I know that heat kills transmitions, but I don't really understand what happens. Could someone fill me in on what happens to the trans fluid and how it affects the tranny? Thanks

Brian

The overheated oil is as much a result of problems as a cause. This is a self distructive circle where high loads, slippage, and hot oil conspire to destroy the transmission. As several of my friends found out, it can happen in moments.

Hot weather is one problem, it raises operating temperatures by virtue of the air passing through the radiator has less ability to absorb heat, therefore, the engine coolant temp goes up. This then increases the transmission oil temperature because the oil passes through a heat exchanger in the radiator. This in turn causes the transmission to get hotter. There are several hot temperature transmission  killer situations such as high speeds, heavy loads, and stop and go driving on a hot day.

High speeds requires more power be generated by the engine which means more heat must be rejected, if that's possible within the design constraints. The faster components must turn, the more heat is generated that has to be gotten rid of. This is true right down the driveline. The transmission gets hotter, the U-joints, the rear axle, every bearing, every gear, everyplace there's friction the temps go up in squared relation to RPMs.

Pulling a heavy load has the same effect as high ambient temps and high speed. The temps go up everywhere for the same reasons as above, that is more power must be generated to overcome the load, thus more heat must be rejected.

The problem here is that we usually combine high speed and high load often when on vacation when the weather is also hot.

Stop and go driving has its own killer heat profile which looks a lot like a combination of insufficient airflow to take the heat away combined with constantly having to use enough energy to restart the load from a stop or nearly so. Remember Mr. Neuton's laws, one states a body at rest will tend to remain at rest unless a force is applied, the other side is a body in motion tends to remain in motion. The summary of this from an operating engineer's point of view is it takes a lot of power to start a load into motion, it takes far less power to keep a load in motion. Thus the energy to restart the load combined with insufficient speed to move an adaquate amount of cooling air through the radiator core is the major sources of stop and go driving heat.

In the middle of all this is a torque converter. A torque converter is nothing more than a turbine pump. Anytime you pump a fluid be it a gas or liquid, it gets hot because you're using power to change its energy state from less to more. Behind the torque converter is a transmission with close tolerance bearings, high pressure apply pistons which tend to be steel inside an aluminum bore with rubber seals all with different expansion/contraction rates, bands and clutches that often as not have friction surfaces made of specially treated paper, but could be kevlar or a sintered metal. All of this hardware is bathed in the same oil that's being pumped around inside the torque converter and has also been pumped by the transmission's operating pressure pump. So just moving this stuff through the converter and pump insures the oil gets plenty hot. The hot oil form the converter must be cooled as even more heat will be added by the bearings, apply pistons, and slippage of the friction materials. So minimally, this oil is pumped to a heat exchanger in the engine's radiator where under nominal conditions it's temperature is brought down to at least that of the engine's coolant. It can't become lower than that temp because the laws of thermodyanics being what they are you can't get the oil any cooler than the cooling medium, you can, however, not get it down to the cooalnt temp.

So if operating conditions are such that the radiator heat exchanger runs out of capacity to cool the transmission oil sufficiently, the spiral process to transmisson destruction starts. The overheated return oil dumps back in the sump where it pumps to the converter where it acquires more heat. The hot oil can't cool the shafts and bearings, they expand and reduce operating clearance generating more heat and if they touch, metal ripped from them begins to flow through the system with the oil. The pistons and bores of the band and clutch apply system overheats and the pistons loose there ability to apply sufficient force to the bands and clutches. This allows slippage within the clutch packs or between the band and spinning gear cage. The temps go very high very fast from the friction of partial engagement. The friction surface whether treated paper, kevlar, or sintered metal goes beyond the materials strength and failure occurs. At this point the friction materal enters the oil to be picked up and circulated causing further mechanical damage. You vehicle stops because there's no longer a mechanical connection through the tranmission. Smoke pours out and there's the real danger of fire as all this oil is right at the ignition point, probably the only reason it ususally doesn't burn is the smoke replaces air inside the transmission and surpresses ignition. But if some air got in there it would burn or explode.

More and better cooling is one answer; as is asking people to drive slower and carry less weight. But this probably doesn't fit their plans for the day. Just in the past couple years I've had two friends fry their gear boxes in hot weather while on vacation, neither had towing packages. One was so mad at GM, he traded his Surburban for a Ford Excursion and proceeded to fry it the next summer because I couldn't convince him that you can't buy anybody's standard model and go haul  the old lady, 5 kids, a back end and roof carrier all full of gear, and tow a 23 foot travel trailer or boat across the desert mountains at a 120 degrees at 80 miles an hour. He saved money on the original purchase by not getting the factory tow package in both cases!? The -burban went up in smoke on day one while driving through city stop and go traffic on a 100 plus degree day. The Excersion at least got over the mountains and into the desert before it let go. In the -burban's favor, it was 2 years old and he had fried it a little the previous summer without making repairs, so it had a head start to distruction the Ford didn't. A third friend of mine has what's now a 3 year old Dodge Durango with a tow package, he's had non of these problems, not because it's a Dodge, but because he got the tow package and even added more cooling capacity before tackling the desert. All these guys are engineers who know better, but 2 out 3 always try to cheap it out, and as number 3 always says, "The easy way out leads back in".

