I've noticed the past few months that when I am towing I get some ATF on the back of the truck and the front of the trailer I am hauling. It's a fairly fine mist but still cause for concern. It's enough to wet the safety chains connected to the hitch.

Best I can see it is coming from the front of the trans...looks like the converter seal/bellhousing area. I try to clean it up every time it leaks.

However, the leak is ONLY present when towing...I can drive down the interstate at 85 all day long and never leak a drip as far as I can tell.

My trans temps never get much over 200 degrees...I have a 11x13 B&M cooler bypassed around the radiator.

I do have a yank 2600 stall and a wheatley tune which turned up the line pressure and changed shift firmness etc. However all this has been in place for the last 2 years, and only the last few months have I noticed the leakage.

I thought it may be the front seal around the converter, but if it were that, wouldn't it leak all the time?

Could it be the seal/gasket/o-ring between the pump and trans case? IF so is that something I can change at the same time I change the converter seal? OR will it cause issues if I pull the pump out of the trans and try to put it back? I've never had to do internal trans work...

Thanks in advance for any advice.

OH, btw...the last time I had a vehicle do this was my 94 silverado 4l60, and the problem continued to grow until the trans roasted 2nd and 4th gear and had to be rebuilt. But, that truck always had problems keeping the fluid cool when towing, even with the same type of big trans cooler I run on my current truck.

 

sounds like the front seal

 

which one? between converter and pump, or pump and trans case?

 

between the converter and pump. i could be wrong but thats what it sounds like to me

 

Is the fluid level correct? If it's overfull it could be blowing out the vent and running down the trans...

 

Well, it's obviously getting slightly lower as it leaks, but it was at the same level it's been at since I put in the 'verter about 2 years ago...perfect on the dipstick when warm. I've never overfilled it.

 

I got to looking at it closer tonight while the truck is on stands putting on my big brake kit.

My transfer case seems to have ATF on the bottom of the rear edge. None on the front at all. That is where most of it is coming from. The top is relatively dry, but there is a vent tube that ends right on top of the tcase, and I bet that is where the fluid is coming from.

Other than overfilling, what causes fluid to exit the vent tube?

I think I am going to get an oil catch can and hook it to the vent tube instead of just letting it vent all over the undercarriage.

Also, following this vent tube to the front, it has a tee, one end goes to the top of the trans, one end has another tube that goes toward the engine and then connects to nothing. What should this tube connect to in the area of the engine?

 

The vent tube that towards the motor is just a vent that hangs behind the block. You might check you transfer case to see it its overfilled,if it is then the double sided seal in be tween the T-case and trans in leaking through and making it vent once it gets hot.Check you transmission to see if it is low also,if so then you know where the fluid is going provided theres no other leaks.

Most of the times when we see overheating transmissions...is because of poor cooling.I see you said you have a cooler but it might not be big enough. Make sure your fan is working correctly and that you don't have any strange things in front of the radiator,just a few thing to look at.

 

UPDATE:

I decided to change all of the oil seals in my entire drivetrain to try and remove the leaks. Changing to all new fluids at the same time. I've already done the r/r, seals and fluid in the front diff. I was also going to do the rear seal on the transfer case, which looked like it could be the source of some of the leakage.

I also bought an ebay oil catch can to hook this trans ventilation tube up to, to catch anything that gets sprayed out.

So I pull my rear driveshaft, to start on the rear differential pinion seal, ujoints, etc. I notice the yoke is scored up some. Uh oh... So I guess that the bushing in the transfer case is also bad.

Now, my GM service manual says that my transfer case (NP 149 1 speed full time) is supposed to have this Auto Trak II special fluid in it. I drain the fluid from my tcase, and ATF comes out. WTF? Then there is a sticker on the tcase itself that says to use Dex II in it. Not sure which I should put back in.

Based on the look of the tcase fluid I drained, I decide to go ahead and take it out and apart. Getting it out was actually really easy and I did it by hand. Got it out and apart. Getting it apart was easy too.

The pump stayed in the rear tcase housing, and the rest of the guts stayed in the front. I saw no immediate problems. Then I checked the oil pump pickup. The magnet beside it was FULL of microscopic metal about 1/8 inch thick. Also the oil pump pickup was CLOGGED top and bottom. There was even a large chunk of black **** about 1/4" thick under the pickup. As well as I can tell, this chunk of **** is actually tiny pieces of RTV where the case halves were sealed at the factory, and they've come loose from the inside and gathered there at the lowest point in the case (at the pickup).

Believe it or not, the factory service manual gives NO information about disassembling or rebuilding the tcase. I searched online for a while and found a writeup about removing the oil pump for an upgrade. There's a snap ring inside the rear case that has to be removed from the top, under a rubber plug. Once that snap ring is released, you pry forward on the gear inside, and the pump assembly pops out of the case. I would have NEVER figured that out.

Once I get the pump out I examine that rear bushing, and sure enough it is badly worn and scarred up. And the reason is likely because the oil pump wasn't pumping, because the pickup was clogged.

This writeup I found about removing the pump was written for jeep and diesel owners with similar tcases but it applied to this one as well. Come to find out the pump body tends to RUB THROUGH THE TCASE HOUSING and cause pin holes, which lets the fluid leak out of the hole.

Sure enough, my case has the pump rub and at least one pin hole in the case. So there is where a lot of the atf was coming from.

Now I have to see if I can:

1) weld or otherwise repair this case housing

2) locate and purchase the pump upgrade to prevent further "pump rub" from eating into the case again

3) replace all the oil seals in/on the tcase

4) repair or replace the driveshaft yoke

5) replace the bushing in the tcase housing for the driveshaft yoke

6) figure out whether to refill with ATF or the expensive auto trak fluid I already bought

7) install my oil catch can on the trans ventilation hose.

 

front case half with front driveshaft output shaft/sprocket still in it, drive chain laying on floor



speed sensor, rubber plug, and main shaft between trans and rear driveshaft with viscous coupler (sealed black unit). the viscous coupler is what provides slip and tension between front and rear axles. I think this unit splits torque 38/62 front/rear from what I've read. If you run this tcase with no front axle, this coupler is what you'd tear up.



pump assembly with output shaft out of tcase. the pump is the thin piece in the middle with 4 "legs".




clogged pump pickup




**** that clogged the pump, small pieces of RTV from the factory seal.






rear case where the pump body sits.