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Old 03-09-2007, 12:40 PM
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ATF Level Thermal Expansion/Contraction Question

Finally concluding that checking the ATF level when HOT is too much of a moving target (who really knows if the TRANSMISSION is at operating temp?), I've decided it makes most sense to check the level when cold. Reason: I know that, when ATF is at "shop temperature" (68-86F), the correct level is approx. 1/2" below the minimum mark on the dipstick.

My question: if it's really cold outside, will the fluid level contract appreciably? Let's say is 20F outside, where would I expect correct level to be? Still at 1/2" below or significantly further down? Do these things behave in a linear fashion?

FYI, I've got a leak I'm trying to keep a close eye on.

Thanks.

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  #2  
Old 03-09-2007, 12:48 PM
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Quote:
Originally Posted by shertex View Post
...........................................
FYI, I've got a leak I'm trying to keep a close eye on.

Thanks.
Your problem is using light seeking fluid.

Here's and idea Could it be the internal parts getting bigger as they heat up, taking up more room,( Increase in displacement.) increasing the fluid level?





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Old 03-09-2007, 04:37 PM
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Quote:
Originally Posted by shertex View Post
Finally concluding that checking the ATF level when HOT is too much of a moving target (who really knows if the TRANSMISSION is at operating temp?), ..

My question: if it's really cold outside, will the fluid level contract appreciably? Let's say is 20F outside, where would I expect correct level to be? Still at 1/2" below or significantly further down? Do these things behave in a linear fashion?
My manual for my 1983 300dturbo specifies 80C. Why not just get a non-contact thermometer and measure it at 80C?

When I had my tranny pan off to to do the filter change, I looked at where the fluid dipstick ended up and where the fluid level inside the tranny must be. The fluid rises from just below the gasket when cold to just above the top edge of it when warm. The valve body sticks down into the pan though, so there's no real good way to estimate the volume around and inside it that is factoring into the level change. Moral to this story? Just measure it at 80C and be done with it
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Old 03-09-2007, 09:04 PM
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Join Date: Oct 2004
Location: Johnson City, TN
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10mm below the add fluid mark when transmission is cold. Don't know about -20 question.

Drive the car for 20 to 30 minutes and the transmission is at temperature. Check the level and fill. Has worked for me on two different 124's. Level doesn't have to be exact.

Now on my '97 S320 with the electronic transmission, level is critical. I have the special dipstick and an infrared thermometer.

Steve

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