Of all the various fluids that flow through your 4WD's engine and components to keep it running smoothly, the transmission fluid in your gearbox is one of the most important. With more and more modern 4WDs coming fitted with automatic transmissions, and several modern manual gearbox models using automatic transmission lubricants, familiarity with automatic transmission fluid is becoming increasingly valuable knowledge among 4 wheel drive transmission enthusiasts.
Automatic transmission fluid is primarily designed to draw damaging heat away from vital transmission components, which is achieved by conducting heat away from the components and lubricating moving parts to prevent excessive friction heat. However, transmission fluid also has another use -- it can be the first sign of mechanical problems in your automatic transmission system, as the fluid often changes colour when these problems occur.
What problems can the colour of my automatic transmission fluid signify?
Under ordinary circumstances, automatic transmission fluid is red or dark pink in colour, and should be relatively transparent with no appreciable odour. Any deviation from these parameters can signify a range of problems with your transmission system and your 4WD as a whole.
If your transmission is starting to darken and turn brown, it has probably aged and needs to be replaced. An accompanying burnt odour is common, and is caused by the fluid degrading as it loses its lubricant qualities and stats to boil away under excess heat.
Brown, slightly burnt fluid is is a relatively common problem seen in 4WDs that have not been serviced in some time, and is not an undue cause for concern as long as the fluid is replaced promptly. However, running for extended periods with this degraded transmission fluid can begin to cause problems.
If you exchange your transmission fluid and your new fluid begins to darken more quickly than it should, the problem may not be with the fluid itself. The new fluid may have been darkened by dirty, clogged fluid filters within the transmission box; these filters are intended to trap solid detritus to prevent it damaging delicate moving parts, but can become clogged and release trapped matter into your new fluid if not cleaned and replaced when needed. Exchanging your filters at the same time as your fluid is generally the best way to avoid this problem.
Black transmission fluid usually has an acrid, overpowering smell, and has been burnt and degraded beyond the point of functionality. 4WDs running with black automatic transmission fluid will often have problems shifting gears promptly, and should be taken off the road as soon as possible. Unless the 4WD has been running for a very long time without a transmission fluid change, you should assume that your fluid has been burnt black by heat generated by a mechanical problem.
Diagnosing the cause of badly burnt transmission fluid can be challenging, as the mechanical problems that can cause it are numerous. Failed transmission solenoids are a common culprit, as they prevent transmission fluid from flowing correctly and dissipating heat evenly. You may also be dealing with a faulty torque converter, or slipping, misaligned clutches within your transmission box. If your 4WD is suffering from overheating problems in general, its radiators and cooling systems may be causing problems and allowing the gearbox to overheat.
With so many potential causes for prematurely burnt automatic transmission fluid, your safest bet is to take your 4WD in for inspection and servicing by automatic transmission specialists. They will have the tools and experience needed to solve tricky transmission fluid problems, and will be able to check for and repair damage causes by running with burnt transmission fluid.