Trans Converter Lock-Up Question

[Exocet]

My 94 Legacy's automatic transmission takes longer to lock-up the torque converter in the winter. I know that it should, as it waits for the transmission to get up to temp before locking up. However, it seems a bit erratic in terms of the distances you have to drive before lock-up occurs. For similar ambient temps (20-30 deg F), most of the time it locks-up within 1-2 miles of driving, other times it goes 10 miles without locking up. It only behaves this way in the winter. During the summer, this doesn't happen. Lock-up is almost immediate. This would appear to indicate a possible problem with the temp sensor in the transmission. But, it could be more serious I suppose. Anyone have anything similar happen?

[alias20035]

Originally posted by Exocet

My 94 Legacy's automatic transmission takes longer to lock-up the torque converter in the winter. I know that it should, as it waits for the transmission to get up to temp before locking up. However, it seems a bit erratic in terms of the distances you have to drive before lock-up occurs. For similar ambient temps (20-30 deg F), most of the time it locks-up within 1-2 miles of driving, other times it goes 10 miles without locking up. It only behaves this way in the winter. During the summer, this doesn't happen. Lock-up is almost immediate. This would appear to indicate a possible problem with the temp sensor in the transmission. But, it could be more serious I suppose. Anyone have anything similar happen?

 

Possibly a bad engine coolant temperature sensor. The torque convertor will only begin to lock when the engine is warm, not when the transmission is warm. As far as I know the transmission sensor is just there to indicate transmission overheat conditions.

 

By not locking up the torque convertor, the engine will warm up faster and more importantly the catalytic convertor will "light up" faster to reduce emmissions. This fact is stated in your owner's manual, and I believe that there are a few TSB's out there to provide to owners who complain about odd cold temperature transmission performance.

 

The engine coolant temperature sensor is a very routine item, practically affecting all Subaru's at one point or another. Erratic transmission low temperature performance is one of the signs of trouble, but usually improper idle is the primary symptom.

 

But if the torque convertor eventually does lock up, it's very possible that nothing is wrong. Track how long it takes the engine to warm up, if the engine takes a long time to warm up the torque convertor should also take longer to begin normal operation.

 

To help your engine warm up faster turn off your heat until the engine is warm, with the exception of blowing air through the defrost vents to avoid frost buildup on the windshield.

 

If the engine takes a really long time to warm up, or you never get "hot" heat, then it is possible that your thermostat is stuck open, and your engine is running too cold, which will in turn prevent the torque convertor from locking up.

 

At 0 C (32 F) both my 93 Legacy and 01 Outback were at temperature within 3 or 4 minutes of driving.

 

At -10 C (14 F) it takes about 6 minutes of driving to reach temp.

 

Even at -30C (-30F) it only takes about 10 minutes.

 

If your engine takes more than 10 minutes to warm up you either are taking away too much engine heat to warm the interior or have a thermostat that is stuck open.

 

Below -7C (20F) it is advisable to plug in your cars engine block heater (if you have one, if not consider getting one). You only need to plug in the block heater 3 hours before startup, you do not need leave it plugged in overnight, and timers are availble to turn on the block heater automatically. By using the block heater you will cut your engines warm up time by half or more, improve cold weather starting, reduce fuel consumption (saves way more $ of fuel than the $ electical usage), and reduce engine wear. Subaru block heaters usually replace the driver's side engine coolant drain plug.

 

Given that you are in central NY you should have a block heater already installed? They are standard items on all Subaru's in upstate NY, VT, ME, and NH, and have been since at least 1990. SNE (Subaru of New England) orders all Subaru's from the factory with the block heater already installed.

[Exocet]

Thanks for the informative response. The car seems to warm up normally, within 3-4 minutes of normal driving. The car doesn't have any noticeable cold weather driveability problems. The coolant sensor went bad about 40K miles ago and was replaced. That doesn't mean it isn't going bad again, it just isn't bad enough to cause driveability problems or trip the "check engine light".

 

This morning, after sitting all night in a 35-40 deg F garage, I drove the car about 10 miles at 65-70 MPH. The torque converter didn't lock up until after driving 7-8 miles. The outside temp was about 35 deg F.

 

The car does have a block heater, but since it is parked in a garage, it has rarely been used.

 

I suppose I should pull the coolant sensor and check it, or just replace it, if it isn't too expensive.