1964 c20 292 3OTT upgrading to OD - need education please

[CousinTone]

I have a stock 1964 C20 with a 292 3 speed OTT. I was lick to get hold of a 50’s gearbox that has an OD unit. It has a wire harness on it and I’m not sure how this is to be properly installed in place of my standard box.

Any advice would be appreciated. Thanks in advance.

[cwcarpenter98]

First picture is the standard 3 on the tree that I pulled out of my 63. Second picture is the 3 on the tree with overdrive that I put back in. The main case is pretty much exactly the same between the two transmissions. It bolted up to my original bellhousing with no problems. Input shaft is the same 10 splines, so I didn't have to mess with the clutch either. It came out of a 65 truck, so the shift linkage was the same on both transmissions. I used the side cover from the 65 trans, but swapped the shift levers from the 63 trans on because they were in better shape. On your transmission, you might need to swap out the shift levers and shafts. You want to use the side cover that came on the overdrive transmission because of the linkage built in that is used to automatically lock the transmission out of overdrive when shifting into reverse. The overdrive unit is not designed to go in reverse. You will break something if you try to back up without locking the transmission out of overdrive.

The third picture shows where I mounted the extra relay that I added to work with the HEI dizzy. If you are still using a points dizzy, you won't need that extra relay. The fourth picture shows where I mounted a new Studebaker kick down switch on my Quadrajet. I wasn't about to spend $200 for a NOS GM switch, so I went with the $50, new Stude switch that operates the same, just uses different connectors. The last picture shows the lockout lever in the cab. It's the knob closest to the steering wheel on the left side that's pulled out. When pulled out, the transmission is locked out of overdrive, so it operates as a standard 3 on the tree.

Length wise, my two transmissions are the same. I did not need to get my driveshaft shortened, but I did have to change the slip yoke from the coarse spline 63 to the finer spline 65. I believe it was in 65 when GM made the change on the output shaft. If I had found a 55-64 overdrive trans, I would not have needed to change anything on the driveshaft.

For the wiring, what is your intention with it? Do you want it to be wired as GM did it, or some other way? I chose to set it up as GM intended, but I've made a couple small modifications to make the drivability better for myself. There's several other ways to make it work, many of which are talked about on this site and others across the web.

Last but not least, post up some pictures of what ya got

[Gianyveedub]

https://67-72chevytrucks.com/vboard/...=313886&page=2


Post #35 is a simple relay control for the overdrive unit. I later changed the design slightly. Basically when the Governor reaches approx. 28 mph it will ground and close the circuit for the overdrive solenoid. I recommend using the Governor rather than bypassing it to prevent you from accidentally putting the truck in reverse while in overdrive( bad thing happen when you do). If you have any questions feel free to ask me and I will help you the best I can.

[CousinTone]

Originally Posted by Gianyveedub

https://67-72chevytrucks.com/vboard/...=313886&page=2


Post #35 is a simple relay control for the overdrive unit. I later changed the design slightly. Basically when the Governor reaches approx. 28 mph it will ground and close the circuit for the overdrive solenoid. I recommend using the Governor rather than bypassing it to prevent you from accidentally putting the truck in reverse while in overdrive( bad thing happen when you do). If you have any questions feel free to ask me and I will help you the best I can.

Wow, another member in CA, we’re rare (but work tells me I’m probably going to Dallas for an assignment...at least I can get parts cheaper!!). My only question would be in wiring the unit. I’m good with electrical and am seeing a push button in the referenced thread. On yours, did you use a push button or is it an on/off circuit...”flip the switch” to turn the OD on? My spline count is 10 in and 29 to yoke. I’d like to wire it as original as possible but keeping ease of operation in mind. Thanks for all the advice so far. Thanks.

[cwcarpenter98]

Did your transmission come with the wiring harness?

[CousinTone]

Originally Posted by cwcarpenter98

Did your transmission come with the wiring harness?

