Fluid selection

Motor, Nov 2003 by Layne, Ken

Can long-life coolants be used in older vehicles that originally used green coolant? Some chemists have said yes, but contamination problems also have been reported. Some engineers and technicians believe that orange coolant may react with solder and other metals used in older radiators and restrict coolant flow. Because its almost impossible to know the chemical properties of older cooling system metals, it's probably better to be conservative and stick with the type of coolant originally specified by the OEM.

Other Fluids

Other automotive fluids haven't experienced the diverse specifications of motor oil, ATF and coolant, but it's worthwhile to summarize some of the key points.

For brake fluid, we have DOT 3, DOT 4 and DOT 5 grades. The chief difference among them is the boiling point. Almost all carmakers specify either DOT 3 or DOT 4. DOT 5 fluid is silicone-based and should not be used in antilock braking systems (ABS). DOT 3 and DOT 4 fluids are compatible with one another, but its best not to mix them. Its easy and inexpensive to keep a supply of both DOT 3 and DOT 4 fluid on hand and follow specific carmakers' recommendations. For example, Ford high-performance DOT 3 fluid has a higher boiling point than regular DOT 3 fluid. Also, some German carmakers call for a "super" DOT 4 fluid with higher wet and dry boiling points.

Gear oils have evolved to multiviscosity types, such as 75W-90, from the simple 90 or 120 viscosity oils of decades ago. Some carmakers recommend synthetic gear oils for specific applications, and most limited-slip differentials require a few ounces of special· additives. GL-3, GL-4 and GL-5 remain the common basic specifications.

ATF used to be a common fluid fbr power steering systems, but most manufacturers today call for their own special fluids; its best not to mix them. Ford, however, continues to specify Type F ATF for power steering systems through 1995. Ford began to phase in Mercon in 1996 for power steering and specified it across the board for 1999 and later models.

Fluid selection for late-model vehicles isn't as simple as grabbing a can off the shelf. The basic rule remains, however, that the vehicle manufacturer s specifications are the last word. That's the principle that guides vehicle maintenance today more than ever before.

The author wishes to thank Rob Colver, Roger Fennema, Rick DuPuy, Arnold Czarnecki and Capt. Bob Mason for their help with this and other articles for MOTOR.

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