Not exactly a Ford truck, but I know I've seen some Ford transmission pro's in here.

2005 Freestar, 3.9 with the AX4N trans. My radiator started leaking apparently, and killed the trans. I use the van for Uber/Lyft driving. Went to go from a stoplight, and lost all forward gears suddenly. Drove in reverse to a trans shop(15 blocks through Denver).

They dropped it, pulled it apart. Scoring on the inside, milky fluid throughout.

Anyways, I decided the best bet is to bypass the radiator cooler, and just have an external trans cooler. Is there any downside to doing this? The only thing I've come up with searching, is a longer time to normal temps in the trans. But that doesn't seem like such a bad issue to me. And I don't want this $3400 repair to fail again. And since I've put almost 10k miles on the van in 2.5 months, any issues I encounter are accelerated by driving this much.

 

You have a new radiator , right? Worked years with the OEM setup, right? Why bypass it?

 

Because it just cost me a $3400 in repairs. Not looking to have that happen again.

 

There are external trans coolers you can get that will work. Get the plate types, as they're more efficient.

I own an Aerostar, and the members of that discussion group all recommend replacing both coolers, the one in the radiator as we ll as the one in the bumper, with the biggest plate cooler that will fit in front of the radiator.

Having said that, I wonder if you saw signs of the radiator failing? You usually get a check engine light that tells you something had gone wrong. If you had kept driving long enough after that to burn up your transmission, how is your engine doing?

 

No early signs. Trans fluid was always good on the stick. I just replaced it maybe 2 months ago, and it was fine (old, but no water). Coolant is not showing signs of contamination. I noticed it shifting a little different in the week or so before failure, but then it just went into sudden death.

I put a Hayden cooler up front. The step below the high end one with fan.

 

Out of curiosity, what did your transmission shop recommend? I would not bypass the factory cooling system, since it is designed as a heat exchanger. This is how your car was designed to operate.

Obviously, they replaced the radiator. So that's brand new. Your 14 year old van radiator failed. Which isn't unusual with a 14 year old van. I don't think that you need to bypass a brand new radiator, just in case it fails in the next 14 years. Your paranoia is that the brand new radiator unit will fail like the 1st unit. That failure took 14 years of wear & tear, corrosion, and bad luck. Keep your transmission system clean with regular fluid exchanges. Flush out your cooling system and maintain it with regular fluid exchanges,. This will prevent 14 years of buildup of contaminants.

The best thing for your transmission, actually, all transmissions, is to add a cooler. Add an external filter which you can change easily, Add a drain plug to your pan or change to a pan with a drain plug, so that you can drain & fill. Monitor the temperature. Use a Scan Gauge, or add a dash gauge.

 

Did you replace the radiator? You really want the radiator hooked up because it's a water to oil cooler and it cools much better than an aux air to oil cooler can. The main issue with running only an aux air cooler is,that you'll have little to no cooling while idle & slow driving and if you don't have a temp gauge and aux fan on it,you could burn up your trans.
Your best bet is to replace the radiator and hook it back up.