[Huskymaniac]

But that still puts the stock cooler in series with the external cooler. As Tom has mentioned, the stock cooler has very high flow resistance. IF flow is an issue then you wouldn't want to do that. I think what we would want is coolers in parallel until the ATF is up to temperature and then just the external cooler at temperatures above that. Then you could have the benefit of a low resistance, high efficiency, external cooler and the benefit of heating the ATF at low temperature.

Alternatively, you could have the stock cooler in line until the ATF comes up to temperature and then have the external cooler in line. You still would have more flow restriction but only until the fluid is up to temperature.

The TStat that has been discussed above is nice in that it actually provides some flow in both directions, most of it bypassed, until the fluid is up to temperature and then only straight through flow when it is up to temperature. In that way, it sort of does put the coolers in parallel when the ATF is cold and then only uses the external cooler when hot. The diagram is attached.