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Automatic Transmission and Power Steering Filters

Ford F150
Jim Allen | Photographer
Posted February 1, 2010

Keeping Your Automatic Transmission And Power Steering Alive Forever

For all of its complexity, the automatic transmission has been almost forgotten in the oil filtration department. For many of its 70-plus years, minimal filtration was considered "good enough." No longer. Today's electronically controlled transmissions are much more sensitive to lubricant contaminants, yet the OE filtration systems haven't evolved all that much. And if you think the auto trans is forgotten, realize your power steering has no filter at all.

With people keeping their trucks longer these days, preventive maintenance becomes even more important. If you want to keep your truck longer or work it harder, we thought you'd like to explore some filtration options for extending the life of your automatic transmission and power steering and keeping them going virtually forever.

Fluids and Contamination
The majority of automatics have only a screen or panel filter on the pump suction in the pan. Modern pan filters average about 100-micron filtering efficiency (see the sidebar), with older screen filters as poor as 200. For comparison, an average engine oil filter can remove particles about 25 microns in size. In nearly 30 years of study, engineers John Eleftherakis and Ibrahim Khalil defined the failure modes of automatics due to contaminants and determined the typical types and allowable amounts of contaminants. The results of those studies have been published in various Society of Automotive Engineers (SAE) technical papers since 1990.

While an improvement in filter efficiency on the suction side would be beneficial to an auto trans, that's been a very difficult engineering problem to solve because the flow needs are so high and the available space is so small. SPX Filtran, an industry leader in filtration products, will soon be introducing a major upgrade in that area, a pleated panel filer of about 40-micron capacity, but according to them it's not likely to trickle down to older applications.

Where does this contamination come from? Of the total amount found in the average 70,000-to 120,000-mile transmission with no fluid changes, some 25 to 30 percent is from normal wear (called Type 2 contamination). The more significant portion comes from the manufacturing process (or remanufacturing process, as the case may be) and is called Type 1 contamination. Better manufacturing processes have reduced Type 1 levels, but Type 2 will always be an issue.

According to Eleftherakis and Khalil, the typical makeup of contaminants is about 51 percent steel, 21 percent copper (copper wool clutch linings), 11 percent aluminum, and 7 percent lead (from bushings). Some 82 percent of the particles are larger than five microns in size, though only 15 percent of them are larger than 15 microns. Some are as large as 400 microns.

Trans failures come when contaminants decrease the operating efficiency of the internal valves that operate the transmission. The long-term issues are from the abrasive wear of small particles, which causes internal leakage and lower pressures. Larger particles can cause valves to jam outright. Anything that reduces apply pressure allows more clutch slippage. The advent of electronically controlled solenoid valves has made the problem worse because a solenoid is nothing more than an electromagnet that attracts the ferrous contaminants.

The most user-friendly method of measuring oil contamination is the ISO Cleanliness Code. There are two codes: the earlier two-digit (e.g., 18/14), which is still in common use in the automotive field, and the three digit (e.g., 18/14/11). For the two-digit code, the first number indicates the number of particles larger than five microns, and those larger than 14 for the second. The three-digit code shows the particles above 4, 6 and 14 microns in size. The actual number in the code (e.g., the "18") is the range of counted particles, in this case indicating 1,300 to 2,500 particles (see sidebar). Dropping from 18 to 17 indicates a 50-percent drop to 640 to 1,300 particles.

So what's "clean?" That varies according to the equipment. Even new oil is not perfectly clean. The manufacturer may offer some minimums, but the basic truism according to Magnefine inventor Len Kelsey and others is, "No system ever failed from being too clean." Ford likes to see counts no higher than 18/15 and GM 19/16, and the import manufacturers prefer cleaner numbers than those. During decades of destructive testing, Eleftherakis and Khalil determined that around 19/16 or better is safe, but anything above 21/18 is a danger area in all transmissions.


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Royal Purple
Porter, TX 77365
SAE International
Troy, MI 48084
Blackstone Labs
Fort Wayne, IN 46806
Magnefine/Boss Products U.S.A.
Spring Lake, MI 49456
Cleveland, OH 44124
SPX Filtran
Des Plaines, IL 60016
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