Transmission / Overdrive: Overdrive Solenoid problems after 6 to 12 volt conversion

[GTFan]

I've recently completed a 6 to 12 volt conversion on a 51 Champion, which went pretty slick with the help of Randy Rundle at 5th Ave. Antique parts. Problem that arose is this: I don't have a 2 terminal 12 volt solenoid so, after study came to the conclusion the 3 terminal solenoid would do the job by using only the two corresponding terminals, thus leaving the lock-out switch wired between the governor and kick-down switch. Nope. No worky. I've been through Ethan Peterson's instructions and overdrive tips, but find no reference to this problem. Questions is: Must I locate a 2 terminal 12 volt solenoid or is there something I can do to the 3 terminal to make it work? Anyone? Anyone?

[Corvanti]

i really don't know, but thought i'd "bump" this up to the top.

i'd think with the 12V conversion, the 3 "prong" OD part wouldn't work, unless there's a wire between 2 of the connections to make it so. and the right way - either positive or negative on the 2.

i ass-u-me it went from positive to negative ground from going to the 12V.

maybe someone that has done this will let you know...

good luck, and i'd like to know the outcome.

[gordr]

If you use the 3-terminal solenoid, you have to dispense with the relay, and wire the OD circuit as per the diagram in the '62 to '64 shop manual.

[TWChamp]

Why not just keep the 6 volt solenoid and use a dropping resistor to cut the voltage in half? You can put an ammeter in series with the solenoid power to find the amp draw, then use that to figure out the ohms and watts resistor you need to use.

[gordr]

That doesn't work, because the overdrive solenoid has two windings, a high-current "pull-in" winding that engages momentarily to make the plunger move smartly, and a low-current "hold" winding, that stays powered up as long as the car is in overdrive gear. A resistor that will pass enough juice for the pull-in winding will allow the hold winding to cook. A resistor properly sized for the hold winding would pass enough juice for the pull-in winding to do its job. Now maybe you could open up the solenoid, and add a suitable resistor to the ground leg of each winding (provided they are accessible). But simply swapping in the right 12V solenoid is by far the easier fix.

[TWChamp]

I thought about the pull-in winding, but figured it is active for only a split second and could take the brief 12 volts applied. Or is the pull-in winding active until the throttle is released and the overdrive allowed to engage, which could be a few seconds? I've never had a problem with any of my overdrives, so I've never had them apart.

[gordr]

Well, "voltage dropping" resistors don't drop voltage. They drop current, and that reduced current can be expressed as a voltage. E=IR, it's the law, Ohm's Law, and it rules here. The two windings are tied together at the "hot" side, and the pull-in winding is grounded by a switch within the solenoid, so it takes itself out of the circuit as the solenoid plunger reaches its full extension. So in order for the pull-in winding to get enough current to do its job, a current-dropping resistor would have to be small enough to pass the current requirements of both coils. Once the pull-in coil takes itself out of the circuit, the hold coil is going to "see" much closer to 12 volts than to 6, and will probably run hot.

There just isn't an easy way to do it with resistors, besides which, all the current dissipated in the resistors is being shed as heat; heat that you pay for by buying the gasoline that runs the engine that turns your alternator. Not much, in the overall scheme of things, but waste is waste.

And since 12 volt solenoids are readily available, it all seems so pointless.

It occurs to me while writing this, that there is a work-around. You could build an electronic voltage reducer that works like a DC motor speed controller. Have a timing device with a 50% duty cycle drive a switching transistor to feed 12 volts intermittently to a big fat electrolytic capacitor, and run your solenoid off that. 12 volts "on" half the time, plus 0 volts half the time nets out to six volts all the time. Run the switcher at about 400 Hz, and the electrolytic capacitor could smooth out the pulsations, as well as supply enough current for that pull-in coil to do its job. Such a device could probably be built for about $20 worth of parts. I'd have to research a bit to find the best sort of switching transistor(s) to use, and how to build the circuit that feeds them. I would probably use a 555 timer chip to generate the pulses, though. If you know somebody who is an electronics tech or hobbyist with experience in power supplies, motor controllers, and whatnot, this description should be enough for them to whomp up a reducer for you. I'm pretty sure I could build one myself, but I don't need one, and I don't know if the market is big enough to justify building one to sell.

[GTFan]

The latest: Spent the weekend pondering and decided to look at the wiring diagram for the systems using the 3 terminal, 12v solenoid, which as it happens was suggested by gordr yesterday. After comparing it to the '51 diagram, I arrived at the same conclusion suggested by gordr, that being, to remove the relay and rewire using only the kickdown switch, governor and solenoid, as suggested. This, of course leaves the lock-out switch un-used. I think it is irrelevant in the later wiring scenario and it's function is, I ass-u-me, taken over by the newer solenoid. Having performed this rewiring yesterday, I haven't yet driven the car as we are waiting for the radiator to come out of the shop. I'll update this thread when we drive it. Two thoughts. Gordr also suggested that 12v solenoids are readily available. 1) I don't think S.I. has the two terminal 12v solenoid (which would have been the simplest solution if I had had one) but I did find one on ebay for over $400!!!! No thanks! 2) I like the re-wire to the 1958-63 schematic shown in the 1958 shop manual supplement (I think that's where I found it). It's much simpler, less copper wire and I'm sure it will work, but will let you know. The conversion to 12v did include converting to negative ground which was a whole 'nuther learning experience.

[gordr]

Sounds like you are on the way, Russ! Yeah, the lockout switch was discontinued after '55. It's basically superfluous. With the overdrive shift rail in the locked-out position, where it needs to be to operate the lockout switch, overdrive cannot possible engage, even if the solenoid were powered up. And you'd have to exceed 27 mph in reverse for that to happen. If your transmission has a lockout switch, just take it off and make a nice little metal plate to cover the hole, since the old lockout switches usually leak oil.