Help arrives in the form of additional oil cooling. This is usually an add-on cooler plumbed ahead of the radiator heat exchanger. The reason for this is that automatics don't like to be too cold either. So plumbing the cooler ahead of the radiator heat exchanger allows a lot of heat to be taken out of the oil. If the resulting oil temp is lower than the engine's operating temp, then the radiator heat exchanger can be used to re-warm the transmission oil up before putting back in the sump. This is mostly a winter thing and if you're willing to do some extra plumbing it probably wouldn't hurt to bypass the radiator heat exchanger in hot weather, thus delivering cooler than engine temp oil back into the transmission. What ever size you compute for the external cooler, buy the next larger. Mount it where air can flow through it and add a thermostatically controlled fan to make sure air gets pulled through it during times of stop and go traffic.

While you're spending money, it's not a bad idea to install an add-on engine oil cooler with a thermostatic bypass valve (use a fan here also to insure airflow across the core under all conditions). A therostatic by-pass can also be used on the transmisson cooler to isolate it when it's not needed in cold weather. This helps warm the engine and transmisson faster. If you have a late model vehicle with engine oil circulated through a radiator heat exchanger (which is actually there to heat the oil to force a faster warm up, which is good for emission reduction and the engine). You will find that taking this cooling load off the radiator during hot weather will help prevent overheating. Also replace the 210-220 degree thermostat the factory put in with a 180 degree, unless you've already chipped the computer and made all the performance changes that go with the chip/preprocessor. 180 degrees is sufficient to take the computer out of warm up (choke) mode and buy's some operating space to work the engine harder without overunning the radiator's cooling capacity when combined with the previously mentioned changes.

Installing a gauge to track oil temps is a pretty good idea. You can use one gauge with a switch to monitor engine or transmisson temp. Both are happy from 180 to 220 degrees.   

Bogie

"... plumbing the cooler ahead of the radiator heat exchanger allows a lot of heat to be taken out of the oil. If the resulting oil temp is lower than the engine's operating temp, then the radiator heat exchanger can be used to re-warm the transmission oil up before putting back in the sump. "

This is also another bracket racing consistency trick.  By routing the hot fluid from the tranny into the external cooler first, then into the radiator's heat exchanger, this helps to ensure the the transmission temps are the same temperature from one round to the next (as long as you are able to control your engine temp effectively). The more consistent you can keep your temps, the more consistent your car will be.

... and thanks bogie for the detailed explanation of why excessive heat kills trannys. 

I learned the hard way, as well, about how to burn up a tranny. I had a 97 half ton 2wd ext cab Silverado with a 350/ 4L60E and no tow  package. I was towing my 5000lb trailer/Chevelle about 160 mi to a drag strip, doing 80mph...   with about 30 mi to go, I noticed the rpm's were rising, yet the speed was decreasing... I had overheated the tranny and pumped out a couple quarts of fluid (all over my Chevelle). I was afraid to stop, afraid it would freeze up and strand me, so I slowed down to about 25mph on the shoulder for a while. When the temp came down, I could drive about 55mph.  I made it to the drag strip.  Fortunately, a friend of mine was there without his racecar or trailer, and was nice enough to tow my car home for me. My truck made it home just fine without pulling a load, but later that week, when I had the tranny rebuilt (and cooler added) the mechanic said he'd never seen a convertor so discolored from heat.

Thank you very much for the replies, gentlemen. I'm seriously considering buying a cooler because of all the stop and go I do in traffic every day. Also, is synthetic ATF worth the money, and if so, what's the best brand to go with for a 90 4L60? Thanks!

Brian

God, I just reread what I wrote, nobody said anything about the spelling and sentence structure. I must' have been really in a hurry. But then way back in high school I took autoshop not typing. Today I have a car with a computer that can't be worked on and I sit in front of a computer keyboard I can't type on.

Anyway, yes a synthetic oil will buy you a few more degrees like maybe 20 to 40 before things go up in smoke.

Actually it's the smoke that makes things work or at least that's the Phosgene Theory and there seems to be good proof of that. You will note that when smoke leaks out of your transmission it stops working. When the smoke leaks out of the alternator, or stereo they stop working. You can go in the house and see that when the smoke leaks out of the TV or your computer they stop working. In fact everything that the smoke leaks out of stops working. Therefore, smoke must be what makes things work.

Bogie