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My recently-acquired 160 doesn't go into overdrive. Is there a most likely cause for this?

You need to determine if the problem is electrical or mechanical. Some but not all ODs have an inline fuse in the wire between the starter solenoid BAT terminal and the relay box terminal 1. If a postwar service article was performed there may also be a reverse safety switch operated by the reverse shift lever at the base of the steering column along with the fuse both inserted in that wire. If 6v is not at terminal 1 check for a blown fuse or open switch.

Does the indicator light come on when the car reaches engagement speed of approximately 20-25 mph? If it does that confirms the electrical command is trying to bring it in. If it does not and there is power into the relay box at terminal 1 then it would be a matter of finding out which component in the string between the governor and relay box might be preventing ground supplied thru the governor from reaching the relays. With the car sitting still, the lockout and kickdown switches can be eliminated by grounding the relay box at terminal 5. If the relays, indicator light and solenoid then energize it would be a matter of working back in the wiring between the governor and relay box by grounding at each switch to figure out which one is not working. The lockout switch is one known place where a worn switch plunger could be preventing the switch from closing when the knob is pushed in to enable OD.

If the relays do click when grounding terminal 5 but nothing is heard from the solenoid under the car or light does not come on then check and clean the main relay solenoid contact before going under to verify voltage is reaching terminals 3 and 4 on the solenoid. Both terminals MUST have 6v present for the solenoid to pull in. There are some inline connectors joining sections of loom between the relay box and solenoid that could be causing an issue either with the solenoid power or ground from the governor.

If the voltage is present at the solenoid but there is no sound of it trying to pull in or if OD still will not engage then the solenoid or a mechanical issue in the OD unit is the most likely problem. The solenoid possibly just needs a contact cleaning or at worst replaced but if the issue is mechanical inside the unit then that will require removal and probably a rebuild.

Copied and printed for use tomorrow. Many thanks!

Also make sure the lockout lever is moving the cable so OD mechanical lockout and enable is operating properly. If the cable is rusty and hard to move in the housing the stiffness has often caused the cable housing to be pushed out of the crimp fastening at the end of the lockout knob bracket. If that has happened even with the knob pushed in and switch working the OD will still be mechanically locked out. Look under the dash to make sure the cable housing is fastened to the end of the bracket. If that looks OK then check on the overdrive case passenger side and look at the lockout lever. When the knob is pulled out to the lockout position the lever end should be just forward of the straight down 6:00 position -- maybe at about 5:00. When the knob is pushed in to enable OD the lever end should be toward the rear at around the 7:00 position or maybe a little farther toward 8:00.