Transmission
Overdrive Relay
An overdrive relay lets a small control signal switch the higher current that engages overdrive on many automatic transmissions. We carry 36 relays from $5 to $42 by Standard Motor Products, API, Forecast, AC Delco and Wells, covering Dodge Ram 2500, Ram 1500, Corvette and Volvo 240 models from 1984 to 2002.
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What a relay does
A relay is an electrically controlled switch. A small current from a switch or computer energizes a coil, which closes contacts that carry a larger current to a solenoid or motor. In an overdrive circuit, the relay powers the solenoid that engages or releases overdrive. Many relays share the same body and pin layout, so one part number can serve several circuits. That is why some listings here also name A/C, ABS lamp or shutdown relay uses.
Symptoms of a bad overdrive relay
The most common sign is overdrive that will not engage, which can cause higher engine speed at highway cruise. Overdrive that cuts in and out, or a relay that clicks without the function working, are other clues.
A blown fuse, a faulty overdrive switch, a bad solenoid or a wiring fault can cause the same symptoms. A quick test is to swap the relay with an identical one from another circuit in the same box.
Matching the right relay
Relays differ in pin count, pin layout, amperage rating and whether they include a mounting bracket. This group covers 20 A, 35 A and 40 A ratings. Never install a relay with a lower rating than the original.
Compare the part number and the pin diagram on the old relay's case with the listing. Turn the ignition off before pulling or installing a relay, and press it fully into the socket.
Overdrive Relay questions
How do I test an overdrive relay?
The simplest check is to swap it with an identical relay in the same fuse box and see if the problem moves. You can also bench test it by applying 12 volts to the coil terminals and checking for continuity across the switched terminals with a meter.
Can I use a relay from another circuit?
Only if it has the same part number or identical pin layout and rating. Many relays look alike but switch differently inside. The diagram printed on the relay case shows the internal layout.
Why does my transmission not shift into overdrive?
Possible causes include a failed relay, a blown fuse, an overdrive switch that is off or broken, a faulty solenoid, low fluid or an internal transmission problem. Start with the simple electrical checks before suspecting the transmission.