First of all, when the transmission is stuck in "limp-in" mode, a problem has been detected by the Transmission Computer, it will stay in second gear, and a daignostic fault code has been stored in its memory. In general, if the car starts out in second gear right after starting the engine, it's usually a sensor problem. If it starts out in first gear, like normal, then pops back into second gear right after an upshift, it's often internal slippage in one of the clutch packs.
The advantage / disadvantage of this system is unlike older transmissions that gradually developed sloppy or mushy shifts with high mileage, the computer in your car constantly watches how much fluid it takes to apply each clutch, and updates how it engages the next clutch and releases the previous clutch to provide nice clean solid shifts. It's making up for normal wear so it always shifts nicely. The problem is the day comes when there's too much wear on some of the clutch plates, and no amount of updating will prevent slippage from taking place. On older cars, you knew the wear was taking place for a long time. On your car, the wear has been masked. One day it slips, and defaults to second gear; the day before it was shifting fine.
To get an idea of how much wear has taken place, your mechanic can connect a hand-held computer called a scanner, to read the "Clutch Volume Index" (CVI). This is a set of four numbers indicating the volume in ccs of fluid it takes to apply each clutch. An experienced transmission expert will know from those numbers where to start looking for the problem.
Along with the CVI, your mechanic can access the Transmission Computer to read the stored diagnostic fault code(s). Since all the car's computers talk back and forth to each other, the Engine Computer feels sorry for the Transmission Computer, so it turns on the Check Engine light. There could very well be nothing wrong with the engine sensors.
Don't worry about it not staying running. Get the transmission issue handled first. When the battery was disconnected, either for its replacement or the alternator replacement, the Engine Computer lost its mind. It must relearn "minimum throttle" before it will know when it must be in control of idle speed. To meet the conditions for this to occur, drive at highway speed with the engine warmed up, then coast for at least seven seconds wthout touching the brake or gas pedals. I don't think this will work when the transmission is stuck in limp-in.
The alternator can fail suddenly, but they usually stop working intermittently while you're driving. This can go on for a long time due to worn brushes that don't always make good contact. 102,000 miles seems a little soon for this to happen, unless you drive on a lot of dusty roads. The brushes can be replaced separately, but for reliability of the repair, most people just replace the entire alternator. You saved the labor to repair an old part. Instead, you have a professionally rebuilt part with a warranty.
Crankshaft position sensors have been known to fail too. Often they become heat-sensitive. They will fail while driving, or when you stop for gas or groceries, the engine heat migrates up to it causing it to stop working until it cools down for an hour or two. It appears to me you just ran into all of the common problems at the same time. Hopefully no more problems for a while.
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Sunday, February 28th, 2010 AT 8:55 AM