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Old 03-30-2020, 01:39 PM   #1 (permalink)
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Fact or Error? Don't change old automatic transmission fluid

My daughter drives an '06 BMW 325i. (Yes, apples can fall quite far from trees.) We do all the maintanence/repairs together. At 120,000 miles it has a less-than-smooth shift habit, intermittent, between first and second and second and third. Shifts slightly hard. Other times normal and smooth.

Fact or error? Changing out the AT fluid will damage the transmission further?

I have heard mechanics I admire, like the YouTuber EricTheCarGuy, say it is a fact (sometimes). My neighbor insists it is true. I do not believe it. I cannot conceive of a situation in which replacing polluted oil that has poor lubricating and heat resistance qualities with new and better functioning oil can somehow do damage--unless the transmission was so far gone already that it was about to fully fail. My daughter's is suffering a mild shift bump, not a major problem of slippage and random shifting and high revving...

Your thoughts welcome...

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Old 03-30-2020, 01:58 PM   #2 (permalink)
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I have always kept the transmission fluid Clean in my wifes in my wife's old Hyundai clean.
I think it for changed 4 or 5 times over 260,000 miles.
If "changing the fluid caused the transmission to fail" it was going to fail anyways.
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Old 03-30-2020, 02:26 PM   #3 (permalink)
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I am not a believer in the myth that clean oil will damage a transmission.
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Old 03-30-2020, 02:45 PM   #4 (permalink)
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That isn't even a myth, it's the opening line of a "sinsha". Sinsha changed my oil, the engine's making this noise... even though I haven't changed the oil in 20k miles...
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Old 03-30-2020, 03:28 PM   #5 (permalink)
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The basis behind this is that the old fluid contributes to clutch friction(less slippage) due to the contaminants. If new fluid causes it to slip, it was already done.
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Old 03-30-2020, 04:57 PM   #6 (permalink)
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Just what I've read:
When shops change the fluid they typically use a pressurized pump that goes in the opposite direction of flow when the engine is running. This knocks debris loose and causes this issue.

Whether this is true I don't know, but if you want to change it I would recommend disconnecting a cooler line and let it pump a quart or two out. Turn the car off. Add the same amount. Rinse and repeat until the fluid coming out of the cooler line is clean.

I don't know if this applies to all A/T's, because of the cooler location in the flow path.
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Old 03-31-2020, 01:48 PM   #7 (permalink)
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That's what I thought, too. "Error."
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Old 04-01-2020, 02:53 PM   #8 (permalink)
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ATF

All I can recall about ATF has to do with 'burned' fluid.It's been reported in the past that,for instance: when stuck in a snow bank,simply switching from reverse,to drive,ten times simultaneously,attempting to extricate the vehicle is enough to 'burn' the fluid,after which it would compromise lubricity as well as all clutch pack performance,necessitating immediate flushing and replacement.It's only a 10-weight fluid to begin with,and any compromise to viscosity could endanger gear faces with high-pointing,heat-welding asperities,fracture,and release of metallic shards circulating within the galleys.
Also,should the trans cooler heat exchanger within the radiator breach,coolant will enter the transmission,and clutch bonding agents,while not oil-soluble,are water soluble,and would ultimately detach unless the leak was fixed and fluid flushed and replaced.A very bad day!I've seen this on a Toyota Corolla.

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