Could braking hard in an emergency cause my engine to stall?

Dear Car Talk

Dear Car Talk | Feb 01, 1995

Dear Tom and Ray:

The other day I had to brake hard attempting to avoid a vehicle turning in front of me. When I did, the engine on my '91 Toyota Celica stalled and I lost my power steering and brakes. I figured cars shouldn't stall when the brakes are applied so I went to my friendly Toyota dealer. After four days of investigation, I was told stalling like this is "normal." The dealer says above 45 MPH, the transmission locks in fourth gear and anytime the brakes are suddenly applied, the car will stall. Is this true or are they the ones who are stalling?

RAY: They're not just stalling, Kristin, I think they're giving you a major league song and dance. The dealer claims the normal operation of the torque converter is what caused the car to stall. But I don't believe it.

TOM: Every car has a clutch of some kind which connects the engine to the transmission. On cars with stick shifts, the clutch is mechanical, and you have to physically disengage it (by stepping on the pedal) when you stop the car. If you don't, the car will stall. Automatic transmissions have "torque converters," which are essentially liquid clutches. And the fact that they're liquid allows them to "slip" at low engine speeds, so you can, for instance, stop at a red light in gear without the car stalling.

RAY: The problem is, at higher speeds, when you don't need the slippage, it's still there. And that lowers your fuel economy. That's why engineers invented the "lock up torque converter."

TOM: The lock-up torque converter does everything the regular torque converter does, except that when it gets into fourth gear above about 40 miles an hour, it "locks" the engine and transmission mechanically, just like the clutch does on a manual transmission. That's why automatic transmissions deliver much better gas mileage than they used to.

RAY: Then, once you step on the brake or slow down below 40 mph or so, a sensor tells the lock up converter to "un-lock," so you can slow down and stop without stalling. Your dealer thinks that when you slammed on the brakes suddenly, the torque converter didn't have time to un-lock, and the car stalled--just like a manual transmission car would stall if you stopped it in fourth gear without stepping on the clutch.

TOM: But that shouldn't happen. The instant you stepped on the brake, the torque converter should have unlocked. And if it didn't, something is wrong. It could be a lazy converter that's taking too long to unlock or a sensor that's not telling it to unlock in time.

RAY: Or it could be an engine problem that's completely unrelated to the torque converter. It could be that your idle speed was too low, or that the tune up specifications were off. Your dealer is just going to have to look harder to find the problem.

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