ShareThis Page

'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.

TribLIVE commenting policy

You are solely responsible for your comments and by using you agree to our Terms of Service.

We moderate comments. Our goal is to provide substantive commentary for a general readership. By screening submissions, we provide a space where readers can share intelligent and informed commentary that enhances the quality of our news and information.

While most comments will be posted if they are on-topic and not abusive, moderating decisions are subjective. We will make them as carefully and consistently as we can. Because of the volume of reader comments, we cannot review individual moderation decisions with readers.

We value thoughtful comments representing a range of views that make their point quickly and politely. We make an effort to protect discussions from repeated comments either by the same reader or different readers

We follow the same standards for taste as the daily newspaper. A few things we won't tolerate: personal attacks, obscenity, vulgarity, profanity (including expletives and letters followed by dashes), commercial promotion, impersonations, incoherence, proselytizing and SHOUTING. Don't include URLs to Web sites.

We do not edit comments. They are either approved or deleted. We reserve the right to edit a comment that is quoted or excerpted in an article. In this case, we may fix spelling and punctuation.

We welcome strong opinions and criticism of our work, but we don't want comments to become bogged down with discussions of our policies and we will moderate accordingly.

We appreciate it when readers and people quoted in articles or blog posts point out errors of fact or emphasis and will investigate all assertions. But these suggestions should be sent via e-mail. To avoid distracting other readers, we won't publish comments that suggest a correction. Instead, corrections will be made in a blog post or in an article.

click me