Re: "Transmission Flush": Needed ?

Ask them if they drop the pan and replace the filter.[1] This is
important because good shifting depends on good flow through the filter
and... well, filters get dirty; it's what they *do*. It's also an
opportunity to wipe the layer of pond scum from the bottom of the pan.
(Unless it has big chunks and/or is there in vast heaps, or is
contemporaneous with gross transmission misbehavior, this is assumed to
be from normal wear.)


A flush that *doesn't* involve that, usually accomplished via the
transmission-cooler lines, changes the fluid, which is good, [2] but
does nothing about the status of the filter or about the stuff at the
bottom of the aquarium.

I personally prefer to drop the transmission pan every few years,
change or clean the filter or screen (whichever it has, if either), and
refill with however much fresh transmission fluid is needed to make up
for this partial drainage. The disadvantage of this is, it leaves a
fair bit of the older fluid in the system.

Nowadays (plus or minus model- or transmission-specific caveats) GM
often seems to suggest this at 50,000 miles for severe service, 100,000
for normal service. Back in 1997, Buick apparently billed these
transmissions as requiring no maintenance under "normal" conditions:
http://www.buickclub.org/BMD_PR/1997overview.htm

But then again, long maintenance intervals are widely regarded as a
selling point. I'm betting that a good mechanic with field experience
knows a thing or two (almost always skewing toward the conservative
side) that didn't appear in press releases. If done decently well it
shouldn't hurt, and might well help (as evidenced directly by smoother
shifts closer to the expected points, and indirectly by longer system
life).



Cheers,
--Joe

[1] Some transmissions have a screen that you clean, not a filter; and
I guess some have none at all. And some have adjustments that get
tweaked while you're at it (in a precise fashion according to the
maintenance manual). I am guessing that your car has a filter and no
such adjustments.

[2] Since it doesn't have an interface with the combustion process,
transmission fluid doesn't get nearly as nasty as quickly as motor oil
unless there is something grievously wrong with the transmission, and
is hardly consumed at all barring a leak; but it does oxidize, and the
properties that it's supposed to have degrade, so in addition to
occasionally keeping an eye on its level, it helps to change it as a
long-term maintenance item.

.

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