Having just serviced my gearbox and having endured the rather convoluted refill procedure, I was wondering if all this was really necessary. Using some leftover ZF Lifeguard 6 Fluid I filled a graduated cylinder to 40ml at a room temperature of 23C. I then heated the cylinder to 52C and noted that the fluid had expanded to about 41ml. This works out to an expansion of 2.5% over about a 30C temperature change or 0.083% per degree C. You can see all this in the pics I attached. I also put it in the freezer and took a reading at -3C just for confirmation which caused it to contract to 39.2 or so. This works out to a shift of 2% over a 26C temperature change or 0.077% per degree. Averaging the two results = 0.08% per degree C.

The difference between room temperature 23C fill checking and the specified 35C fill checking the difference in fill volume would come out to 12C X 0.08 shift per degree = 0.96 or 1% So, if my Super V8 holds 10L of fluid then 10L X 1% = 0.1L or 100ml, or 0.42 Cups.

It seems like 1% is a pretty small shift in volume to have much of an effect but I'm surmising that there may be other considerations such as viscosity or ZF wanting the tech to run the transmission long enough to make sure all the passages are filled.

Jeff

 

Glad someone's doing this. But how could you remove the .4 of a cup after circulating the fluid that would be easier than the current procedure? Maybe fill the car off level (on ramps perhaps) so that you start it cycle once and fill not waiting for it to get hot. If someone could figure that out we'd have something. When I did mine all I kept thinking is there has to be an easier way than waiting, cycling the shifter and watching the thermometer..

 

Seems that back in the 70's & 80's that the old Chevrolet cars i had.
Auto transmissions from then 10+ year old cars.
That the fluid level on the dip stick, changed very LITTLE from cold, to warm.
But i as well as others went through all the suggested steps with good results.
No change in the shifting, but the leaking plastic filter pan, was gone, and with it, the transmission leak.

 

Very informative, thanks....takes me back to the old physics days..... unfortunately, those brain cells have long since perished......but, your data is easily understandable

 

I have done the oil change as ZF advised. My problem has been that once the temperature starts rising then it does rice quite fast.
Taking off and putting back the plug has been a struggle once it's getting warm or even hot.

Here is the ZF procedure with my thoughts:

1) Fill as much as possible with the engine not running
2) Start the engine and add as much oil as possible. "Then put the plug back."
3) Shift through the J-gate once with a short stop at each gear.

Why do they advise that you put back the plug?
If you don't put it back then you can monitor how oil starts dripping as when the temperature is rising.
If it is not dripping then you just add some more without waiting for it to reach the specified 35C level.
Once it reaches 35C put the plug back and your done.

There has been very little said about how much of a problem it is if there is not the exact amount of oil in the gearbox.
I did put 1.5L more oil in than I took out when I changed filters and solenoids. The gearbox seemed to work fine.
It is of course not advisable to run with too little oil. But is it really a problem if you are not able to fill it up exactly to the right level?
Or is it a problem if you fill it cold and put back the plug? If it expands 0.1L can it really be an overfill that does matter?

 

Just do the best you can to follow the absurdly convoluted fill procedure and don't worry about it. As long as it's dripping out while the car is level and running warm you will be fine.

It confounds me how millions upon millions of automatic transmissions (including very high tech modern units) have been made with a dipstick and work just fine and yet the German engineers at ZF had to make filling theirs akin to rocket surgery. SMDH

Jeff

 

The question was why put back the plug for the time the oil is warming up?

My point is that you save yourself from much hassle if you leave the plug out.
This as it gives you a broader time window working (monitoring and filling oil) as the oil temperature is rising quite fast.

The plug can be fiddled to get out and put back if you are working on a car on stands as your working position will be difficult with a hot exhaust tube beside the plug hole.

 

I actually do not put the plug back while warming up the car, I must have skipped that part of the instructions when reading them... ;-)

The one very important thing is to NOT switch off the engine before fitting the plug when the oil starts leaking from the filling hole, because when you switch off the engine with the plug undone a lot of oil will rush out of the gearbox...

Best regards,

Thomas

 

I am not sure, but is there a thermal valve that allows oil to flow through the cooler once it reaches a certain temp? They may want you to get to 35C to ensure this is open and the cooler is filled.

People complain about how car companies don't do this and that--the plain truth is, the car manufactures don't care if you can service it. It's cheaper to not include a dipstick, then the dealership can charge the customer more labor for the under vehicle check procedure. They also don't really care what happens once the car is past the warranty period. They have no incentive to make cars last 25+ years.

 

This is the question I also have been considering. But I have come to the conclusion that this can not be the case.

I have monitored the gearbox oil temperature (iCarsoft) when driving. It seems to set at about 80C deg.
We also know that the gearbox software is designed to warm up the oil by "lurking". So the gearbox is designed to work better at a higher temperature than what you have on startup.
If this is the case then there would be no point in opening up the circulation to the cooling radiator as low as 35C.

 

Thank you Thomas-S now we are two who don't put back the plug while warming up the car.

I also want to warn everybody never to open the gearbox plug if the engine is not running (the exception to this rule is of course if you already drained the box of oil).
And if the plug is out never stop the engine.
The inner circulation of oil in the box will stop and the oil level will rise so that lots of oil will spill out.
If the oil is hot or even warm then you will not be able to put back the plug.

 

Also be SURE to get both plugs loose BEFORE trying to do a service!
For some reason those ZF fill/drain plugs are installed very tight!
.
.
.

 

Very wise reminder!