Auto Service World
Feature   August 1, 2003   by CARS Magazine

Avoiding Transmission Disaster

Two experienced SSGM readers discuss a transmission-saving aftermarket accessory: coolers. What's the right way to install one?

Michael Craig, shop foreman at Jack McGee Chev-Olds in Peterborough, Ontario kicks off:


I just thought I would send a note to review the instructions on page 32 of your CAT article in the June 2003 issue. The article states that to properly install a transmission cooler, it should be inline with the original cooler pipe, in the return side. I feel this is not correct and could lead to further transmission problems.

Almost every car with an automatic transmission comes from the factory with a transmission cooler in the radiator. The purpose of this cooler is to both cool hot fluid, and warm fluid which is too cold because as we all know, automatics may behave quite differently when they are cold. The fluid level may also be affected because generally the fluid level is controlled by a bimetallic metal strip.

Let’s use an example vehicle with round temperature numbers to illustrate a point.

Your way, if a vehicle is pulling a trailer and the transmission fluid is very hot, say 140C, and you pass it through the rad mounted cooler, it is cooled by the coolant first, which itself started at the far side of the rad at about 100C and has been cooled to about 70C. (You will notice that all transmission coolers are in the cold side of the radiator) So the trans fluid temp is reduced to about 110C, while the engine coolant is back up to about 85C. (Notice how this way will also cause the engine to run hotter while it is pulling a trailer) Then the trans fluid goes through the aftermarket cooler, reducing it to maybe 75C or colder, just inside of operating temperature.

Then on a cold winter day, with no trailer, when the coolant in the rad is reduced to almost freezing unless a piece of cardboard is placed in front of the rad, the trans fluid is already cool coming out of the transmission, where it is passed through the cooler in the rad, maybe losing or gaining a few degrees from the coolant since it will be quite cool anyway. The trans fluid is then routed through the auxiliary cooler where it is further reduced in temperature, possibly to below 0C depending on the vehicle speed and ambient temperature. This overcooling can’t be good for bearings depending on lube fluid, or for just plain old fuel economy.

My way, on the hot day with the trailer, where your cooling problems are the largest, the super hot trans fluid will first be routed to the auxiliary cooler where it will be out in the open air blast, and should have its maximum effect in reducing the fluid temp. In the best case, the fluid would be reduced to below 70C (or approximately the temp of the coolant in the cold side of the rad) so that on its way through the rad it can also remove some of the BTU’s from the engine coolant thereby making the rad more efficient to keep from burning up the engine as well. Or if the fluid is still above that, the rad cooler will still have its chance to help cool the fluid, but both coolers will be working together to cool both the engine and the transmission.

And finally, on that cold day, the already cool fluid will first pass through the auxiliary cooler, being reduced to below its operating temperature, but will then pass through the rad to be warmed before returning to the transmission.

This is just my opinion, but it is also what I have been taught by all of my instructors since auto shop in high school.

After reading Michael’s detailed E-mail, SSGM Editor Jim Anderton checked back with the transmission technician of the original article, Mister Transmission’s Dieter Schmitz. Schmitz replied:

Hi Jim:

I read the e-mail and did some fact finding, with some help from a design engineer at Long Manufacturing.

First off it’s a known fact that transmission temperatures can reach up to approx. 310F and if sustained for long periods of time will cause the fluid to break down as well as cause problems with internal seals. Add to this the additional strain of towing and you also add to the temperature of the transmission and the engine. Today’s modern cars also run hotter than those of 15 years ago do and at the same time they seem to reduce the size of the grill area for airflow. Suffice it to say that additional cooling of the transmission fluid is essential.

Let’s look at what happens when an auxiliary cooler in connected ahead of the radiator.

The transmission fluid coming out of the transmission will be at its maximum temperature. The difference in the temperature of the fluid and the temperature of the airflow is great enough that the airflow will only cool the fluid to slightly above or the same as the coolant in the radiator, (as tested by Long Manufacturing) thereby nullifying the cooler in the radiator. With this in mind it only makes sense that the auxiliary cooler be installed after the radiator. With this arrangement the hot fluid will first be cooled down to coolant temperature and then be cooled even further by the auxiliary cooler. This will maximize transmission fluid cooling. As far as the threat of the fluid being over cooled, you have to consider the situations that can cause this. Transmission fluid as well as engine coolant is engineered so that under normal conditions it will not freeze. I also believe that our climate conditions are such that this would likely never happen. Most people will warm their cars up in the winter months before they drive off anyway. Also with the advent of self-regulating coolers and cooler bypass valves, this threat is virtually non-existent.

Hope this clears things up a bit.

Two different views, both well argued. Who’s right? E-mail SSGM editor Jim Anderton at to register your opinion, and check back with SSGM as we explore this issue further.SSGM