This quote was too good not to include in this thread.
At least you have some sort of factory trans cooler. My '00 Vulcan/AX4S only had a non-finned bar for the PS. The transmission just went straight in and out of the radiator heat-exchanger. I put an auxiliary cooler on it the week after I bought it with a 180 degree bypass. That was at 42k miles. Zero problems 132k miles later.
FWIW though, the entire purpose of the trans cooling system is to get the fluid to operating temperature as quickly as possible and keep it there. As a whole it does a good job of this. Though we always get all excited about lowering our trans temps, mainly because of the fragile nature the the AX4S/AX4N, temps below 160 can be just as detrimental to the transmission as high temperatures. I can't tell you how many transmission issues I've seen in truck applications that were caused by the owner installing a large transmission cooler without a bypass. This is a really big issue in colder climates.
Ideally the trans fluid temperature would be a steady 180 +/- 20 degrees. With Mercon V, fluid temperatures really don't get dangerous until you hit the 250 degree range. Fluid life begins to diminish at a much faster rate above about 220 degrees though because of oxidation. Rarely does the stock cooling system exceed this. Unless you live in a very warm climate or are constantly under heavy load (towing, hills, etc.), the stock system is sufficient in most cases. By the time the Gen 4's came around Ford had been building these transmissions for 14 years. They knew what they were doing, at least as far as cooling them. Fluid temps are the least of the AX4*'s typical problems. If they could have fixed the issues with a simple one or two bar finned cooler they would have in a heartbeat. That said, an auxiliary cooler can be good insurance to prolong fluid life if you routinely see trans temps above 200 degrees or so. You just have to be mindful that your temps don't drop too much. A bypass is a really good idea.
I have started looking at the temps on my 03 and comparing them on the same commute to my 06. The O6 had only the radiator tank cooler and I added air to fluid cooler to it. My 03 has a single pass factory cooler. I have not yet had a day where it was really hot and the traffic was slow yet for testing so I will have to update this thread. Trying to decide if I want to make a change on the 03 or not at this point.
22 Mile 95% highway commute. Shorter times are no traffic and longer times are with stop and go traffic. AC
in use on the hot days.
2006 Taurus with only the radiator tank cooler no fluid to air cooler.
25 min drive 67 degree day 160 final trans temp
40 min drive 75 degree day 164 final trans temp
35 min drive 68 degree day 167 final trans temp
35 min drive 70 degree day 171 final trans temp
45 min drive 79 degree day 189 final trans temp
50 min drive 90 degree day 200 final trans temp
212 and 214 were the max temps I saw on the hottest days for a short time climbing a grade.
The 30 plus minute drive in hot weather seems to be enough to get to the point where the temp has stabilized to the condition.
2006 Taurus with Derale dual pass finned transmission cooler and radiator tank cooler
25 min drive 96 degree day 177 final trans temp
35 min drive 92 degree day 189 final trans temp
40 min drive 89 degree day 178 final trans temp
35 min drive 79 degree day 168 final trans temp
40 min drive 92 degree day 178 final trans temp
45 min drive 79 degree day 180 final trans temp
With the Derale cooler installed I have never seen the 200 degree plus temps on the same route on that grade on a hot day.
2003 Taurus with factory single pass finned cooler and radiator tank cooler
25 min drive 68 degree day 144 final trans temp
30 min drive 82 degree day 184 final trans temp
30 min drive 91 degree day 189 final trans temp (84 deg starting trans temp)
26 min drive 56 degree day 147 final trans temp (78 deg starting trans temp)
30 min drive 56 degree day 141 final trans temp (75 deg starting trans temp)
32 min drive 82 degree day 182 final trans temp (90 deg starting trans temp)
30 min drive 54 degree day 140 final trans temp
36 min drive 74 degree day 182 final trans temp
26 min drive 42 degree day 125 final trans temp
29 min drive 70 degree day 161 final trans temp