Automotive and transmission folks...

[230RN]

Why are such weird rituals required to check the transmission fluid in a vehicle? 

Sometimes you have to bring the vehicle up to temp, run it through the gears (or not)  with the parking brake on (or not)  and then check the dipstick with it in gear or in neutral or whatever.

Why can't the dipsticks simply be calibrated for the levels which establish themselves when the engine is cold?  Or even just with the engine off for a half hour or an hour or whatever and the dipsticks calibrated  for that?

Surely, given enough time for the fluids to drain, shouldn't the levels just establish themselves for that standard measurable condition?

Terry, 230RN

[Tallpine]

I have no clue.  Power steering cap stick says "full cold" and "full hot" 

I seldom have a good chance to check transmission when it's hot.  Apparently the shop didn't either, because some years ago while towing it started blowing fluid, which then got on the exhaust and smoked a lot 

After pulling off on the shoulder some miles from town on a sunday morning, I couldn't find an obvious leak.  I started it back up and checked the level, and surprisingly it was "still" full.  We eased it into town and checked again, and since it seemed to be doing fine, continued several hundred miles on our trip.  That was 10+ years ago and no more problems so I think the shop over filled it.

[Phyphor]

Because some engineers are retarded.

Seriously, some I've seen are set up to be checked when cold.  Others have to be hot & with the engine running (after cycling through the gear range, which would be closer to the actual operating amount of fluid.  Others can't even be checked since the fluid will supposedly last the lifetime of the transmission.

Don't get me started on the friggin' internals (BMW, I'm looking at you.  3 goddamned bands to make 4 forward and 1 reverse gears? )

Then there's the non-standard way they interface with the engines these days.  Used to be a series of bolts held the bell housing onto the block, and they all screwed forward.  The last trans I helped install (on a 2005 Ford Ranger pickup) had half the damn bolts go in from the front, half from the back.  Real weird crap.

[Tallpine]

Our little car has no way to check the transmission fluid. 

Supposed to not need service unless "extreme duty" like hills, traffic, blah blah blah (IOW, most everything).

We had the transmission serviced at about 60K.

[coppertales]

My 2010 Ford F150 does not have a dip stick.  You have to take it in to have it serviced.....chris3

[charby]

I think because the fluid expands when it gets warm, why need to check it hot.

Also perhaps the dipstick can't got low enough when the fluid is cold because if it is that long it could cause damage.

It seems like anymore, the fluid is good for x number of miles and you change it when you get there.

[230RN]

Seriously, some I've seen are set up to be checked when cold.  Others have to be hot & with the engine running (after cycling through the gear range, which would be closer to the actual operating amount of fluid).

Well, that's what's bugging me.  Why should one level with a certain, specific operating condition be any more accurate than just letting it sit for a half hour?

It's sort of like there are separate volumes in there which get filled differently with different operating conditions.  Still, it would seem more accurate to just set the conditions for "Drive it normally until warm or 10 miles, stop, shut off engine, look at your watch, go get a cup of coffee, come back in a half hour, and read the dipstick."

Period.

Or simply, "Check transmission fluid level in the morning before starting the engine."

Period.

I mean, it's not like ATF is the consistency of Vaseline or anything.  Hell, it's almost like water.  It ought to just drain to standard levels pretty quickly.

It's been niggling at me for years, having had different cars with different rituals (and some with different rituals for the same car depending on where you get the information.)

But recently I figured I ought to check it on my Subie just on general principles.  So the question came up again.

Just posing the question is a problem, since the general answer from several professional sources boils down to "Because that's the way to do it."

OK.

But why?  

So thanks for the answers so far, but the presumption is that the guys who designed the transmissions knew what they were doing when they wrote the ATF-checking specs (rituals), so it sure seems strange.

It was interesting to find out here that some transmissions now have no way to check the ATF --I'm not too familiar with more modern cars.  Done a lot of wrenching, but not lately because I'm physically limited to light topside servicing nowadays.

Terry, 230RN

[mtnbkr]

Also perhaps the dipstick can't got low enough when the fluid is cold because if it is that long it could cause damage.

