Re: Overdrive not working

Posted by HH56 On 2016/7/24 21:21:48

It should drop out on its own when you tromp the accelerator all the way to floor activating the switch. Part of the switch kills power to the relay letting the solenoid relax and the other part of the switch completes a ground coming from solenoid to short out the ignition for a few sparks. With the ign shorted the engine loses torque letting the spring pull the plunger and pawl out of engagement. Once the solenoid plunger moves out a bit contacts inside the solenoid open the ground so engine fires on all cylinders again and you are out of OD.

Not sure exactly which switch yours has but there should be 4 terminals. If it is the typical rectangular box, to check the switch remove the two wires on the terminals closest to plunger. Those are in the relay power circuit. Use an ohmmeter and with the accelerator relaxed there should be continuity between the two terminals. Have someone push the accelerator all the way down. There should be a small tab on a part of the linkage that hits the plunger and pushes it almost all the way into the switch. The bottom two terminals should lose continuity which would drop out the relay. The upper two terminals are for the cutout circuit and work just the opposite. If you want to check those then the procedure is the same except they should be open when accelerator is relaxed and closed when the plunger is up.

I would also check the travel of accelerator. For some reason there have been a couple of posters lately who have said the switch plunger is not being moved enough or even at all when accelerator is mashed.

Assuming the switch is good, causes for mechanical engagement problems have been varied. One was the little tab that hits the plunger was bent and missing the plunger. The linkage adjustments can be incorrect so the accelerator is not moving far enough before the carb throttle plates are all the way open thus stopping the action. Another possibility is the accelerator pedal is bowed in the middle so the middle of pedal is hitting the carpet and stopping the upper end of pedal from moving the linkage far enough. On the 51-4s there is a bracket on the firewall above the accelerator about even with the head near the left side of engine. That normally has a rubber isolator so it is free to move slightly but is still fairly snug to firewall. If the rubber has deteriorated or fallen out the bracket could be cocking and preventing a full range of motion. A final possibility if the switch has the long threaded neck is perhaps the height of switch is misadjusted and too high for the plunger to activate.