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Automotive Transmission Care and Maintenance

Submitted By: Kit Sullivan, Director of Training
Southern Express Lubes, Inc.


Take care of your transmission and it will last a long time.

The typical example of a ‘classic’ muscle-car is one with a big squeeze high-compressionV-8, a fossil-fuel swilling 4-barrel carburetor, a pavement-wrinkling ‘locker’ with grippy rubber and a rock-crusher ‘U-Shift-It’ manual transmission, all in pursuit of 12 second ¼ miles. Revisionist history as well as some hazy recollections of the ‘good old days’ would seem to illustrate that all of the so-called ‘Super-Cars’ of the 60’s and early 70’s were manufactured to be just such a beast. To be sure, there were plenty of our beloved chariots that were assembled in exactly this fashion. However, fond remembrances be damned, most of our classic muscle-cars from back then were in reality assembled with components that were only slightly more ‘high performance’ than the basic-transportation grocery-getters that they were derived from. The standard package on most of the go-faster brethren included a little higher (but still fairly reasonable) compression than normal, a medium-sized 4-barrell carburetor, a limited-slip differential and a choice of a manual 3 or 4 speed transmission or an automatic transmission. To the purists, an automatic transmission in a performance car is a sacrilege. Why, how can you be in control of your ride if you don’t even have control of when the gears change?

What the auto manufacturers realized early on is that for every bona-fide ‘gear head’ that wanted the ultimate drag-strip terror, there were many, many more who wanted the association of high performance, the look and feel of a real barnstormer, without the bone-crushing, teeth-jarring reality and consequences of an authentic killer street-machine. No, what most of the car-buyers then wanted (and needed) was a machine that would take them, at street legal speeds, to work, school, and any other mundane excursion they might have in mind, including (shudder)…the grocery store.

The real beauty of the typical muscle-car from that era is that it could do all the things that the typical family sedan could do, with the same relative ease and comfort of operation, and still have enough of a performance ‘attitude’ built in to be an attention-getting, fun to drive ‘excitement machine’ whenever the lucky owner got the urge. This dual-personality nature is what marks the difference between a typical ‘backyard hot-rod’ and a true factory-built ‘Supercar’ from the 60’s and 70’s. The flexible abilities of these cars were due in part mostly to the ‘civilized’ features that were included on most of them. That desire for ‘civilized performance’ resulted in a large percentage, if not most, of the classic muscle-cars being equipped with an automatic transmission.


To some of the most knowledgeable mechanics there are, the inner workings of an automatic transmission are nothing but a big, murky, hard to comprehend combination of valve-bodies, torque converters, modulator valves and planetary gear-sets. Fortunately, automatic transmissions are relatively durable and long-lasting devices that need only a modicum of maintenance to deliver many trouble-free miles of service.


The ‘killer’ of automatic transmissions is heat. Excessive heat causes the fluid to lose its ability to lubricate, which leads to higher internal-component friction, all resulting in rapid wear-and-tear and premature transmission failure. Keeping the transmission and its fluid cool, and in the correct operating-temperature range is crucial for long life.


Automatic transmission fluid is probably the single-most complex and highly-engineered lubricant used in an automobile, including even the most sophisticated, synthetic motor oils. They are typically made from as many as 20 different elements, whereas a common motor oil may only contain 8-10. At higher, elevated operating temperatures, automatic transmission fluid can oxidize rapidly, leading to complete transmission fluid failure in a short amount of time. ‘Oxidation’ is simply the result of the transmission fluid absorbing oxygen, which in turn causes it to dramatically increase its viscosity, or thickness. Heavily oxidized automatic transmission fluid is not the nice and thin, relatively low viscosity fluid that is needed to properly lubricate and cool your transmission. No, oxidized transmission fluid is thick and ‘gooey’, does not lubricate well, causes a dramatic increase in heat and friction, and…well, you know the rest. Severely oxidized transmission fluid leads to deposits of sludge and varnish in the transmission, a rapid increase in the corrosion of the copper-alloy bearings, hardening of the various elastomeric seals, plus excessive glazing, flaking and wear of the clutch-plates and bands. Any one of these conditions is bad news for your transmission and wallet, but all of them can be avoided by simply changing your transmission fluid before oxidation can alter its protective abilities. The big question is: How do we know when the time to change the transmission fluid has arrived?Most of the auto manufacturers have recommended intervals for the changing of the transmission fluid in the owners manuals. Most often the recommendations call for fluid changes at somewhere between 12,000 and 30,000 miles. If you follow those recommendations, you will probably get many years of good service from your transmission. That, of course, applies to a regular street driven passenger car.


