ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by Laters » Wed Jan 15, 2020 9:18 pm

When I bought my car I was told by the previous owner that it had a issue with the ATF level going down.
He claimed there was never a puddle though.
The vacuum modulator was faulty and wet along with the gearbox sump & what looked to be coming from the somewhere at the back of the gearbox but the fluid always seemed to be on the yoke of the propshaft yet the part that slides into the gearbox appeared dry.

I have replaced the vacuum modulator, tried to make the joining faces on the gearbox sump as flat as possible after its been overtightened at some point, changed the filter while the sump was off, put the sump back on with a new gasket and some flange sealant to be on the safe side.
I replaced the gearbox rear seal with a new seal even though the old seal looked very new.
The propshaft was refitted and I put about 3 litres of AQF in the gearbox a few weeks back.
When the gearbox sump was off the gearbox drained at a guess most of its fluid as all the drips stopped after a few week.

The car hasn't even been started & is still on stands but there is a small puddle formed under the rear of the gearbox.

Looking at the propshaft and the rear of the gearbox the fluid looks like its running from the yoke that slides into the gearbox but the part that is slid into the gearbox that the rear seal operates on looks dry.
It looks to be coming from the area where the welch plug is but cant see too well due to the u/j of the propshaft.

My only thought there is could the welch plug on the yolk be the cause of the leak?
Anyone ever had anything similar?
It looks like the previous owner tried to find the leak and gave up. When fitted the propshaft isn't loose in the bushing in the gearbox & yes I did check the seal was the correct size before I fitted it.
Last edited by Laters on Thu Jan 16, 2020 12:53 pm, edited 2 times in total.


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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by AJL Electronics » Wed Jan 15, 2020 10:14 pm

How are you checking the fluid level? It only takes a little bit of overfilling to blow it out of the vent on the top rear of the gearbox. If it isn't the rear seal, then that is the only other place it could be coming from. The reason that the leak could never be found is that it will stop ejecting once the level is right.


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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by Laters » Wed Jan 15, 2020 11:25 pm

When I got the car it drove and ran but there was some delay in the gears, which I put down to low fluid.
When checked the fluid was low as soon as it was topped up with fluid it started to leave puddles.

Since the sump was off and the box was drained it hasn't been run at all. Its just had 3 litres of fluid put in it * left for a couple of weeks.
Now there is a small puddle on the floor right under the propshaft yolk with the only place fluid is looking to drip from the yoke of the propshaft but the rest of the propshaft and gearbox are still dry.

It hasn't been started as yet so cant check the level hence only adding 3 litres of fluid from empty & not adding the amount that came out.

I got over 4 litres of fluid out of the gearbox when the sump was off, plus quite a bit spilled etc, plus it had been dripping with no sump on for a week or two so I guess a bit more than 4 litres came out. ATF spreads nicely on a painted floor.

I added 3 litres of fluid just to check the sump gasket was sealing & didn't expect a leak from the rear of the box/propshaft area.
Last edited by Laters on Thu Jan 16, 2020 2:13 pm, edited 2 times in total.


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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by philhoward » Thu Jan 16, 2020 4:41 am

To answer your question; if fluid can travel down the splines of the yoke (or does it even have “missing splines”), then if a blanking plug is used and it’s loose then fluid could indeed leak “out” of the yoke.


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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by Laters » Thu Jan 16, 2020 8:13 am

Thanks Phil that was my thoughts as to the leak.
Could be wrong and it seems a pretty strange one too.

I will pump as much ATF out the box and remove the propshaft as I think that's the only way to get the answer.

I was hoping someone would say there is a seal or something as the information in all the books and net seem to say it might have fluid or as you said might have a missing spline.

I will update as and when I can but might explain way leak has been never found as when prop was removed by me first time everything was saturated in ATF.


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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by Tinker man » Thu Jan 16, 2020 8:52 am

good morning, just as a slight aside, when i was working on the gearbox i didnt enjoy the pumping out process and getting ATF dripping everywhere so added a drain plug. might be something worth considering whilst you are getting stuck in down there, so to speak. useful mod i thought.

good luck with finding the leak. Ian.
sump plug 1.jpg
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sump plug 2.jpg
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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by philhoward » Thu Jan 16, 2020 11:50 am

A quick look (I've been looking at propshaft yokes for something else) and Ford don't use the missing spline - seems to generally be a Japanese thing.


