As complex as modern automobiles are, there's almost no such thing as a single-system failure. Today's cars are an amalgamation of integrated systems, all carefully interlaced to give the computer as much control as possible of every aspect of your car's performance. The short answer is that, yes, an alternator can affect a modern car's transmission just as it can the radio, catalytic converter and even tire pressure.
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You can divide your car up into three basic systems (engine, drivetrain and chassis management), all of them codependent to some extent. The engine-management system communicates with the drivetrain to optimise shift points and fuel efficiency, and it communicates with the chassis to modulate anti-lock brakes, traction, cruise and stability controls. If anything goes wrong with the engine, every other system may need to change parameters to maintain performance, fuel economy and emissions compliance.
Voltage Variation - Programmed Strategies
Voltage drop is the most obvious culprit where alternator-related transmission woes are concerned. If your alternator's voltage stays consistently below the required output, the transmission may alter programming to keep the engine rpm low to reduce load on the engine. Alternatively, some transmissions may have programming to lock the engine into a lower gear than it should to raise engine rpm and alternator speed with it. Most of your transmission woes probably have more to do with a programmed strategy than an incidental failure.
Voltage Variation - Incidental Failures
Alternators with integrated voltage regulators may surge prior to failure, which can send unwanted power through the system. The computer may reroute power away from delicate components like the power train-control module in the event of a surge, causing the transmission to either fail outright or use its best guess as to proper clutch pressure and shift points. The worst-case scenario is that a random voltage drop can deprive the transmission servos, torque converter lock-up and fluid controls of the power needed to keep the transmission functioning properly.
A bad alternator can cause a number of engine symptoms, which can by extension cause the transmission to act up. If the alternator cuts current to the ignition system, the engine will spit unburned fuel out of the exhaust and into the catalytic converter. If the engine computer reads the high converter temperature as a sign that you're towing or otherwise placing a load on the engine, it may drop the transmission out of overdrive to reduce engine load.
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- "Understanding Automotive Electronics, Sixth Edition "; William Ribbens; 2003
- "The Haynes General Motors Automatic Transmission Overhaul Manual"; Eric Godfrey; 1996
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