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'Fifth gear' likely torque converter clutch

| Saturday, July 23, 2011

Q: I have a question about transmissions. I recently bought a replacement car that has, I believe, a four-speed automatic transmission. Sometimes when I am climbing a hill, I can feel the transmission downshift to what seems to be a gear between third and forth. I have noticed this occasionally when I manually up-shift between third and fourth. What's going on• Is it actually a five-speed• From what I've read, they don't make those.

-- Vern Bueller

A: Vern, that shift you're feeling is probably your torque converter clutch disengaging or re-engaging. The torque converter in an automatic transmission equipped vehicle is a fluid coupling between the engine and transmission. This gadget allows a vehicle to come to a stop with the engine running and transmission in gear and allows greater engine torque (twisting force) to be applied to the drive wheels during acceleration.

Imagine two propellers in a liquid-filled chamber. The engine is connected to one; the transmission joins to the other. The fluid churned by the first propeller drives the second one, but in a flexible way. This works great for coming to a stop and accelerating, but at steady highway speed, there's an efficiency loss that cuts into fuel economy and generates needless heat. Most cars and light trucks built since the 1980s add a lock-up feature or clutch to the torque converter. During light load, warmed-up, mid-speed and higher operation, the clutch is engaged, locking both propellers together. An attentive driver may feel the engagement or disengagement, which feels like a transmission shift. One might notice this in the tachometer, about a 300 RPM change.

A gentle torque converter clutch thump is a normal occurrence. One can check to be sure the converter clutch works by very lightly applying the brake pedal at speed with your left foot while maintaining a steady throttle. Brake pedal input cancels torque converter clutch lockup, which should be noticeable.

Q: Do cars still "blow head gaskets"• I was curious because I had a car back in the '80s that ate these things, but haven't heard much about it since.

-- Don Geller

A: Head gaskets seal the cylinder head to the engine block and must resist tremendous combustion chamber pressures and varying metal expansion rates when an aluminum cylinder head is bolted to an iron engine block. High-tech developments in gasket technology and improvements in engine design have reduced a lot of the problems, but failures can still occur.

Pre-ignition (pinging) and detonation, along with engine overheating, are leading causes of head gasket failure. Maintaining proper engine and cooling system performance is vitally important to minimize the chance of failure.

When a failed head gasket is diagnosed, there are many factors and procedures that should be followed to prevent a recurrence. Some engines have a track record of head gasket issues and benefit from the use of enhanced aftermarket replacement gaskets and diligent installation methods.

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