It did. It’s simply a set of wires going from the connections to a gang plug (the receiver plug being on the older Belair I think it came out of). I’ve not checked my new harness, still in box, from 2019, from American Auto Wire. I’m doubting if there’s a provision/wiring for the OD being such a rare option. I wanted to get info before removing the wires from the harness gang plug and get it into place for my old rust bucket.

I’m guessing here, but what I’m taking away from all of the great input, is to install a hidden (not drilling my virgin dash!!) on/off switch when I want to go faster than 40 (LOL) I’ll flip the switch and get to 60, maybe.

Am I thinking/ on the right track here? Any pics needed?

Thanks to you all for the help, I sincerely appreciate it.

[jayoldschool]

Hidden switch? Can't get better than a second floor dimmer switch!

[Riverstory]

If you don't install a handle to manage the OD from freewheeling, when you park the truck, put the transmission in reverse. If you put the transmission in a forward gear, you'll be freewheeling and the truck will roll down an incline.

Make sure to utilize the governor in the electrical system. If you don't and only use a switch to turn the solenoid on and off, you can ruin the needle bearings in the OD unit if you put the truck in reverse with the OD engaged.

Good Luck!

[Gianyveedub]

Having the governor tied to the ground of the relay allowed me to drive the truck as I would, if I forgot to turn off the OD when slowing down, it would automatically unground to relay and disengage the OD when below 28mph. This prevented me from accidentally trying to take off in 1OD or going into reverse in OD (will break the transmission). I did not have the original harness, so I had to come up with one that served the purpose I needed. If you are pretty savvy with wiring and relays, you could make your own that acts just like the factory one.

I wanted to as the saying goes, Keep It Simple Stupid.

[cwcarpenter98]

The way GM installed them, there were no switches inside the cab to do anything with the overdrive. The only component you saw inside the cab was the lockout cable. On your truck, it would usually be mounted at the bottom of the dash to the right of the steering column. The knob would probably have been a T handle of some sort that said overdrive on it.

Everything else wiring wise was under the hood or by the transmission.

For the factory wiring, GM used a single, fused power wire going to a 3 prong relay. The power wire is a constant hot. One wire that goes to the relay is the ground wire straight from the governor. It grounds the system once you reach 28 mph, thus allowing the relay to send power out the 3rd wire to the solenoid. On the way to the solenoid, the power goes through the kickdown switch.

There's 2 wires on the solenoid. One wire is to engage the solenoid thus automatically shifting the transmission into overdrive. Your foot must be off the gas for the solenoid to do its job. The other wire on the solenoid is connected to the other circuit of the kickdown switch and the wire that goes to the negative side of the coil. When the kickdown switch is hit, the power normally going through to the solenoid gets transferred to the other wire which disengages the solenoid, thus pulling the transmission out of overdrive. The wire to the coil server a very important purpose. The solenoid can not disengage with a load on the transmission. The moving parts don't like that. So, when the kickdown switch is hit, in stock form, the coil gets grounded out for a split second. That cuts the ignition, thus taking the load off the transmission and allowing the solenoid to pull out of overdrive. That whole process of the kickdown switch is faster than you can say banana .

In the picture below, you can see the rusty looking relay that is under the brake master. That's the 3 prong relay that was under my hood when I bought the truck. That's one of the indications that my truck had one of these transmissions many moons ago. Thankfully, that relay still works well for me. The relay that you see on the passenger side firewall is to make the kickdown switch component work with my HEI distributor. Points don't care if you ground out the coil. HEI coils do care if you ground them out, so I wired up a relay that turns the power off like I do with the key instead of just grounding out the coil. It has worked great that way for me since I installed the transmission over 2 years ago. IF you want to know how to wire up that extra relay, check out the overdrive wiring link in my signature below.


I hope that helps you understand a little more about how the factory wiring worked. If you're still confused about it, I might be able to explain it better over the phone