When is it too late to change automatic transmission fluid?

I've heard that once you reach a certain mileage, it is better to keep the original transmission fluid instead of changing it.. Is this true? If so, when is it "too late" to change the auto transmission fluid?
(in relation to what's best for the engine, not warranty)
I ask because I recently changed the fluid for the first time on my 78,000 mile Sonata (2000, V6). Before the change, the car was getting stuck in gear, not shifting well, and the crank sensor code came up during a scan. Now, it doesn't even start. I've ordered a new crank sensor and really hope that changing the tranny fluid didn't screw up the engine beyond the damage that was already there...
..Thanks you.
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Dmitry of NJ wrote:

Changing the transmission fluid cannot affect the engine. The problems are most likely unrelated, unless they're electrical. The engine and trainsmission are both computer controlled and could malfunction due to voltage irregularities and/or bad connections.
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Right. What I meant was: How does changing the transmission fluid at a late stage (in my case at nearly 80,000 mi) affect the transmission itself?
... I've heard that doing this dislodges dirt and can be worse than keeping the original fluid, but would like to hear from people who have experience with engines and Hyundai's in particular.
Sorry for the misstatement though- I understand what you're saying but it's a bit different than what I was thinking of in my post.
Best, Dmitry
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You never really know it was too late until you find out the hard way. Realistically speaking, though, you've only provided an acceleration to a trans failure which was going to happen anyway.
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Wow.
..... so you're telling me the mechanic at my dealership had me "accelerate" a trans failure by advising a fluid change?
I really, really hope that the reason my car isn't starting is the busted crank sensor.
Hyundaitech- your post scares the crap out of me, and if you're right then I will go absolutely crazy on the guy who told my to do this, esp if a new trans is not covered by warranty.
best, dmitry
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Well, I think you're looking at it incorrectly. By your own admission, the transmission was already not operating properly. If your fluid was in poor condition, changing it could have very well have had the opposite effect of helping the situation. The real acceleration of trans failure came when the car was driven 78k without changing the fluid.
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On Tue, 29 Nov 2005 11:46:43 -0500, hyundaitech wrote:

Interesting (to me). My 1990 (bought new then) Excel GL with 110 thousand miles, still with the original transmission fluid (check levels but never added or replaced) operates perfectly. I wonder if it's because I let the dirt stay in there and don't disturb it. :O)
-- Throttle Vamp
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Dmitry of NJ wrote:

Old wives tale. Keeping bad fluid in there is never a good thing.

The tranny fluid certainly didn't hurt the engine. Something else is wrong, probably that sensor.
John
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