The most important thing, make sure the supply line/ inlet hose to the transmission is on the bottom of the cooler.
You want your trans oil to flow entering in the top and exiting at the bottom.

That should not cause any issues with the existing radiator.

Curious as to why you're wanting to bypass the factory radiator set-up?

 

Curious as to why you're wanting to bypass the factory radiator set-up?

Unless you are towing a trailer with that Passat, I don't see the need for the cooler. My old 2.8 A4 had 270+K miles when I sold it, trans working just fine. My current 3.0 A4 with the AWD version of that trans has 230K, working great. Just did routine ATF changes with Valvoline MaxLife.

 

I agree. Why add? I am at almost 200k. Mine is still in good working condition. I use my car 2001 b5.5 2.8l for simple service work. The trunk is like a tool box. Just have the trans serviced. Have them put in a new filter, new fluid. Just my thoughts.

 

1-)The factory cooler is leaking at the radiator entrance. I replaced the o ring, still does. So I decided to install an external cooler.

2-)To be clear, the warm fluid from tranny should enter the cooler at the bottom, travel upward and go back to transmission from the top. Am I correct?

3-) I don’t want to start this again but did you use MaxLife and your transmission is Ok with it? I have been using Pentosin, crazy expensive.

 

3-) I don’t want to start this again but did you use MaxLife and your transmission is Ok with it? I have been using Pentosin, crazy expensive.

MaxLife is all that I use for these ZF 5HP19 transmissions. When I first used this years ago in my B5 A4, a first-gear shudder was completely cured. My impression is that the shifts get a little smoother and less distinct than OE fluid, but I'm fine with that. The best local transmission shop, in my opinion, rebuilt my daughter's Jetta transmission probably ten years ago (a previous owner had chipped the thing and the clutches just wore out). He used MaxLife, says that's what he uses in most transmission rebuilds and service, and buys it by the 55 gallon drum.

 

100K on my son's wagon in the last few years and most of that was with Maxlife in the trans. I have it our Golf (24K in the last 6-8 years) too.
The only issue I see with bypassing the oem trans cooler is that trans temp is no longer regulated to 190 (roughly). I added a similar setup to an older Taurus wagon that would dump trans fluid in stop and go traffic as the trans got too hot. Worked fine, but the Passat has a temp sensor inside the trans and I don't know how that would respond to being too cool at times. I know the engine CTS affects shift points, maybe the trans temp sensor does the same.
It will not matter which hose is hooked on top or bottom. Fluid will flow through anyway.