While racing at the dragstrip, I was getting an ATF leak from somewhere. The fluid level is/was filled perfectly level with the fill hole. I initially thought the ATF was foaming up and venting out of the vent tube on my T45.

But that wasn't it. I took the cover/boot off the shifter and watch it carefully. As the rpms climb up past 5000 or so, the ATF fills up the well around the shaft of the shifter, and starts overflowing out (see arrow in photo). Its not foaming or anything -it just looks like regular atf, overflowing. Also, no ATF seemed to be coming out of the vent hole.

I pulled the shifter, and checked for the plastic cup-looking thing. Its there, and looks normal. Any ideas?
Shifter = MGW, tranny = Promotion built T45, fluid = regular, nonsynthetic dex/merc

 

A couple of things come to mind:
1. One of the drain-back holes in the trans is clogged, causing fluid to build up up top and thus leaking out of your shifter.
2. You overfilled. I don't know what it is for the T45 but I know at least for the TR3650, the correct fill level is below the fill hole.

 

I know there's a drain vent at the top of the transmission, on the right side of the shifter. Is that what you're referring to, or something else?

 

I'm referring to inside the trans. Fluid is accumulating top side and it somehow needs to go back down, which is done through passages fed by the drain back hole. Maybe that's not the right terminology since that's what's used in engines but the point is, if something clogged the way the fluid drains back, it will build up.

 

I have a theory...clogged vent tube. When the atf heats up (high rpms), it expands. The air in the tranny needs somewhere to go, but since the vent is clogged pressure builds and pushes out the tranny fluid out the shifter.

Only question now is how to check/unclog/replace(?) the vent tube. It would be nice if it can be removed without having to drop the tranny.

 

My first thought is vent tube as well. On most manual trannys you can't get to them with the tranny in the car.

I would if you can get brake cleaner spray it up into the vent cap move it around so it loosens up and then of course you are going to have to swap tranny fluid in case some of the brake cleaner got into the vent and into the tranny. That's what I would start with at least.

 

Here's an update, for the record.
It turns out that the vent tube is fine, and that the shifter was the culprit. The shifter was acting like an open vent to atmosphere, and was out-venting the vent tube, letting out air (and transmission fluid) from the shifter cavity. I used a little dab of RTV to block the air path in the shifter, repacked it with grease and now its fine.

BTW, the stock vent tube is designed to release air at a few psi, according to Tremec. This makes sense to me b/c an open vent to atmosphere would cause problems when you accelerates hard: tranny fluid would run to the back of the transmission, fill the shifter cavity and come right out through the vent.

Having a vent that releases at a few psi allows a little air pocket to be there, preventing this from happening. At least, thats my take on it.