I recently purchased a Honewell Timer Switch from Amazon. The instructions that came with it for replacing an existing 3-Way switch don’t work. Here is how to install it in place of an existing 3-way switch.

Working instructions

1. The timer switch will replace one of the two existing light switches. The other switch will need to be re-wired.
2. Turn off the circuit breaker and verify there is no power in the switch boxes.
3. Identify which switch box is connected to the load (the lights) and which is connected to the hot (power). a. Each switch box will have 2 black wires. b. The black wire that comes from the same bundle as the red wire is a traveler. c. The other black wire in each box will be either the hot or the load. Disconnect these wires from the existing switches, turn the power back on, and test which one is hot. d. Turn the power back off when done with this step.
4. You don’t have enough traveller wires to install the timer in the box with the power, so put it on the side with the load.
5. Wire the timer: a. Timer blue to load black b. Timer white to bundle of white c. Timer yellow to red traveler d. Timer black to black traveler
6. Rewire the second switch. a. Disconnect the black traveler and connect it to the black hot wire. Either screw it under the same screw, or create a tail as illustrated below. b. Make sure to use one black screw and one gold screw on the switch. Leave the second gold screw with nothing connected to it. It doesn’t matter if the red gets connected to the black or gold screw as long as the black wires are connected to the other color.

Why this works

In a normal 3-way switch, the switch uses mechanical action to choose which of the two travelers (red and black) has power at any given time. The timer switch has an electronic relay (rather than something mechanical) to turn the power to the load on and off. It always needs power coming in on its black wire to power these electronics. It senses a change in state on its yellow wire and electronically switches the state of the blue load wire (light).

When this 3-way configuration is working properly, the remaining mechanical switch is not being used as a 3-way switch. You could replace it with a cheaper single-pole switch and reuse the 3-way switch elsewhere. For wiring purposes, a single-pole switch is missing the second, unused gold screw compared to a 3-way switch.

Honeywell’s Instructions

The switch from Honeywell comes with a jumper wire.

The wiring diagram they provide shows it across two screws of the 3-way switch.

There is no way that is going to work. That is going to cause the 3-way switch to provide power to both the black and red travelers all the time.

You can instead convert the jumper into a tail. Cut one end off and strip it. Use it to connect the hot wires together with a wiring cap and put the other end on the 3-way switch as illustrated above.