There has been allot of talk about aftermarket transmission coolers and low temps. Well recently down here in Houston we have had temps in the low 20's the last two days so I thought it would be a good chance to take some temp measurements from the transmission cooler.

First off I have a B&M transmission cooler 70264 which is 14,000 BTU unit mounted in front of the condenser. I have the transmission lines routed to go from the transmission to the spin on filter which has my temp sender on it, then to the cooler and finally back to the transmission.

The cooler is designed with "bypass feature" as B&M calls it which allows the thicker colder ATF to bypass the cooler by going through two wider channels at the bottom of the cooler.

So now the results.

With the outside temps running in the low 20's I got in the 4runner that had been sitting all night and started driving. My house is just one street off the highway so there no stop and go, just back out of the driveway, go down the street and turn and get on the highway.

Well after 10-15 minutes of driving on the highway the transmission temp was hanging between 105-115 degrees and stayed that way for the rest of my 50 mile trip. On the way back I got into some stop and go traffic and finally with a good hard acceleration on the on-ramp I was able to heat the transmission up to 165F.

As soon as I got on the freeway I got in the left lane and put cruise on 75mph. As soon as I put on the cruise and saw from the tach that the torque converter had locked up, the transmission temp started to fall.

In about 1 minutes after the torque converter locked up the temp fell from the previous 165F down to 105-110F and stayed there from the rest of the 50 mile trip back home.

The above test proves that with the "self regulating" temperature design of B&M and most high quality transmission cooler that a in-line bypass thermostat is unnecessary. It also means that keeping the stock radiator/transmission cooler inline is unnecessary and very risky when you consider the ramifications if it fails and allows engine coolant into the transmission.

As far as the issue of how hot transmission fluid needs to be to function properly, I've spoke with several transmission techs and the going answer is 80-90F. My Chevy Duramax (hey I know but Toyota doesn't have a 1 ton diesel) which has just a large transmission cooler (no connection to radiator) runs around 100-115F also in the low 20's weather by factory design.

Another reason not to use a bypass thermostat or keeping the factory cooler piped in is heat. With just my B&M cooler during the 95-100F summer days this year my transmission never got above 125F on the highway with the torque converter locked up. During stock and go traffic or hard acceleration it would get up to 175F max but very quickly cool back down to the 120-125F range.

Any transmission technician will tell you the colder you can keep a transmission the long the transmission and fluid life will be. With the bypass thermostats your guaranteeing your transmission will never get below the thermostat temp which is normally 165-180F. With the stock transmission cooler still in-line your temps will vary with outdoor temp but in most cases will stay around 165-170F.

So if your considering a aftermarket cooler to protect your transmission from the risk of the stock radiator cooler failure or just want to keep your transmission cooler, forgo the bypass thermostat or keeping the stock cooler in line and just go with a good temp regulating cooler like the B&M or the IPT.

I've been getting some baseline temps with my stock in radiator cooler while I've been waiting for a warm enough day to install my aftermarket. Stock temps have been looking like an average between 124-168 with the air temps being between 14-35