Of course, if you have a factory-built high-performance engine, or a big-cube power-maker, they both have a similar result from all that extra ‘motivation’ they produce. That result is HEAT! This extra heat easily finds its way into the automatic transmission, putting an extra burden on the automatic transmission fluid. In fact, the operating range of transmission fluid is so critical, that even a seemingly insignificant 10-degree difference in operating temperature can have severely damaging effects on your transmission. According to ‘B & M’, the optimum operating temperature for automatic transmission fluid is 170 degrees. For every 10 degree increase in operating temperature, the useable life of the fluid is reduced by half.

For example;
If the temperature is: the fluid life is:

170 degrees 20,000 miles
180 degrees 10,000 miles
190 degrees 5,000 miles
200 degrees 2,500 miles
210 degrees 1,250 miles
220 degrees 675 miles
over 220 degrees 0 miles

Obviously, our high-performance cars are putting our automatic transmission fluid to the test!


How then, can we best keep our transmission fluid as cool as possible?

Most cars provide their transmission cooling through a cooler that is an internal part of the car’s radiator. This may provide adequate cooling for the transmission under most operating conditions, but since it is basically sealed from, yet immersed in hot engine coolant, its overall abilities are severely limited. Add to that the fact that whenever the engine coolant starts to run a little hotter, the transmission fluid will naturally run hotter also.

The hard fact of the matter is that if you overheat your engine, or even run it hotter than normal, just one time…then you have overheated your transmission fluid. So keeping the transmission cool is of utmost importance if long transmission life is your goal. The single-most effective method to keep an automatic transmission within the correct operating temperature range is to add an auxiliary transmission fluid cooler, to supplement the radiator’s internal trans-cooler. An auxiliary trans-cooler is simply a small ‘fluid-to-air’ radiator that is dedicated to only cooling transmission fluid. They are most often installed in-line with the regular factory-installed units, and are installed in front of the cars radiator. These are typically good for about a 20-degree drop in the operating temperature of the fluid, which will add many years of life to the transmission.

If your automatic transmission-equipped muscle car does not already have an auxiliary trans-cooler, then installing one as soon as possible is money well spent. Typically, a good quality trans-cooler can be purchased and installed for less than $100.


Heat is bad, but it is not the only condition that can cause automatic transmission fluid to deteriorate. The typical day-to-day life of today’s classic ‘muscle car’ creates another situation that is damaging to transmission fluid. Since we don’t typically drive them every day, sometimes as little as only once every week or two, a lot of these old ‘war-horses’ do most of their ‘grazing’ not out in the ‘range’, but in a closed garage. Simply sitting around and being exposed to the air creates a condition known as ‘evaporative additive depletion’. The additives that are so important to the life of your transmission fluid as well as your transmission will actually ‘fall-out’ of cohesive suspension in the fluid and can no longer offer those much needed protective properties.


If you typically drive your classic cruiser on the weekends only, with an occasional highway run to a somewhat distant location for ‘car show duty’, then changing your transmission fluid about once every two years or 20,000 miles should be sufficient. However, if you drive your pride-and-joy more than a couple of times a week, or do a fair amount of highway driving, then changing your fluid once a year or about every 10,000 miles, whichever comes first, is your best bet. If you like to really ‘stick your foot in it’, or occasionally take your ride to the local ¼ mile strip, you are creating a lot of excess heat, and you should change your fluid about every 6,000 miles.


For many years, the only way to change the transmission fluid was to remove the pan from the bottom of the transmission, which would allow about 40% of the entire amount of fluid from the transmission to be drained. Additionally, some vehicles had a drain plug on the torque converter allowing you to drain that also. Even so, the most that could typically ever be removed from a transmission was around 60% of the total transmission’s capacity. This left a lot of dirty, worn-out old transmission fluid in the transmission, but what else could be done?


In the early-to-mid 90’s, transmission flush machines started to become widely available. Most of them simply hook-up in-line with the transmission cooling lines, and while the vehicle is idling, pumps brand new and clean transmission fluid into one side of the cooling line while the dirty, old and contaminated fluid is pumped out the other line. The beauty of this system is that since the engine, and therefore the transmission are operating during the flush, any debris or other contamination in the fluid is held in suspension in the fluid as it is being pumped out of the transmission. This is far more effective than when the fluid is drained out of the pan, for any debris and contamination that are present are just going to sit in the transmission and help to contaminate the new fluid as soon as it is put in. A phenomenon known as a ‘hypothermic barrier’ is created during the flush procedure which helps to make sure that the new fluid does not mix with the old fluid as it is being flushed. The laws of physics dictate that two fluids of identical chemical make-up cannot mix with each other until they are the same temperature.

