I have an auxiliary cooler fitted before the radiator cooler. On these cold winter days the transmission is not warming up quick enough to lock the torque converter. It takes over 10 minutes of driving, most of which is freeway before the revs drops to 2000rpm @ 100km/h. This is causing the fuel consumption to be higher than it should be.

I have done some research and found a Derale 4 way thermostat that will bypass the coolers.
http://www.ebay.com.au/itm/181431068591?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Looking at my setup it would easily fit on the 2 lines going to the aux cooler, this would mean that fluid is always going through the radiator cooler no matter what (stuck thermostat etc.). Other option is to put it right at the lines from the transmission, bypassing the coolers entirely. I think this would help it warm up even quicker as I don't think the radiator adds much, if any heat untill the engine has been running for a while. Thing is if the thermostat jams in bypass then the oil will overheat. Plus it would be a tight squeeze with the Magnefine filter.

Anyone know how reliable these are? I was going to change the oil before installtion as it would be more difficult with the thermostat in.

I have one very similar to that.
The way it is connected is that the ATF goes to the radiator cooler then to the thermostat and then if it is still too hot then it goes to the auxiliary cooler, otherwise it bypasses it.

I would assume it to be reliable, seeing as a thermostat for the coolant lasts a long time.

My observations though are that the ATF gets quite hot quite easily as the auxiliary cooler is hot to the touch most of the times.

I have my auxiliary cooler fitted right in front of the AC fan, so that when the AC is on there is always airflow over, even in traffic. And the AC is on in warmer weather when the ATF is more likely to be hot.

I was thinking of adding an additional thermo switch where the AC fan switches on if the ATF gets too hot

I went to Autobarn today. They had one that looked like an older model but much the same thing. It came with 3/8" hose barb fittings and hose clamps for $140. I asked the guy if he would swap them for 5/16" fittings, he said he couldn't as they were part of the pack but he would sell me the fittings for extra money. That would have made it rather expensive so I left it.

It appears that these things don't actually block the flow of oil to cooler, they just open up a bypass to give it an easier path to return to the transmission. Then when hot the bypass closes off forcing the fluid to go throught the cooler. I got to thinking that you could put a T-piece in each line with a solenoid valve between to open and close the bypass. The valve could be setup to shut the first time the toque converter is instructed to lock, or even by hooking it up to temperature sensor of the transmission. Problem is that solenoid valves tend to be quite restrictive meaning you would need a bigger one which would weigh a lot (requiring a support bracket), also the wiring would add extra complexity. If you could find a light weight non-restrictive valve it would work. Even adapt a small thermostat from something else and do away the electrical side entirely.

I like that idea to be honest, but as you mentioned the complexity is starting to get pretty high.

Another way would be to get one of those ATF coolers that have fans on them and a thermo switch, and mount it in a place with minimal air flow.
That way when the oil is cool there is no airflow over the cooler, and once it starts getting too hot the fan kicks in and cools down the fluid but since you've already installed a cooler it seems like a bit of a waste.

I think the ATF still gets hot even if the torque converted is locked though, and it is not a case where if the torque converter is locked then the fluid cannot overheat.

Do you have pics of how the cooler is installed?
And what size is it?

This is the thermostat bypass I have on my car:
http://www.ebay.com/itm/Tru-Cool-Thermal-Bypass-Transmission-Cooler-By-Pass-Cold-Weather-Use-Long-Brand/201363680994?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%2 6asc%3D31192%26meid%3De600b018b243417584cc489b5191 580c%26pid%3D100005%26rk%3D3%26rkt%3D6%26sd%3D1216 60116494&rt=nchttp://www.ebay.com/itm/Tru-Cool-Thermal-Bypass-Transmission-Cooler-By-Pass-Cold-Weather-Use-Long-Brand/201363680994?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%2 6asc%3D31192%26meid%3De600b018b243417584cc489b5191 580c%26pid%3D100005%26rk%3D3%26rkt%3D6%26sd%3D1216 60116494&rt=nc