspended in the fluid again where they can clog things up later.
So ideally, you’d use both methods. Drain the transmission by removing the drain plug and cleaning the magnet like they tell you to, and then hook up the machine to change all of the fluid
On the other hand, if I were just going to use one method or the other, I’d probably opt for the drainplug on this vehicle.
Honda did have some transmission problems, and I’m guessing that the magnet -- and the cleaning of the magnet -- is a key part of their solution. So at least get the magnet cleaned. And if you’re feeling fastidious, use the machine after that.