Seems plausible to me.

Chris

[cassandra and sara's daddy]

I wish I could find a sensor I could install ro check atf fluid without me crawling under car to pull plug and use finger

Sent from my SAMSUNG-SGH-I537 using Tapatalk

[Brad Johnson]

A lot has to do with the trans design.  Some trannys allow fluid to settle back into the pan after the engine has been shut off for a bit.  Others, not so much.  Calling for a hot fluid check ensures that the all circuits are full and the fluid is properly circulating.  Cold checking a tranny that calls for hot checks will likely get you and overfull reading.

Brad

[Tallpine]

A lot has to do with the trans design.  Some trannys allow fluid to settle back into the pan after the engine has been shut off for a bit.  Others, not so much.  Calling for a hot fluid check ensures that the all circuits are full and the fluid is properly circulating.  Cold checking a tranny that calls for hot checks will likely get you and overfull reading.

Brad

It's sort of problematic unless you check your ATF at the end of each day's drive when everything is thoroughly warm.

A low ATF condition is most likely to present itself in the morning when cold (vehicle doesn't move) when you have no clue how much to add.

[230RN]

"Aaand," Terry asked again, "why not under cold standard conditions  instead of under hot standard conditions which must be attained by following a complicated routine?"

[Firethorn]

"Aaand," Terry asked again, "why not under cold standard conditions, instead of under hot standard conditions?"

I wonder if that might have something to do with tradition?  IE way back in the day they checked levels when hot, for whatever reason, and have thus never felt the need to change?

[230RN]

^  Y'know, that was my suspicion from the outset, that it was the old "But that's the way we always did it" syndrome. 

[drewtam]

I can't fully explain it, but I know on some engine designs, the oil level when the engine is shutoff can be erratic. What I mean is that the engine has a large remote oil filter, and lines, and all the oil suspended in the engine internals.

If the oil stays in the oil filter, it will appear lower than if it syphons out back into the pan, that can make ~2L difference. Or if the engine sits a long time, the oil in all the internal engine passages will take a while to drain, that can make another ~2-3L difference. Or the first time the engine is started, there are a lot of dead pools that get filled with oil, and that can consume another ~1L.

So the first time the engine is started it fills the dead pools, fills the bearing passageways, oil cooler, etc, fills the oil filter and lines; and so the volume in the pan might reduce up to 5-6L! How much of that is returned when the engine sits cold, it can be nearly random. Getting dipsticks right is a lot harder than it looks.


That said,
I wonder if the transmission guys are having the same kinds of problems, dead pools, passageway fill, and random drain back behavior, to the point where they give up on a cold transmission measurement and tell you to fill all the passageways with hot trans oil and measure the height left in the sump.

[Firethorn]

I wonder if the transmission guys are having the same kinds of problems, dead pools, passageway fill, and random drain back behavior, to the point where they give up on a cold transmission measurement and tell you to fill all the passageways with hot trans oil and measure the height left in the sump.

From what I've seen of cut-away transmissions, especially automatic ones, I'd say that this is probably the case.

[Scout26]

Two things.

1)  What is this ATF of which you speak?  Do you mean gear oil? 

2)  Be grateful that said ritual does not involve the sacrifice of small mammals nor family members.   

[Boomhauer]

1)  What is this ATF of which you speak?  Do you mean gear oil?  Tongue

You know, you manual trans guys are like car hipsters.

"I drive a manual, but you've probably never seen one"

[charby]

Two things.

1)  What is this ATF of which you speak?  Do you mean gear oil? 

2)  Be grateful that said ritual does not involve the sacrifice of small mammals nor family members.   

Your boobaru maybe ATF in the manual transmission.

[mtnbkr]

You know, you manual trans guys are like car hipsters.

"I drive a manual, but you've probably never seen one"

For the record, most of my cars have been manuals (2 of the 3 I currently own are 5spd), but I would be just as happy with an auto considering our local traffic.

Chris

[Tallpine]

We are 2 and 2 for manual and automatic, which just so happens to also divide along the line of pickups/SUV types.

