Ask the Mechanic: What’s the function of overdrive?

By: Eric Allen
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Let’s start with a basic explanation of the overdrive on/off switch, which in most cases is mounted on the automatic transmission shift lever or on your instrument panel. When you depress the switch, or button, the overdrive feature of your automatic transmission will be disabled (off), not allowing the fourth gear to engage if your vehicle is equipped with a simple four-speed automatic transmission. After you press the button or switch, a light on the instrument cluster will illuminate letting you know that overdrive is disabled — it usually does this by saying “overdrive off.” The transmission control module or powertrain control module will then command a solenoid to “lock out” the fourth gear ratio and only allow shifts up to third gear. Many of us use this feature everyday when more power is needed to climb hills with various loads to keep the rpms up, and it can also be used as an engine braking feature as well to keep a steady speed coming down hills. When using overdrive, always refer to your vehicle’s owner’s manual as to the correct application and recommended operating conditions. Today, most newer vehicles don’t come with the overdrive on/off feature due to the fact that most automatic transmissions come equipped with an “auto-stick” type feature where you can actually control the shift points by depressing the shift lever or a switch and upshift or downshift when you feel the need. If you own a pickup truck you might also have a tow/haul feature as well that actually controls the shift schedule and firms up the shift points when desired by increasing hydraulic pressure. This feature is also enabled by depressing a button on the shift lever and a light will illuminate on your instrument cluster to alert you. So when you depress the overdrive switch/button in your vehicle you are not turning overdrive on, but rather turning overdrive off. There are many benefits of the overdrive off feature as we discussed, but when not loaded or towing I would recommend that you use the full set of gear ratios available to optimize fuel economy. Eric Allen is the shop foreman for Magnussen’s Dodge Chrysler Jeep, 1901 Highway 49, Auburn. He can be reached at (530) 885-2900.