Stepdown Overdrive Kick-down

In the OD solenoid is a set of contacts that are closed when OD is engaged. They're part of the kickdown circuit, and they are what provides a temporary ground for the ignition when you hit the kickdown switch. Sometimes these contacts get out of position or get oil on them and don't make electrical contact, so the ignition doesn't get grounded and the OD doesn't kick out. For a quick check to see if this is the source of the problem, use a test light, with one side powered from the battery or ignition and the other connected to the smaller terminal on the solenoid (you can leave the regular wire to that terminal hanging loose). With OD button pushed in, drive the car and as soon as you get above normal OD cut-in speed you should see the light come on. If it does, then the ignition grounding part of the solenoid is OK. If the light doesn't come on above cut-in speed, you've found the problem. Back to the garage.

Remove the wires from the solenoid so you can remove the solenoid cover. The grounding contacts are the very obvious ones just beneath the cover ... the "movable" contact is at the end of a curved, flat piece of black-colored spring steel. First, wipe the contacts with a paper towel or other dry material, being careful that no debris gets left between the contacts. Then, with the OD button pushed in, use a hot wire from the battery to the larger terminal on the solenoid so it energizes, then look at these contacts to see if they're definitely closed. If not, de-energize the solenoid and carefully bend the curved "strap" a little so the contacts are a bit closer together, then run the test again to be aure they're definitely closing. De-energise the solenoid and be sure the contacts are now open. Then reassemble and drive-test to see if the kickdown problem's gone.