Why Doesn't Checking Xmission Fluid Cold Work?

> levels would change. The car is sitting level so the oil should
> drain back in the same way and the oil volume will shrink by the same
> amount at the same temperature.
>
> So, why won't this method of checking tranny fluid work?

Because there is a lot of unpredictability in how the torque convertor
will drain back into the main transmission pan. Sometimes (and in some
cars) it will drain part of its fill back into the pan overnight, and in
other cars it may not drain-back for days or weeks. Or it may drain
overnight one time, and not for weeks the next time. It depends on how
long it takes air to work past the seals around the TC hub, and
obviously as a car ages the TC will start to drain back faster and
faster. And if there are imperfections that seal better when the engine
stops in one position instead of another, it will be different every
time you shut the car off. When the TC does drain back, its not a
trivial amount of fluid either. It can be 2-3 quarts or even more.

When the car is hot idling in park, the transmission's pump
ise in operation, the torque convertor is full of fluid, and
all the accululators and servos are in a known state with a
known about of fluid content. When the engine is off and the
fluid is cold, you can't be sure how much fluid is left in
the torque convertor or any of the accumulators and
accuators. The fluid level in the pan can vary drasically as
a result.

Ed

Ken Hall wrote:
>
> I realize all cars forever have required tranny fluid be checked with
> the engine hot and running, but I can't understand why. I understand
> that the proper fill level must be done this way but I don't
> understand why a know full level doesn't reflect itself in a
> consistently correlated cold engine off level.
>
> If I arrive in my garage from a run that had the car hot and I check
> the fluid as specified and it is at the correct level. Then the next
> morning, after the car has become cold, I check the level on the
> stick, I don't understand why the relationship between these two
> levels would change. The car is sitting level so the oil should
> drain back in the same way and the oil volume will shrink by the same
> amount at the same temperature.
>
> So, why won't this method of checking tranny fluid work?
>
> Please don't explain why the levels are different between hot/running
> and cold/engine off unless it is to explain why the change from
> hot/running to cold changes from time-to-time even when the car
> position and temperature difference are the same.
>
> Ken

> I realize all cars forever have required tranny fluid be checked with
> the engine hot and running, but I can't understand why. I understand
> that the proper fill level must be done this way but I don't
> understand why a know full level doesn't reflect itself in a
> consistently correlated cold engine off level.
>
> If I arrive in my garage from a run that had the car hot and I check
> the fluid as specified and it is at the correct level. Then the next
> morning, after the car has become cold, I check the level on the
> stick, I don't understand why the relationship between these two
> levels would change. The car is sitting level so the oil should
> drain back in the same way and the oil volume will shrink by the same
> amount at the same temperature.
>
> So, why won't this method of checking tranny fluid work?

Several reasons -- 2 main reasons >>1st, engine: off: >> most A/T torque
converters may drain hydraulic fluid back into the A/T reservoir tank. Thus
filling reservoir to an unknown level.

2nd: A/T pump housing uses check valves that may or may not drain hydraulic
fluid back into It's reservoir. Pump uses a filter (owners never change) as well
that may become dirty and or clogged up with crap and thus will not allow any
fluid back into reservoir.



A vehicles automatic transmissions fluid level may change from day to day. I
mean, if you park your vehicle today >> engine: off >> record transmissions
fluid level!! Next day engine: off >> fluid level reading may differ >> may
depend on the weather or!!!!





BBA