Automotive Automatic Transmission Question

[Timbo2015]

When my car is cold, (colder than maybe 50F ambient or so) the torque converter will not lock up. After driving a few miles, it will lock up. This distance can be as much as 10 miles when it is really cold, say -10F. I understand there is a transmission temperature switch, set to maybe 50F, that disables lockup when the fluid is cold. What is the purpose of this? I realize the fluid will warm up a lot faster with no lockup, but why does it need to warm up quickly?

In the Q&A section of the car magazine I get, someone asked this question a few years ago. The answer given was either total BS, or at best only a partial answer and the question has been bugging me ever since.

I don't know if every automatic transmission does this, but I have driven several GM cars that do it, and at least one Chrysler.

Thanks

 

[jestal]

The simple answer is smoothness. When the transmission fluid is cold the apply/release of the torque converter clutch may not be as transparent to the driver. In addition, the ability of the torque converter clutch to isolate driveline pertubations is not as good when the fluid is cold leading to more roughness in the driveline...which causes customer complaints.

Not all torque converter lockup clutches and apply schemes/controls are the same. There are lockup torque converters that simply lock the pump to the turbine via a spring dampened clutch. There are TCC units with modulated apply/release schemes. There are units that use pulse width modulated clutch controls that are designed to constantly slip the clutch slightly to isolate driveline roughness. Other units use a viscous coupling in the clutch, instead of a spring/damper setup, to isolate the driveline roughness. The viscous coupling units are often refered to as VCC for viscous converter clutch instead of TCC for the more common torque converter clutch. Typically the VCC units will have a much longer cold startup delay to allow the silicone fluid in the VCC unit to warm up completely to maximize the isolation capability of the unit. The more common TCC units will also delay apply longer after a very cold ambient cold start so that the apply/release can be controlled more predictably since the apply/release schemes are calibrated for transmission fluid at normal operating temperatures.

Some transmissions have temp switches, some have temp sensors and others have neither. The delay you perceive might be a timer based on the startup ambient temperature or trans temp or it could be based on actual trans warmup temp if the unit has a temp switch or sensor.

 

[HYCLE]

When my car is cold, (colder than maybe 50F ambient or so) the torque converter will not lock up. After driving a few miles, it will lock up. This distance can be as much as 10 miles when it is really cold, say -10F. I understand there is a transmission temperature switch, set to maybe 50F, that disables lockup when the fluid is cold. What is the purpose of this? I realize the fluid will warm up a lot faster with no lockup, but why does it need to warm up quickly?
In the Q&A section of the car magazine I get, someone asked this question a few years ago. The answer given was either total BS, or at best only a partial answer and the question has been bugging me ever since.

I don't know if every automatic transmission does this, but I have driven several GM cars that do it, and at least one Chrysler.

Thanks

Late model cars usually have a way to increase engine rpm when cold and driving. The purpose is to warm up the CAT sooner. If the CAT gets up to temp quicker then it can clean up the exhaust emissions sooner. Mercedes will have different shift points when cold to the point we get complaints of late and hard shifts. Let me know what make, model and year if you want specific info.

Mike Whalen

Motoring Specialists Inc.

Vacaville, CA

 

[Timbo2015]

Thanks for the answers. The car I described was a 2001 Buick Regal. I have a 2006 Lacrosse now, and it seems about the same, although I have not yet driven it in really cold weather.

Jestal, you mentioned that manufacturers use various different methods to control lockup, and I have experienced that myself. I had a '88 and '93 Thunderbird, both shifted the same, and much different from GM products. The torque converters on the Fords would both lockup immediately upon shifting into 3rd. 3rd and 4th were always locked up. GM cars (at least the ones I have driven) do not do this. I much prefer the GM method of transmission control. I haven't driven a Ford lately, but my guess is their transmissions no longer use the Thunderbird method. It really sucked.

 

[Fred W]

In a BMW 540 that I used to own, the transmission was forced into "sport mode" for the first few miles of driving in an effort to get the engine up to temp a bit quicker. In sport mode the trans shift points were significantly higher and the TC would not go into lockup. The idea here being the faster it gets the engine warmed up the less total emissions. It was also a ploy used to reduce the sulpher deposition in the cylinder walls as on that particular engine the (Nikasil) lining was prone to being corroded by sulpher from the fuel.