AKRON (Dec. 1, 2000)—Tire dealers who perform automotive repairs should know that a transmission only works as well as the engine does. Ignoring or underestimating the engine/transmission relationship can cause costly misdiagnoses.
In my last column, I explained how the traditional "dry" clutch in a manual-transmission vehicle mechanically links the engine and transmission together, which improves overall fuel economy and performance. To grossly simplify, a vehicle with a "linked up" or "locked up" drivetrain will run smoothly as long as the engine´s producing enough power to accommodate it.
Within the last 20 years, designers have improved the fuel economy of automatic transmission vehicles via a computer-controlled "wet" clutch inside the torque converter. As I discussed last time, the torque converter is the fluid coupling between the engine and transmission. When a trans or powertrain computer (TCM or PCM) applies this torque converter clutch, it mechanically locks the trans to the engine.
On many vehicles, the torque converter clutch system is so refined that the TCM or PCM improves overall smoothness by applying and releasing the clutch very, very rapidly. In fact, the well-designed systems work so smoothly that the motorist doesn´t realize a clutch under his feet is rapidly engaging and disengaging while he drives.
Let´s use an extreme example here to grossly simplify the engine/trans relationship. Most readers have seen inexperienced drivers shift a manual-trans vehicle into high gear too soon during acceleration—or leave one in high gear too long during deceleration. The vehicle shakes and shudders for two reasons.
First, the trans is mechanically linked to the engine until the driver pushes in the clutch pedal. Second, the engine is turning too slowly to produce adequate power for operating conditions.
The crucial qualification here is adequate power for operating conditions. Suppose engine performance isn´t quite up to par due to a partially restricted exhaust system, dirty fuel filter, filthy air filter, dirty fuel injectors, late valve or ignition timing etc. (Tired, sloppy timing chains and timing belts are common causes of late timing.) The engine control computer has been desperately trying to maintain engine performance to the best of its ability.
Fair warning, service personnel: As overall performance has dropped off, the driver has mentally accustomed himself or herself to it. On the one hand, they say they don´t perceive any engine-related problems. At the same time, they´re pestering your service writer about a herky-jerky motion the car´s transmission is making. They have already decided—and they know best, right?—that the trans is causing this buck-buck/jerk-jerk sensation.
Regular readers know I´ve carped long and loud about the importance of road testing. Hopefully, a competent service person road tests this vehicle carefully and recognizes the surging condition is only a symptom of a weak engine instead of a damaged transmission.
Some savvy techs approach these complaints by connecting the proper test equipment to the vehicle and observing converter clutch operation during a road test. If, for instance, the vehicle only surges and bucks when the tester says the converter clutch is engaged, the tech performs a thorough engine analysis first. He repairs the engine and/or engine control system and retests the car before blinking at the transmission.
Over the years, I´ve cultivated many sources among top-flight transmission specialists. They report that at least 25-30 percent of the "transmission" symptoms consumers bring to them are actually caused by engine problems. Sometimes, the engine and its computer aren´t checked until an uninformed tech has already overhauled the trans.
Don´t let your service personnel be victimized by bogus trans symptoms. I can´t say this enough: Make it shop policy to road test every vehicle thoroughly first. Then perform an equally thorough engine analysis and do the necessary repairs.
Last but not least, be sure the trans has the correct fluid in it and the fluid level is correct.
Let me know how many transmission problems that fixes!