Auto Gearbox Leak

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DreamCar

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Mar 11, 2008
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Noticed recently hesitant gear changes and over revving when cold. Took to specialist and gearbox oil was low so they topped up. All fine for two weeks now starting again. Presume this means there is a leak? Is it a decent amount of labour for them just to locate the leak or normally not too difficult? Will try get it back to them this week.
 
The 5 speed gearbox commonly leaks from the electrical l plug (sometimes called a pilot bush) that connects the transmission to the Transmission Control Unit (TCU) If left the fluid can find it's way into the control unit and cause problems.

A replacement plug from Mercedes is around £25. I am surprised that the garage didn't check for leaks?

Of course it could be leaking from somewhere else but this is a good starting point.
 
The 5 speed gearbox commonly leaks from the electrical l plug (sometimes called a pilot bush) that connects the transmission to the Transmission Control Unit (TCU) If left the fluid can find it's way into the control unit and cause problems.

A replacement plug from Mercedes is around £25. I am surprised that the garage didn't check for leaks?

Of course it could be leaking from somewhere else but this is a good starting point.

As above, however, I'd expect any mechanic worth his salt to de able to identify a leaking pilot bush (or sump), and so the issue might be less obvious, e.g. a leaking transmission oil cooler etc.
 
I had a leak on both my 2009 S320 CDI and the 2008 ML 320 CDI with 162k and 130k respectively on them at the time. It's the 7g box and both turned out to be porous sumps. Both done at a local independent and c.£140 each
 
If you have a 5 speed 722.6 transmission you have one of the most robust and easy to care for automatic transmissions out there . It is fitted in countless MB cars including the mighty CL65 AMG V12 ,Mercedes SLR McLaren (650 bhp/820nm) Chrysler cars, trucks, buses and everything in-between.

It can only leak from a very few (and obvious to the naked eye) places . The afore mentioned TCU plug , metal sump gasket or seal each end . I have this transmission in my car which is 17 years old and has 153K miles on it . No leaks.

Were you present when they checked the level ? there is a particular dipstick used and a certain method of checking.

Skip to 27 minutes and you will see that exact transmission having seal replaced, but to repeat , any 'proper' leak will be instantly visible to the naked eye.

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If you have a 5 speed 722.6 transmission you have one of the most robust and easy to care for automatic transmissions out there . It is fitted in countless MB cars including the mighty CL65 AMG V12 ,Mercedes SLR McLaren (650 bhp/820nm) Chrysler cars, trucks, buses and everything in-between.

It can only leak from a very few (and obvious to the naked eye) places . The afore mentioned TCU plug , metal sump gasket or seal each end . I have this transmission in my car which is 17 years old and has 153K miles on it . No leaks.

Were you present when they checked the level ? there is a particular dipstick used and a certain method of checking.

Skip to 27 minutes and you will see that exact transmission having seal replaced, but to repeat , any 'proper' leak will be instantly visible to the naked eye.

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For more detailed information, see our cookies page.
I was there yes and the dipstick thing they used was very weird compared to normal ones, am sure it was the specialist transmission one. It's one of the well known specialists recommended a lot on here. I'll get it back to them asap and hopefully it's a leak they can spot easily and not expensive to fix. Car has over 200,000 miles on the clock.
 
As above, however, I'd expect any mechanic worth his salt to de able to identify a leaking pilot bush (or sump), and so the issue might be less obvious, e.g. a leaking transmission oil cooler etc.
Just to update, you were right with one of your ideas above. Transmission oil cooler was leaking, they said expensive to get in there in terms of labour to replace pipes etc. They fitted a bypass connector thing and said don't tow anything, thankfully I never do and just did 850 miles to lake District including multiple mountain passes, car worked perfectly so very happy!
 
Spoke too soon, noticed little bit of vibration and now suddenly it hangs in gear not wanting to change up. Oh dear.
 
Slipping clutch, said not worth fixing for age and mileage of car. Just wondering if it's something an auto transmission specialist can do at a cheaper price.
 
Bizarre thing is that it was perfect after they put the bypass thing in for automatic transmission cooler. Can't understand why it's suddenly gone bad again.
 
Hanging on to gears could be an issue with the conductor plate. When the box hangs does it upshift if you lift the throttle ?
 
Bizarre thing is that it was perfect after they put the bypass thing in for automatic transmission cooler. Can't understand why it's suddenly gone bad again.
So the bypass stops the transmission fluid from being cooled?

Overheated fluid will cause faults.
 
Hanging on to gears could be an issue with the conductor plate. When the box hangs does it upshift if you lift the throttle ?
No, that's the strange thing that it only happened twice, both times parking for a while when I had to go somewhere fixed it. If it stays as it is now I can live with it, it's not great but it's certainly driveable but when it hung at 2200-3000 revs and wouldn't change that I definitely couldn't live with!
 
So the bypass stops the transmission fluid from being cooled?

Overheated fluid will cause faults.
Yes but they said only a problem if towing, more than adequate cooling for normal use which to be fair I read online as well and for the first 1000 miles it was perfect. Drove it through several Lake District mountain passes daily for a week and performed admirably. Maybe it will just crack on like this for a while. 10,000 more miles would be useful with all the work it has had done.
 
Yes but they said only a problem if towing, more than adequate cooling for normal use which to be fair I read online as well and for the first 1000 miles it was perfect. Drove it through several Lake District mountain passes daily for a week and performed admirably. Maybe it will just crack on like this for a while. 10,000 more miles would be useful with all the work it has had done.
I knew that would be their answer! Round objects!

Why do you think that Mercedes engineered and fitted a transmission oil cooler to the car from the factory?
In case someone towed with the car one day?

No, it was fitted because transmissions need the fluid to be kept within an optimal temperature range. range and any fluid /oil will break down and not perform correctly if subject to abnormally high thermal stress and you drove through some steep terrain for a week!

When was the last time the fluid was changed?
 
I knew that would be their answer! Round objects!

Why do you think that Mercedes engineered and fitted a transmission oil cooler to the car from the factory?
In case someone towed with the car one day?

No, it was fitted because transmissions need the fluid to be kept within an optimal temperature range. range and any fluid /oil will break down and not perform correctly if subject to abnormally high thermal stress and you drove through some steep terrain for a week!

When was the last time the fluid was changed?

They changed it when fitting the bypass thing. Before that it was oil and filter gearbox service 46,000 miles ago.
 
They changed it when fitting the bypass thing. Before that it was oil and filter gearbox service 46,000 miles ago.
So the replacement fluid has now probably been degraded by the "experts" removing the factory designed and fitted cooler!
 
Hanging on to gears could be an issue with the conductor plate. When the box hangs does it upshift if you lift the throttle ?
No when it does the hanging in gear thing even lifting off accelerator won't trigger an upshift. Today it hasn't done it, if it cracks on like this for 10,000 miles it will be ok I suppose.
 
No when it does the hanging in gear thing even lifting off accelerator won't trigger an upshift. Today it hasn't done it, if it cracks on like this for 10,000 miles it will be ok I suppose.
I would suggest it is inspected by someone who knows what they are doing such as a Mercedes specialist and get it scanned for fault codes.

If it needs a new conductor plate these are not that expensive but as minimum you really should get the fluid changed and the transmission cooling circuit reinstated.
 

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