Power window · motor reversing · relays

Power window wiring diagram: motor, switch and relays

A window motor has two wires and no fixed + or −: it goes up with + on one wire and down with + on the other. The trick is to give both wires ground at rest and switch one of them to +. Two changeover relays — or the window switch itself — do exactly that.

Open the editor · ▶ Run the live demo

The relay circuit

Each motor wire goes to the common terminal (30) of its own 5-pin relay.
On both relays 87 → fused +12 V, 87a → ground. At rest both motor wires sit on ground: the motor is stopped and braked.
Up energises one relay: its motor wire switches to +12 V, the other stays on ground. Down energises the other relay.
Both at once: both wires on +12 V, no current — the motor simply stops.

Fuse and wire

A window motor runs at about 8 A but draws 15–20 A when it stalls at the top or bottom, until the switch is released. That stall current sets the fuse (20 A for one window) and the wire (1.5 mm²).

Window does not move?

Relays click but nothing moves: the fuse on 87, or broken wires in the door hinge — the most common place.
Moves one way only: the relay or the switch contact for the other direction.
Slow: low voltage at the motor (corroded door connector) or a dry window channel.

Reverse the motor in the simulator

Run the live demo: the window motor between the common terminals of two 5-pin relays, 87 on +12 V and 87a on ground.
Hold Up: one relay pulls in and the motor runs; hold Down: the other relay, same current the other way.
Hold both: the motor stops — both wires are on +12 V.

Questions

Can the switch drive the motor without relays?

Yes: most door switches are double changeover switches doing the same job as the two relays, and they carry the motor current themselves. Relays spare the switch contacts.

Why does 30 on this relay look “reversed”?

Here 30 is the output to the motor and 87/87a are the inputs (+ and ground). That is the usual way to reverse a motor with changeover relays.

How do I find which motor wire is which?

It does not matter electrically: if the window goes the wrong way, swap the two motor wires.

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