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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by willholderogri » Thu Jan 16, 2020 12:07 pm

Tinker man wrote:
Thu Jan 16, 2020 8:52 am
good morning, just as a slight aside, when i was working on the gearbox i didnt enjoy the pumping out process and getting ATF dripping everywhere so added a drain plug. might be something worth considering whilst you are getting stuck in down there, so to speak. useful mod i thought.

good luck with finding the leak. Ian.
sump plug 1.jpg

sump plug 2.jpg
i did that to my BW35 but used a metric magnetic plug instead there is enough material to tap it out metric (I can remember the size off hand ) the original is a slot head screw


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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by Laters » Thu Jan 16, 2020 12:52 pm

I'm hoping the gearbox sump doesn't have to come off again but if it does the drain plug seems like a pretty good solution.
Pumped a fair amount of fluid out last time but it was still messy when the sump came off.
I wasn't too worried about keeping the old atf as I was unsure if it was the correct type hence the new fluid being AQF.
Going to stick the electric oil pump down the dipstick tube and try and save as much ATF (AQF) as I can before the propshaft is removed & hopefully there wont be too much lost.

I am sure we used to have a plastic gearbox plug for use of the manual gearbox's of the older cars for when the propshaft was removed but not seen the plastic plug for years & haven't seen any for sale to fit the current auto.

If it is the propshaft yoke that's causing the leak that might explain why the previous owner never managed to find it, not sure why he wasn't 100% honest about the leak but hindsight cant really help here.

When the propshaft was off last time the rear u/j was replaced as it was notchy and the propshaft cleaned up & painted hence why I think the leak is from the yoke as its now wet with atf.
The front u/j feels ok but doesn't have a grease nipple so as the shaft is coming off & at a guess the front u/j is going to have to come off it might get changed for one that can be greased anyway.


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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by AJL Electronics » Thu Jan 16, 2020 2:48 pm

With the correct level in the gearbox, pulling the propshaft will not result in much loss. As long as the car is level of course. I still think that it is being overfilled, it doesn't take much for it to vent. There is no way that oil can come out of the rear with a yoke pushed in unless the seal has been damaged. There is no way through the solid yoke casting.


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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by AJL Electronics » Thu Jan 16, 2020 2:53 pm

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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by philhoward » Thu Jan 16, 2020 3:45 pm

That’s not the same yoke though, Andy?

Random googling yoke images shows some might have a plug in the back, so it could be a leak point:
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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by Laters » Thu Jan 16, 2020 5:07 pm

With the greatest respect Andy I know what you are saying but my yoke looks like the one in the photo Phil posted.
I have a quick photo that's just been taken.
Hope you can see the point I am making.
You can see the plug on the inside of my yoke and when a finger is put in the yoke plug/u/j area it comes out slightly wet with atf.
The propshaft was cleaned before it was put back on hence why I am thinking the plug in the yoke is leaking.

Fingers crossed the propshaft will be taken off sometime this weekend & I will be able to answer this with 100% certainty.
The area around the rear seal in dry as is the rest of the gearbox.
The car is sat slightly nose up for access underneath at the moment but its not a massive amount.
The leak isn't huge but the car has been sat for a few weeks since the fluid was put in with no leakage at all then noticed a small puddle (really noticeable on the painted floor).

You can see by the state of the underside of the car that its not been oil tight for some time.
IMG_0196.JPG
propshaft yoke with drip
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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by Laters » Mon Jan 20, 2020 7:16 pm

As a bit of a update.
The propshaft has been removed from the gearbox.
Propshaft was wiped dry around plug and u/j before it was stood against wall,

The propshaft was stood it against the garage wall & filled with fluid to the top.

After a few hours just had a look to see if the fluid level has dropped or if there is any drips.

A couple of photos.
IMG_0215.JPG
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IMG_0222.JPG
IMG_0222.JPG (126.74 KiB) Viewed 903 times
To me the photo shows that the welch plug in the yoke is the point of leakage.
Next step is disassemble the u/j, I may as well put a new u/j on while its in bits, remove the welch plug & clean up yoke where plug fits, use some sealant, reinstall welch plug (new plug if I can get one the correct size) and add some epoxy sealant over welch plug for good measure after everything is cured.


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ATF leak from rear of auto gearbox (C3), possibly the propshaft itself?

Post by philhoward » Mon Jan 20, 2020 7:24 pm

Looks like a winner - and a job for some JB Weld to me?


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