The B&M speed sensor unit is the hands down best unit we have ever used for TCC installations. We have one in the yellow 72 Formula (Avatar pic) for many trouble free years now. The simple control unit mounts under the dash and the driver can select the speed at which the torque convertor locks up.
We leave it at the max setting all the time in case "an opportunity" comes up alongside us. But for cruising on the highway, set the speed (usually @ 50 mph for us) and forget it. Any speed below that setting will automatically unlock the TCC and we wired the whole thing through a later model brake pedal switch.
We originally tried a vacuum switch in the late 80's but found them to be too tempermental. Then went the route with the 4th gear pressure switch. Problem with that was the TCC was always locked when the trans is in OD no matter what.
The B&M Speed Sensor lockup kit is an easy install too if you must have a lockup converter. However I agree with Cliff that a high quality non lockup converter is ultmately the way to go. And our next unit will be just that.
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