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June 4, 2009

Car Talk

Couple's stalling Saturn sedan is plagued by dark nights of the solenoid

Syndicated columnists

Dear Tom and Ray:
Our 2000 Saturn sedan has died twice in the same exact spot (stopping at the end of a highway exit ramp). The first time, it took 10 minutes or so to get going again; the second time, it took 30-40 minutes. Then it happened again on a straight stretch when slowing down at a stoplight. An engine scan showed a torque converter solenoid problem, but the mechanic does not think this is the problem. Could it be a fuel pressure regulator? Please help! My wife is stranded at home with two kids. Thanks!
-- Jim

Tom: Well, I hate to disagree with your mechanic, but I think it probably is the torque converter solenoid.

Ray: Modern automatic transmissions have a device called a lock-up torque converter that improves your fuel economy. Normally, in an automatic transmission, power is transmitted through the (how's this for an inventive name?) "automatic transmission fluid."

Tom: By using a viscous fluid to transmit power from the engine to the wheels, the transmission allows for some "slippage." That way, the engine can keep running even while the wheels are stopped (like when you're at a red light).

Ray: If you had a rigid mechanical connection, like in a manual transmission, you'd need to push in a clutch to keep the car from stalling when you stopped.

Tom: But that slippage, which is necessary in stop-and-go driving, is undesirable at sustained high speeds. When you're going over 30 mph or so, you don't want any slippage, because it just wastes energy and lowers your gas mileage.

Ray: So, modern automatic transmissions have a solenoid (which is an electronic switch) that activates something called a lock-up torque converter. That locks up the two halves of the torque converter, giving you a temporary solid mechanical connection, just like you'd have with a clutch on a manual transmission.

Tom: But if the lock-up converter happens to stick in the "on" position, the car will stall when you try to stop it, just like a manual-transmission car will stall if you're in gear and you come to a stop without pushing in the clutch pedal.

Ray: And the car will continue to stall when you put it in gear until the lock-up torque converter disengages.

Tom: Since the scan showed a code for the lock-up torque converter, and the symptoms fit, too, I'm going to go way out on a limb here and suggest that your mechanic check it out. It's easy to do. He can simply unplug it for you.

(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk Web site.)

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