The company has found only one confirmed instance of a transmission failure related to this defect in its passenger cars, with no accidents or injuries. “We are acting out of an abundance of caution to ensure that this doesn’t become an issue for our customers,’” said Tom Elliott, executive vice president of American Honda Motor Co., Inc.

The auto tranny wastes a lot of power, that power has to be turned into heat, and there is only a tiny cooler to get rid of it...

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A few thoughts on CLS transmission getting really hot...

First some assumptions and/or facts (depending on your “views):

1. MAX HP of stock CLS engine = 260 HP

2. Typical max wheel HP of unmodified CLS/TLS on Dynojet dnos: 195 to 205 HP.

3. HP lost between engine and wheels is between 55 and 65 HP.

4. Assume that the bulk of this loss is from the transaxle (torque converter, transmission, diff, etc).:

5. The transaxle has only a tiny fluid-to-fluid intercooler to get rid of a shitload of heat.

6. A fluid-to-fluid intercooler is more efficient than an air-to-water intercooler of the same size (with special exceptions). However, that statement presumes that the gas or fluid that is doing the cooling is at identical temperatures. In the case of the CLS, the fluid-to-fluid intercooler is using HOT engine coolant to “cool” the ATF. An add-on air-to-fluid cooler can have its air flowing past at 0- to 120-degrees F (that’s a lot lower than the temperature of the fluid in the stock cooler.


So, I was thinking, 240 degrees max oil temp is about as hot anyone would want to run in a auto tranny with conventional ATF (and you would probably want a synthetic fluid). I also looked around and some anecdotal info implied that gears and clutches could be close to 350 degrees when the ATF is at 240 degrees F. (The people I called and the web pages I found suggest that auto trannies should be run around 175-degrees F for best results.)

So, what does this have to do with anything?

External transmission coolers are generally indicated for heavy-duty operation (drag racing, hill climbing, high temperature climate, police use, towing, and other situations where the ability of the transmission to get rid of “excess” heat is a must. The B&M ATF coolers are rated in GVW (Gross Vehicle Weight). There are a few coolers that specify the amount of heat that they will “exchange” (this should probably be assumed to be a maximum or limiting value as the ambient temp and air flow will have a dramatic effect on this value). One cooler I found has the following dimensions and specs: COOLER H. D. 8" x 18" X 1-1/2" (1 /2 NPT) 14,500-21,500 BTU/HR.

I wanted to first get a “guesstimate” of how much heat is being dissipated at full output and that is where the 55 to 65 HP figure comes in. I am now going to convert those amounts to BTU/HR. The conversion from HP to BTU/hour is: 1 horsepower = 2,547.2 BTU/hour.

IF I assume that 50 HP is being lost under full load conditions (and under continuous WOT operation), the transaxle needs to get rid of approximately 50 * 2547.2 BTUs per hour (127,360.0 BTU/HR). (That’s ten times the amount of cooling available from a good size EXTERNAL plate cooler.)

WOW, 127,360 BTU per hour is a lot of heat to get rid of at full power. How does all that heat get removed from a stock CLS transaxle? (The CV joints and other non-transaxle drivetrain components account for some energy losses and some of the heat [wasted energy] is going to be transferred through the transmission’s case. For argument sake, cut my figure in half and think about driving uphill at WOT for 20 minutes!

Can the CLS’ tiny fluid-to-fluid intercooler with its “sink” temperature (engine coolant temperature of around 160 degrees) be capable of keeping the tranny’s temp to 175-degrees F?

And if the car is only running down the road using an average of 26HP, that still means that the “potential” loss is still 127,360 BTU/hp / 10 (260/26) = 12,735 BTU/HR.

IF a BIG cooler is sitting in the car’s air stream is only able to remove 14,000 to 21,000 BTU/HR, it goes to reason that this radiator is only capable for removing the heat energy equal to 25HP/hour (average). What does this say for the CLS when it is being driven at 80-90 MPH up hills WITH THE STOCK COOLER?

Quote from transmission repair site:

“Heat is what sends a transmission to the shop 90% of the time according to the Automatic Transmission Association, a trade group that keeps transmission mechanics informed about special problems and fixes for their profession. If you have a transmission with a hard to fix problem, and a transmission mechanic belongs to this organization, you have a better chance of getting the problem fixed. Only about ten percent of the failures of automatic transmissions with less that 100,000 miles on them are from something other than heat failure. Engineers design a transmission to have a normal operating temperature of 175 degrees F. At 250 degrees, the rubber parts are going to loose flexibility resulting in control servos that leak resulting in slippage when the car starts up cold in the morning. At temperatures higher than 250 degrees, the clutches and bands start to slip causing imminent failure. “


AND

” At elevated operating temperatures, ATF oxidizes, turns brown and takes on a smell like burnt toast. As heat destroys the fluid s lubricating qualities and friction characteristics, varnish begins to form on internal parts (such as the valve body) which interferes with the operation of the transmission. If the temperature gets above 250 degrees F., rubber seals begin to harden, which leads to leaks and pressure losses. At higher temperatures the transmission begins to slip, which only aggravates overheating even more. Above 300oF, the metals inside the transmission will warp and distort in varying degrees, depending on the severity of overheat. Eventually the clutches burn out and the transmission calls it quits. Because this damage occurs and fluid life is so seriously impaired, rocking out snow, mud or sand should never exceed a very few minutes. The only way to repair the damage now is with an overhaul -- a job which can easily run upwards of $1500 on a late model front-wheel drive car or minivan.
As a rule of thumb, every 20 degree increase in operating temperature above 175 degrees F. cuts the life of the fluid in half. The approximate life expectancy at various temperatures is as follows:
175F 100,000 miles
195F 50,000 miles
212F 25,000 miles
235F 12,000 miles
255F 6,250 miles
275F 3,000 miles
295F 1,500 miles
315F 750 miles
335F 325 miles
375F 80 miles
390F 40 miles
415F Less than 30 minutes

If you think this is propaganda put forth by the suppliers of ATF to sell more fluid, think again. According to the Automatic Transmission Rebuilders Association, 90% of ALL transmission failures are caused by overheating. And most of these can be blamed on worn out fluid that should have been replaced.

On most vehicles, the automatic transmission fluid is cooled by a small heat exchanger inside the bottom or end tank of the radiator. Hot ATF from the transmission circulates through a short loop of pipe and is thus "cooled." Cooling is a relative term here, however, because the radiator itself may be running at anywhere from 180 to 220 degrees F.!
Tests have shown that the typical original equipment oil cooler is marginal at best. ATF that enters the radiator cooler at 300 degrees F. leaves at 240 to 270 degrees F., which is only a 10 to 20% drop in temperature, and is nowhere good enough for extended fluid life. “