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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / October 2006Torque Converter, when to replace??
Up until a week ago, I had never had a torque convertor go bad. This relates to one of several Geo Metro ATs I have. No advance warning of a problem. Drove at highway and traffic speeds for 60 miles or so and pulled into a parking space. Returned an hour later, tried to back out and had significant slippage. Same in all forward ranges. Checked fluid levels and they were normal. I could feel it shift into each gear but not enough power to get up to highway speed. Went home and got my dolley and towed it home. Figured I might have a blocked filter screen, so I drained the fluid and dropped the pan and screen. Not trash on the magnet or in the screen. Cleaned everthing and closed it up/refilled. Still the same slippage. Pulled the engine and tranny out and inspected the torgue converter before swapping ATs with a spare I had. When I shook or "swished" the convertor around to get the impellor to move, I could hear a noisy bearing sound. Did the same with the replacement convertor, nothing like the old one. The sound was like loose bearings in a race. This was, of course, with the tranny fluid still in both convertors. Replacement/rebuilt convertors are to expensive to replace, just on principal on these cheap little cars. However, used convertors and readily available. How can you tell if a convertor is really bad before putting it in?? No way to take it apart. Can I put a shaft into the hub and spin it up to see if there is bearing noise, etc. ?? If they are selling rebuilt convertors, they must cut them open to replace the bearings. To be honest, I've never seen the inside of a torque convertor and all the home shop manuals fail to show a cutaway of them. Just curious. Steve > Up until a week ago, I had never had a torque convertor go bad. > [quoted text clipped - 34 lines] > > Steve Its rarely bearings that fail in a TC, its usually the "sprag" or overrunning clutch on the stator or the lockup clutch. There are actually two shafts (at least) that poke into the TC from the transmission side and engage it- the ouput or turbine shaft (which as its name implies is driven by the turbine section inside the convertor, which in turn is driven by fluid impelled by the impeller side of the convertor). The other shaft is the reaction shaft, and in most auto trans designs its actually rigidly mounted to the case and doesn't rotate. What it does is provide a fixed reference for the stator to act against through its overrunning clutch. Under certain conditions, the stator is held locked rigidly aginst the reaction shaft, and in other conditions it rotates freely (the other direction from the direction it tries to spin when its being locked). The stator being still or rotating changes the torque multiplication factor of the convertor. A broken overrunning clutch on the stator that allows the stator to spin backward will cause an incredibly "loose" convertor that barely will get the car moving, but once up to highway speed it will be fairly normal. Conversely, a seized stator clutch will be fairly normal at "breakaway" from a stop, but will cause the engine to feel like its dragging at higher speeds. Convertors are indeed rebuilt by cutting them open around the circumfereence where the original assembly weld bead is. As far as I know, it is ALWAYS standard practice to replace the convertor when rebuilding a transmission, unless the transmission has some other identifiable failure at very low mileage such as a broken band or burned clutch. The torque convertor on most modern (post-1975) cars has a lockup clutch which wears about the same rate as the rest of the transmission clutches and bands, so it should always be replaced as part of a rebuild. Its false economy to rebuild a transmission and not replace the TC. The only way to FULLY diagnose a convertor is to run it in the car or open it in a shop capable of rebuilding it (with all the equipment that entails). You can kinda tell if the OR clutch is working by engaging the splines that the reaction shaft would engage and seeing if it will only turn one direction, but a person can't apply the same amount of torque as an engine so its not a very good test. You can't really test the lockup clutch on the bench, either, as the convertor has to be pressurized and de-pressurized to switch the clutch on or off. |
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