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Repairing a TR Overdrive Solenoid

This article was kindly made available by the author - Ronnie Babbitt© (8th Apr 2004)

I was able to repair the solenoid, for those of you who are familiar with the construction you might be able to follow the repair as I try to explain.

Solenoid operation:

First the solenoids are basically two coils, one high current that draws the plunger in and the second a low current that holds the plunger once in place. If you remove the protective boot covering the solenoid there is a plastic cap held in place by two small screws. Once the screws are removed the cap can be lifted. Under the cap are the soldered connections that connect the coils to the breaker contacts. It is similar to the points inside your distributor. During normal operation when the high or pulling coil energizes, it pulls the plunger in to the solenoid's bore. When the plunger bottoms out it strikes a nylon pin located in the center of the bore. Upon striking the nylon pin it pushed it backwards or to the rear of the unit. This backward movement then pushes against the contact breaker causing it to open the breaker gap. This is the switching mechanism that allows the pulling coil to switch to the holding circuit. I hope this all makes sense for it is my best explanation to how the solenoid works. By measuring the different contacts you can measure the individual coils to determine the health of the solenoid. You should have readings on one coil of 0.44 ohms and 11 ohms on the other coil. My measurements were as indicated.

Now to the repair:

First of all I polished the plunger to remove burrs and potential drag. I then polished the bore of the solenoid.

After removing the plastic cap and exposing the contacts I noticed that the nylon pin had become over heated and had melted. This melting diminishes the ability of the pin to open the breaker gap once the plunger is engaged. I carefully lifted the breaker contact using tweezers and removed the nylon pin. Once removed my first thought was to make a new pin by turning one on the lathe. However the pin is so thin it would flex while turning. So I decide the only thing to do was to modify the original pin. I was able to chuck the pin in the lathe exposing only the end of the pin. I drilled the center and tapped the pin. I found a 6-32 nylon screw and threaded it into place. This provided the additional length I needed to allow the plunger to open the breaker gap once engaged. I reinserted the pin, re-bent the contact back into the correct shape and correct location and then re-flowed all the solder connections and measured the coils resistance.

The solenoid was reassembled and tested using a 12v power supply. Electrical circuitry is not one of my fortés so I enlisted the help of one of our engineers. He assisted me with all the necessary testing to ensure the unit was working correctly. Prior to installing the solenoid back to the overdrive I applied a small amount of lithium grease to the plunger and the solenoid bore.

One other mod:
If you have ever replaced one of these solenoids, you know that access to the bottom screw is nearly impossible. The first time I tried I spent two hours on my back under the car using a very long screwdriver and flash light. Not this time, I located a scrap piece of octagonal rod about five inches long I turned down about a half inch and threaded it to a 8-32 screw. I could then insert the rod into the bottom hole of the solenoid and turned it into place using a < in socket.

Once the solenoid was installed I adjusted it according to the shop manual making sure to have the correct gap between the adjusting nut and the adjustable stop.

I connected the wiring and applied power to the unit, I repeated the switching several time and it never failed. After engaging the unit many more times decided a test drive was in order, so far so good.. I hopefully prolonged the life of this solenoid for a while however I will obtain a spare for the shelf.

Thanks to friends Dave, Michael, Joe, Fred and Randall for their input.


Ronnie       EMAIL