I much prefer a stick and clutch, but they are hard to find in an SUV anymore on the used market.

[KD5NRH]

"I drive a manual, but you've probably never seen one"

It's so nice to get to say that to mechanics sometimes.

I did once pick a car by asking my favorite transmission guy what he was least familiar with.  Got it with 180k miles, sent it off to the salvage yard with 260k and part of a Chevy emebbed in the passenger side.  Still wish I'd pulled the engine and transmission out of it before letting them haul it off.

[Parker Dean]

It's sort of like there are separate volumes in there...

And there ya go.

The biggest contributor is the torque converter. The majority of fluid is in that. With the engine off, and therefore no pump action actively filling the converter there will be drainback. But at what rate? Way too many variables to anticipate. Then there's a not-insignificant amount in the cooler circuit, which will also drain back at varying amounts and speeds. Then there's some in the various passages in the valve body and servos. To top it all off you also want enough reserve to cover the pickup in all likely conditions since air and hydraulics don't mix too well while at the same time not so much that it gets into the rotating components and foams up, increasing heat and adding air.

FWIW, it seems to me that the domestics were usually pretty lax about how fluid levels were determined. Pretty much something close to the right temp and somewhere in a fairly wide range was good enough. Some of these German things actually spec an actual temp as shown on a scan tool along with specific drive cycles to achieve that. I had to laugh at my old A6 that required a spec temp as shown on the scan tool while also being in the air, level, and with the fill plug out of the side of the pan. Proper fill was then determined as being a dribble from the fill port.

Oh and olde domestics frequently had a dot, either drilled through or stamped into, the dipstick about a quarter-half inch below the Full Hot range. That's the Cold Full mark but that should always be verified against the Full Hot range

[bedlamite]

1)  What is this ATF of which you speak?  Do you mean gear oil? 

http://www.pennzoil.com/other-car-products/transmission-fluids-and-gear-oil/manual-transmission-fluid/

On a side note, After I took my current truck for a test drive, and was about to leave the dealer, the manager came by and asked if I could move the truck, because there was nobody in the place that could drive a manual.

[230RN]

bedlamite remarked:

On a side note, After I took my current truck for a test drive, and was about to leave the dealer, the manager came by and asked if I could move the truck, because there was nobody in the place that could drive a manual.

LOL.

I guess I'll have to go along with the variable trapped volumes theory, but it still seems that with the relatively low viscosity of ATF, it should all drain down within a reasonable time to result in a "standard level."

My dipstick... wait... my "measuring stick for the transmission fluid"... has three holes in it and just looking at it cold (60ish°F), having been driven normally the previous day and having sat in the parking lot most of the afternoon, the level is about 1/8in below the top hole.  I'm going to figure that's good to go.

In addition to his excellent explanation, Parker Dean said,

Some of these German things actually spec an actual temp as shown on a scan tool along with specific drive cycles to achieve that. I had to laugh at my old A6 that required a spec temp as shown on the scan tool while also being in the air, level, and with the fill plug out of the side of the pan. Proper fill was then determined as being a dribble from the fill port.

Both my two Audis had similar persnickety requirements for various things.  I had a full shop manual (Bentley, Benjamin?) for my last one and when I was going to service the air conditioner  I looked it up and they gave a whole set of conditions to measure this and that.... specific temperatures coming out of the vents, with engine fully warmed, at various positions of the control knob, with the cabin temperature sensor covered and uncovered, all kinds of nonsense like that.

What really got my goat, though, was that the initial requirements before performing these diagnostics were to have the car sitting in a room with a controlled environment set to like 65°F (actually stated in Celsius, of course) for 24 hours to stabilize the conditions.

Yeah, right.

My first one was also a little over-controlled.  For example, you could not get any heat at all out of the heater until the engine had warmed up to normal operating temperature!  

"Vaht?  You cannot expect our enchin to operrate effizhiently until it comes up to speziffications!  Vy shoot you expect heat before zen?"

Oh, I got other stories about that one.

And, Parker Dean, of course your point was, "how did they define the 'dribble?' "  One-komma-seven-three cc per second?

Terry, 230RN