[AceSpades]

So last year I added an external cooler to the existing cooler and maintened the flow through the coolant radiator as well. Towing my boat up the hills I managed to keep my temps at 188F everything looks good. Last month the trans fluid started leaking from the coolant radiator, so I thought to bypass it and see how it goes...This weekend I tow a single PWC and on slight inclines and it only peaked at 160F. Yes this was an apples to oranges comparison (and it wasn’t intended to be) but the cold temps had me worried as optimum cooling happens at around 180F. With that I’m thinking the coolant radiator acts more like a regulator, where if it’s too cold it actually warms it to ideal operating temps, and when it gets too hot it helps to cool it down. In other words it’s purpose is to get it to 180 or whatever value it may be. I was expecting it to be warmer because I removed a cooling device, not so.

[Drcoffee]

So I also have an aux atf cooler and run at 130-140* in 90* F ambient. Ideally you want the temp to be at 160-170*. This prolongs the life of the fluid and tranny and help rid condensation. If you are running cold, attach a cover to cooler and adjust it based on outside temp. I cover 75% of my cooler during the winter. I used a plastic yard sign board and cut it to shape. Zip ties work to hold it in place.

[UrbanRunner651]

Likely just due to the lighter load & your aftermarket being more efficient. I wouldn't even consider that cold really..
Your rad is different because it is limited by the thermostat as well.
I monitor the pan on my v6 and she likes to stay right around 164ish when driving under normal conditions in the summer. Jumps about +10 with a 4x8 trailer. Longer trips/larger loads with small hills on warmer days it will get closer to 190-200. (no aftermarket cooler)
I drive it all winter through freezing temps and have yet to experience any trans issues during shorter trips where it doesn't reach temp.

[firebirdguy]

doesnt sound like a major issue running just the aux cooler & being you are in socal the extreme cold of winter temps shuldnt be an issue, but keep in mind the rad cooler is always better to keep hooked up & use the aux cooler in series with it. one thing to consider is when towing or super hot outside temps & prolonged driving is that the aux cooler doesnt provide any cooling at lower speeds or when stopped like in traffic jams or off roading climbing hills etc. that is where the rad cooler is needed & being liquid cooled its much more efficient at removing heat than air cooled.

running too cool with just an aux cooler is an issue but just as much if not more important is running too hot in situations where the cooler cant get proper air flow & when out driving or especially towing you never know when that can happen.

did you determine why the rad cooler was leaking? could have been a loose fitting or bad clamp/hose, could be an easy fix to get the rad cooler hooked back up or worst case scenario a new denso radiator is about $100 & a quick & easy change, if the rad is higher mileage or has a leaky trans cooler issue, a new rad would be the best option IMO.

im sure you have seen this but for those that haven't here is a trans temp chart & diagram of how every aux cooler company recommends installation.