The new, clean room-temperature (70 degrees) transmission fluid being pumped into the transmission cannot mix with the old and dirty, hot (170+ degrees) transmission fluid being pumped out. This makes for an amazingly effective flush. And since all the old fluid in the entire system is being replaced with fresh new fluid, it is far more effective than simply changing 50 or 60% of the fluid. A 100% transmission fluid flush is definitely the most effective method for changing your transmission fluid.


Logic would seem to dictate that if there is a removable filter in the transmission that it should be replaced periodically. In actuality, this is not always the case. When most people think of a transmission filter, they apply the same logic as they do towards an oil filter. The difference between the operational characteristics of a transmission filter and an oil filter are so different that they cannot be compared. An oil filter is considered to be a ‘full-flow’ filter, meaning that the entire amount of your engine’s motor oil passes through the filter on each pass through the oil pump. The oil filter will trap any particle of about 10-15 microns or larger, which is roughly the thickness of a human hair. Eventually, the filter will become so filled with particulate matter that it can become too restrictive, causing it to go into ‘by-pass’ mode, which simply allows unfiltered oil to circulate through your engine. Obviously then, frequent oil and filter changes are necessary.

In contrast to that, a typical transmission filter is NOT a full flow filter. Only a small portion of the transmission fluid passes through the filter as it makes its way through the transmission. In fact, most of the fluid flows around the filter and up into the valve body. The idea is that eventually, most of the fluid will find its way into the filter, and any particulate matter that may be ‘swimming’ around in there will be held in the filter. When the engine is turned off, all of the transmission fluid that is being held up inside the valve body and filter will drain back into the transmission pan. This all has somewhat of a ‘backwashing’ effect on the filter, as a lot of the debris in the filter will be forced down into the pan. Many manufacturers put a magnet in the bottom of the transmission pan to collect and hold these metallic-filings and debris, to keep them from traveling through the transmission. This design is what allows manufacturers to make claims of anywhere from 50,000 miles for a filter change recommendation, all the way up to 100,000 miles.

The best recommendation for a classic muscle-car owner is to flush the transmission as often as described above, and to change the transmission filter about every 30,000 miles.


There are externally mounted, auxiliary transmission filters available that do an exceptional job of giving an automatic transmission that extra ‘edge’ when it comes to long life.

‘Fluid-dyne’ corporation offers it’s ‘Magna-Fine’ in-line full flow transmission filters. Mounted in-line on one of the transmission cooling lines, the ‘magna-fine’ filters work much the same as a typical in-line fuel filter. In addition, these particular filters have a built in ‘catch magnet’ to trap any metal filings or wear debris. Simply change this external filter every year or two, and you will add years of life to your ‘slush-box’.


Even though all automatic transmissions operate essentially the same, there are real differences between brands, and each manufacturer specifies a particular fluid for its own transmissions. Your owners manual will most likely tell you the exact type of fluid your transmission takes, and the information is almost always listed directly on the transmission fluid dipstick.

Typically speaking, most General Motors vehicles use G.M.-spec transmission fluid referred to as ‘Dexron’. Dexron can have a numerical suffix such as II, II-E, III, III-e , IV or no suffix. Ford products typically used a Ford-spec fluid called ‘Type-F’ for many years up until about the mid 70’s. The major difference between these two fluids is that the ‘Type-F’ fluid had different frictional properties which allowed the Ford transmissions to shift a little faster and firmer than their G.M. counterparts. G.M., on the other hand, felt that their typical customer wanted a smoother, easier shifting transmission. The Dexron fluid allowed for these easier shifts. Easier shifting with smooth gear-changes may be more comfortable for the average driver, but it definitely increases the amount of ‘slippage’ in between gear changes, which creates more heat, and accelerated wear and tear. Through continual refinements to their transmission design, Ford was able to offer the smooth shifting capabilities that consumers demanded, and these newer-style Ford transmissions called for a new Ford fluid called ‘Mercon’. In reality, Mercon is nothing more than Dexron with a Ford engineering specification and trade name. In fact, the fluid is often sold aftermarket as ‘Dexron/ Mercon’ fluid. There are many other brand-specific types of fluid out there for each manufacturer’s specific transmissions. Make sure that you use a fluid that is specified for your particular transmission.


There are fully-synthetic transmission fluids available, just as there are fully-synthetic motor oils available. The advantages that a synthetic transmission fluid offers above a conventional fluid are the fact that a synthetic transmission fluid is extremely resistant to oxidation at high operating temperatures. A synthetic transmission fluid is also far less likely to ‘desolubilize’ at low temperatures. Both of these factors give synthetic transmission fluid much more ‘durability’ at extreme operating temperatures, as well as a much longer useable life. At any rate, make sure that whatever fluid you use in your automatic transmission is of good quality, and of the correct specification.

With just a little bit of care and attention, your automatic transmission will continue be a ‘shifty’ character for years to come!

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