Changing auto box oil - what have I done wrong?

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plip1953
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Changing auto box oil - what have I done wrong?

Post by plip1953 »

I feel a bit sheepish about posting about this because I feel I must have missed something very obvious!

A few weeks back I changed the gearbox cooling pipes that run forward into the radiator. And at the same time I had the gearbox sump off and changed the filter. At the time I collected around 5 litres of old fluid and then put the same back in.

Being conscious that the fluid hadn't been changed for quite a while, and that whatever is in the torque converter stays where it is, I decided to do another drain down and re-fill. With the car off the ground and engine running I cycled through all the gear combinations and warmed the fluid a bit in the process. I then removed the sump plug and waited for another 5 ltrs to come out.

But barely as much a 2 litres actually deposited itself in my bowl!

Have I done, or not done, something obvious?

And before anyone mentions it, there isn't a big puddle of oil underneath the car to account for the other 3 litres!
Phil
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Re: Changing auto box oil - what have I done wrong?

Post by plip1953 »

Could really do with some help here guys!

After settling overnight I managed to drain off another 0.5 litre directly via the sump plug, but the other 2.5litres remains elusive.

Would there be a problem if I ran the engine for a few seconds? Strictly speaking there should still be around 4.5 litres of fluid lurking somewhere!
Phil
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Re: Changing auto box oil - what have I done wrong?

Post by hornhospital »

Won't hurt a thing as long as you don't drive it.
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Re: Changing auto box oil - what have I done wrong?

Post by plip1953 »

hornhospital wrote: Sat Jul 25, 2020 3:41 pm Won't hurt a thing as long as you don't drive it.
Thanks, I'll give that a go and see if it somehow forces more fluid into the sump area.
Phil
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Re: Changing auto box oil - what have I done wrong?

Post by plip1953 »

I ran the engine for around 30 secs and then once again opened the sump plug. And a further 0.5 litre or so fluid came out.

I still don't understand why it's short of what I put back in at the first change, but I'm not going lose too much sleep over it and hope that filling back up and setting the correct final level will be ok.

On this latter point, am I right in saying that I need to test the level with the engine running and having first cycled through all the gearbox positions?
Phil
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Re: Changing auto box oil - what have I done wrong?

Post by plip1953 »

Just to conclude, I re-filled the box and set the level with the engine running, although technically the gearbox fluid would still be cold so I needed to take that into account. It took around 4.5 litres so I'm reasonably happy with that.

Just one last question in case anyone can provide an answer - if you end up over-filling with fluid is it detrimental to the box or does it simply mean that you run the risk of it being vented out of some kind of breather/overflow arrangement?
Phil
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Re: Changing auto box oil - what have I done wrong?

Post by plip1953 »

wfwright2 wrote: Mon Jul 27, 2020 6:35 pm If it is grossly overfilled, there is the opportunity for the rotating parts to cause foaming of the fluid in the sump. This foam will prevent proper pressures to build as well has having air which has no lubricating or cooling properties enter the ant farm of passages in the transmission, possibly causing excessive wear on friction and steel disks and improper lockup of the torque converter.
Get the transmission up to operating temp, run trough the gear selection a couple of times and check again. If it is much above the full hot mark, some should be drained.
I wiped a GM turbohydramatic 350 by not watching filling level carefully after a rebuild. Even on the Buick it was installed in, no fun replacing it twice.
Good advice, thank you :-)
Phil
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Re: Changing auto box oil - what have I done wrong?

Post by sansouci »

Could the original fill that you drained have been overfilled so that you are chasing a ghost? DON'T run it to get more out of the box. Rely on the proper dipstick reading technique.
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Re: Changing auto box oil - what have I done wrong?

Post by plip1953 »

sansouci wrote: Wed Jul 29, 2020 1:34 pm Could the original fill that you drained have been overfilled so that you are chasing a ghost? DON'T run it to get more out of the box. Rely on the proper dipstick reading technique.
The original drain down produced over 5 litres of fluid and I replaced with a like amount. But second time round it was only around 3 litres initially and then I managed to get out another litre. I've now put back in around 4 litres and I think I may be a bit overfull, so will drain some off again and re-check.

Is there a good reason why level checking has to be done with the engine running? Why is it different to engine oil level checking?
Phil
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Re: Changing auto box oil - what have I done wrong?

Post by plip1953 »

wfwright2 wrote: Wed Jul 29, 2020 1:58 pm A running engine has fluid filling the torque converter, with a non-running engine, the torque converter drains down to about half of total volume which adds to the level in the sump/pan. This volume is a large percentage of the total fluid capacity of the transmission unlike an engine that has a relatively low percentage of oil out of the pan when operating.

An example is that a newly rebuilt transmission will take 7 to 8 qts to do an initial fill, but a drain and fill for filter change will nominally take only 4 qts. These are example, nominal numbers, I do not have my manual available for the actual numbers on your specific transmission handy.

Also transmission fluid expands more when heated than one would suspect, which is why the fluid level is checked at or near operating temperatures.
Thanks, a very helpful explanation. Nevertheless, I still don't see why it isn't possible to have a dipstick that shows where the fluid level should be when cold and engine off ie somewhere higher up on the stick. Or perhaps the drain down of the torque converter isn't always by a consistent amount - which could explain why on the first occasion I drained down I got 5 litres out, but only 4 litres second time?
Phil
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Re: Changing auto box oil - what have I done wrong?

Post by plip1953 »

wfwright2 wrote: Thu Jul 30, 2020 2:04 pm The drain from the torque converter is a function of time, as the fluid gets to transit through valves, pump, control tubes and channels, etc. on its way back to the pan.
The scheme of a hot, engine running level check goes back to the 1940's and 50's first hydromatics, torque flytes, and ford o matics. Be thankful that you do not have a new ZF aurptobox where there is no dipstick for transmission fluid level, nor a fluid level sensor, and level checks and refills are done through a plug in the side of the case. Engine running at temperature of course and an even more tightly packed tunnel. My e70 4.8 is also blessed with no dip stick, but does monitor the oil level for me.
Again, thank you :-)
Phil
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