Piston Slap: A Touch-Down for the Tri-Five’s overdrive?

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Jack writes:

Where can I source electrical parts for the overdrive on my 1955 Chevrolet Bel Air? My car is bone-stock with a three-on-three standard transmission, and the likely culprit is the wiring from the relay to the transmission and/or the solenoid. Likely, but I’m not sure.


Sajeev answers:

If, like me, you embarrassingly had no idea an overdrive transmission existed for Chevys of this era, here’s a screen-grab from the 1955 Chevy brochure explaining the “Touch-Down” overdrive system and its rudimentary wiring diagram.

Now that we’re on the same page, detective work is necessary to acquire parts for the Touch-Down Overdrive system. Since you mentioned the wiring, any signs of decay/wear/rodent infestation suggest a prompt installation of a new harness. From there, if the relay tests bad, getting a new one is either expensive for a correct part or very cheap with a quick rewire of the harness (it’s only three wires, looks easy) to accept the far-more-common General Motors horn relay.

Now let’s discuss eBay Motors, the go-to place for owners in need of obscure, hard-to-find classic car bits. And let’s do it from the perspective of a Touch-Down Overdrive component hunter:

  1. Make this eBay query into a saved search, as it might be your only hope for acquiring parts at a good price, or at any price at all.
    1. For example, take this used kick-down switch from a Tri-Five vendor for $250, versus this brand-new one (auction live at the time of writing) for $50 less.
  2. Since the Touch-Down system was used by other manufacturers, save an eBay search for terms like Packard overdrive parts, too.
  3. Sometimes you get lucky, and you’ll find an NOS relay for a reasonable premium over the GM horn relay. Wow!

The search for difficult-to-find parts like the Touch-Down system should be treated like a treasure hunt. Keeping a vigilant eye on eBay is one of the best ways to get the part you need quickly and cost-effectively. So good luck, and happy hunting in your quest for overdrive in your Tri-Five.

Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.com, and give us as much detail as possible so we can help! If you need an expedited resolution, make a post on the Hagerty Community!

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