Auto Repair Q&A



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Automatic Transmission Locking Repair

Transmission Fluid Flush

Most automatic transmissions on passenger vehicles today are electronically controlled, often referred to as "electronically-controlled automatic tranmissions." These are a transmission control unit (TCU) that works in a way similar to the engine control module (ECM or ECU) and performs diagnostics with triggered error codes like the ECM does.

An electronic transmission actuates shifts the transmission by triggering the hydraulic changes that trigger a shift after receiving enough information from the engine and drivetrain to do so. The electronic shifting mechanism has almost entirely replaced the vacuum-based engine compression systems used in earlier automatic transmission designs.

In vehicles that have a geared transmission (not those equipped with a CVT) made within the last two decades or so, it's very likely that the electronic transmission is being utilized.

Most TCUs can be read by standard code readers and are often accessible through the ECU port in the car. Some may require that you specifically connect to the transmission and a very few will require specialized readers or equipment to access the TCU. Remember that most parts stores and many friendly mechanics will pull codes for free if it requires no real work. Spot them a tip or pack of smokes and most mechanics will gladly pull codes for you off the books.

Most TCU codes are standardized and can be found online. Many will be make/model specific, though, so search carefully or find the technical manuals for the vehicle for reference. Most late-model vehicles will be using the SAE standard PCM codes in their TCU, though.

In a transmission that is locking out of gears, the problem is almost always going to be a sensor issue. A sensor may have gone bad, its circuit may be dysfunctional, or the issue could run deeper and be a fluid circulation problem or leakage. Getting the codes is a good start. Checking fluid levels and consistency is another easy job that may find the problem. If the TCU is indicating a specific sensor, check it and its circuit.

In most electronically actuated transmissions, sensors are the likely problem in gear locking or mis-shifts.