The valve body won't allow overdrive except through high gear so no OD in 1st or 2nd. There are only 2 or 3 pins on the VB connector and diagrams are posted in several places. One pin is power in which is hot with the key on. Then OD is activated by grounding the other pin on the OD only 2 pin trannies. On 3 pin trannies that have OD and lock-up, then the lock-up is activated by grounding it's pin. There are a lot of ways to do the grounds. Most are really simple.

I have a 2 pin connector which is a non lock up converter version. The 518s with lock up converters are more plentiful because they made them longer. I went with the non lock up version only because the converter companies said that I had more and better choices with the non lock up converters. Besides, a few percentage points of efficiency wasn't important to me.

I actuate OD with an on/off button on my console. I also wired in an adjustable vacuum switch on the engine, so that when I crowd it and the vacuum drops, it drops out of OD and then back in when I let off. Much like passing gear. All of it is simple, cheap, and effective. And fun.

I don't do a lot of hiway running. But it is an exciting new dimension to my Cuda to pull onto the Interstate and cruise with anything out there even with 3.91 gears. I actually use the OD most of the time on 2 lane roads. In the 50 to 60 MPH range. The car is much quieter, you can hear the radio, talk without yelling, the wife is more comfortable, and the engine just sounds more relaxed. And if I feel rowdy, I drop out of OD and crack open the cut-outs. The best of both worlds.

I wished that I had installed the 518 years before I did.

Here’s what I did



Completely automatic, wanted my daughter to be able to drive it.

The control module automatically turns on/off the overdrive and lock-up for the transmission.

The 518 (46RH) is hydraulically controlled and uses 2 servos to enable the OD and LU.
The 3-pin connector on the transmission supplies the control voltages.
The center pin always has 12V power connected. The 2 outer pins are switched grounds. The front pin is the OD ground, the rear pin is the LU ground.

The module works according to the following sequence.
1) The 12V toggle switch supplies voltage to the relays and transmission center pin.
2) When the hydraulic pressure trips (50mph/adjustable) the pressure switch, ground is supplied to the vacuum switch. When speed is below 50mph, hydraulic switch opens, disabling OD.
3) When there is enough vacuum, the vacuum switch closes, providing the OD relay ground. Under heavy acceleration, vacuum drops, opens switch, OD is disabled.
4) When OD relay has ground and 12V, it waits 10sec, then supplies 12V to the LU relay and ground to the transmission OD pin, enabling OD.
5) Now the LU relay has 12V, it's already grounded. It waits the 10 sec programmable delay then provides ground to the transmission LU pin. So lock up doesn't occur until OD has been enabled and on for 10sec.
6) The programmable delays for both relays fix the situation when speed is fluctuating around 50mph. This would normally cause the OD to kick on/off, the 10 sec (programmable) delay stops this from happening.