Why Measure Trans. Fluid with Engine Running?

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On another forum I visit, someone claims they check their transmission fluid the same way they check their engine oil. Engine not running.

Please explain to me the technical reason why this does not work. If it is does not give consistent results, why? I'm aware of the fluid level sensitivity to temperature.

Thank you.
 
After you start the car, the fluid gets drawn up into the torque converter and the rest of the transmission.

If you check it with the engine shut off, the level will be much higher than the normal operating range on the dipstick.
 
Originally Posted By: rustypigeon
After you start the car, the fluid gets drawn up into the torque converter and the rest of the transmission.

If you check it with the engine shut off, the level will be much higher than the normal operating range on the dipstick.

Then why not adjust the marks on the dipstick to reflect the proper (Higher) level with engine off?
 
To service the transmission fluid in my car:

1. Fill the pan until fluid runs from the filler tube.

2. Start the car and run through gears P, R, N, D, and 1-6 in tiptronic mode.

3. Fill the pain when the fluid is between 30-40° C under measured values until it begins to run from the filler tube.

4. Drive the vehicle and ensure the measured value reaches at least 90° C.

5. Let transmission fluid cool back down to 30-40° C and perform one more filling.

The point here is, measure the transmission fluid the correct way, not the way that you feel is correct.

Also, i envy those with dipsticks!
 
Originally Posted By: doitmyself
Originally Posted By: rustypigeon
After you start the car, the fluid gets drawn up into the torque converter and the rest of the transmission.

If you check it with the engine shut off, the level will be much higher than the normal operating range on the dipstick.

Then why not adjust the marks on the dipstick to reflect the proper (Higher) level with engine off?


Because fluid continues to drain! If you let your car sit with the transmission pan off for a week, 75ish percent of the fluid will drain.
 
Originally Posted By: doitmyself
Originally Posted By: rustypigeon
After you start the car, the fluid gets drawn up into the torque converter and the rest of the transmission.

If you check it with the engine shut off, the level will be much higher than the normal operating range on the dipstick.

Then why not adjust the marks on the dipstick to reflect the proper (Higher) level with engine off?


Simple. It takes a while for all the transmission fluid to drain back into the sump. So a reading shortly after the engine is turned off can be significantly different than one that has been sitting for an hour, and different still for one that has been sitting all night. That's why the procedure to check the fluid can be different depending upon the transmission-some require it to be hot in park, some hot in neutral, some require it to be run through all the gears, etc. The same amount may not get drained back to the sump every time, but the same amount will be in the torque converter and rest of the transmission when it's checked according to the manufacturer's requirements, so it's the best way to get an accurate reading.

It would be a bit annoying to have to wait for hours in order to get an accurate reading on the transmission fluid, and transmissions can be more sensitive to low or high fluid levels than an engine is with engine oil.
 
Originally Posted By: rooflessVW
To service the transmission fluid in my car:

1. Fill the pan until fluid runs from the filler tube.

2. Start the car and run through gears P, R, N, D, and 1-6 in tiptronic mode.

3. Fill the pain when the fluid is between 30-40° C under measured values until it begins to run from the filler tube.

4. Drive the vehicle and ensure the measured value reaches at least 90° C.

5. Let transmission fluid cool back down to 30-40° C and perform one more filling.

The point here is, measure the transmission fluid the correct way, not the way that you feel is correct.

Also, i envy those with dipsticks!


And therein would be my point: each manufacturer could do it differently. Thus read the manual. Who knows, perhaps there is a manufacturer which specifies their automatic to be checked with the engine off.

IIRC Mopar transmissions used to be in neutral for this test. The trans pump wasn't running in Park or somesuch. My Tundra needs to be at 115F or so, but first I need to insert a pin to force the ATF thermostat out of bypass mode so that the ATF cooler is in use. Still others need a computer to check level.
 
Honda says check not running at operating temp. And, wait at least 60 seconds but no more than 90 seconds after engine shut down.
smirk.gif
 
The amount that drains back, and thus the pan level, from the complex fluid path, including valve body, servos and torque converter varies with fluid temperature, time, and even individual transmission casting.

With the transmission off, you simply have no idea if what you're measuring is accurate. It certainly isn't repeatable, even if left overnight, and it varies across individual units.

In the Tundra example, you have to bypass the thermostat to ensure that the cooling line and cooler are full in order to get an accurate reading, they might have drained back and the reading would be off.

Just do it the way that the manufacturer tells you...
 
I hazard a guess that it has to do with dimensional ratio R of oil level surface area to oil height for similar volume of fluids.

Transmission has far lower ratio.
 
Last edited:
Originally Posted By: old1
I have, on my older cars, just remarked my dipstick with a cold not running mark. Has always seemed to work for me.


+1 me too.
 
When I do D&F's on cars with drain plugs I measure the amount of fluid drained out and put about the same amount back in. I use two empty windshield washer fluid jugs - one for the old fluid, one for the new. Seems to have been close enough for me.
 
Originally Posted By: slacktide_bitog
Transmission dipsticks are never accurate anyway, except Honda.
Could not say if any of my cars dipsticks are accurate. Never emptied any Auto transmissions out fully and measured what came out. So I do not know if Honda is more accurate. None of them broke due to fluid high or low. My Honda AT dipstick is a bit hard to read. Seemed accurate enough like the other cars but transmission still failed and the transmission fluid ended up smelly and black and just the right level on the dipstick a few seconds after shutting engine off.
 
Ford Falcons used a Borg Warner 3 speed auto up until 1988 or so, the level was checked within 10 seconds of turning the engine off.
 
BW35/45....so why didn't other uses of that trans use the same method for checking the oil? Chryslers dumped the converter in park, so were always checked in neutral...and the same with some early Mitsubishis.
 
Dunno Silk, it's the way Ford wanted it done though, it's even printed on the dipstick! I know the BW was used in a few other cars (Sigma, some Volvos I think?) but I only dealt with them in Falcons and Cortinas.
 
Originally Posted By: slacktide_bitog
Transmission dipsticks are never accurate anyway, except Honda.


Interesting. Servicing a friend's neglected 2005 Buck Lacrosse with 4T65E transmission with several pan drop & filter changes, it's taken right at 7.5 qts. to refill it to the full operating mark at operating temperature. Pretty much as predicted. Seemed pretty accurate to me.

http://www.lubegard.com/pdfs/2015_LUBEGARD_Conv_Chart_fnl.pdf

My Ram with 68RFE however also took 7.5 qts. after pan drop & filter change, significantly more than predicted, so it depends on the quality of info used to predict the refill amount I think.
 
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