Transmission temperatures 101.

One thing I enjoy on this forum is learning new things. One thing that is not clear is my mind is the general recommendation for transmission fluid temperature ranges and recommended configurations. I just learned a certain combo had recomendations for an indepdant trans cooler. I didn't see that coming but I am not an expert. Can someone chime in? Maybe this becomes a sticky? :3gears:

 

I run a medium sized B&M cooler with the radiator bypassed. After a 25 Mile Interstate crusie in Overdrive with lockup in 40* weather, I noticed the fluid going through the cooler was barely warm.

In the summer time it is usually real hot.

 

So that is interesting, I have read anywhere from ambient + 90F or less than 230F. I have never monitored my trans temp so I have no idea what it is running at in the worst case where the car is sitting~200 water temp. So I believe the 180-210F is the target range. So in what situation do the trans cooling configuration change from the radiator to something different. For example, run an external cooler in the following climate like in AZ where water temp can reach 210F, you have a high stall converter etc etc. Maybe I am over analyzing this...o No:

 

Transmission coolers are placed in the radiator for TWO reasons: one is to cool the fluid and keep it at optimal temperature for longevity of the fluid and thereby the transmission. Secondly and just as importantly it is to warm the transmission fluid after startup as quickly as possible. The transmission will not warm up just by idling the engine in Park to get the heater going if cooled by an air cooled setup but it will warm nearly as fast as the engine once the thermostat opens when circulating through the radiator cooler.

Oil and transmission fluid is designed to be most efficient when operated in a narrow temperature range. Most would be in the range of appx 180 to 250*F. Anything much below that causes parasitics and much above that leads to lubricant breaking down.

Weather and load conditions dictate what is needed. My best results on my tow vehicles were accomplished by running the trans fluid through a large air cooler first and from it to the radiator and then back to the transmission. This allowed the high capacity cooler to do it's job when towing and relieved some heat load from the radiator. I also had the added benefit of warming the fluid when in cold weather or not towing, still keeping the fluid at an optimum temperature. I always put a guage in the transmission pan of any tow vehicle.

If you do not have a temperature guage in the transmission pan, you will never see the huge temperature variances that are possible by using an air only cooler and deleting the radiator circuit. Overcooling is just as detrimental as overheating.

 

I've heard (from ATCO?) that aftermarket people tend to over cool their transmissions, leaving the fluid thicker than the valve body is designed for. Remember it's a hydraulic system that uses orfices and the orfices are calibrated for an expected fluid viscosity. I've also "heard" but not verified that some OEM's actually have thermostats on their transmission coolers. Anyone know of one?

If you have a very loose torque converter or haul a lot of weight for long periods of time I can see needing an aftermarket cooler on top of a standard radiator one.

My 2007 Chevy Truck has a transmission temp gauge and it runs fully warmed up, highway speed driving, unloaded, for hours on end, around 160*F. When I was towing my Skylark home from Colorado it hung around 180-185*F and I spiked it to 220*F a couple times on the long steep grades keeping my foot in the throttle (I stopped doing that on the third rise, I slowed down at that point).

 

Light load temp of 160* seems acceptable to me. I don't know how accurate your guage is but the trans cooler is in the cooled side of the radiator which would be somewhat lower than thermostat temp. The actual temp would depend on how much coolant the thermostat is allowing through and how efficient the radiator is. As the thermostat reaches fully open under load and the radiator is working at maximum capacity, the cool side back to the engine may only be 10-15* cooler than guage temp. The trans temp will reflect this and begin to climb under hard load. A comparison of engine temp and trans temp will show the variables and how efficient the systems are operating. These temps and relationships will vary with each vehicle. My vehicles nearly always ran close to coolant temp but climbed above coolant temp under hard pull with converter unlocked. Locking converter always dropped the trans temp 10 to 20*.

 

Here's a helpful chart from TCI. The closer you can keep the temperature to 200* or so, the better. Too cold though can also be problematic. Some of the nicer aftermarket trans coolers actually have a built in bypass system, meaning no fluid flows through the cooler when the fluid is under a certain temperature. I have decent sized auxiliary cooler mounted in front of the radiator. I've never measured the temperature, but I can usually touch it after a drive without being burned. The same cannot be said for the radiator. It might run a little cool, but I decided to err on the side of caution since my converter is pretty loose. Too much heat is generally more detrimental than not enough.