Lucas DR3 wiper motor
- Wiper motor and wiper switch circuitry

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There is a large amount of Lucas wiper motor information available on the internet. The best electrical circuit diagram I found is based on the Jaguar Mk2 saloon. I have annotated it to make it useful for any XK150 owner. (The 79SA toggle-style wiper switch of the Mk2 has the same connection logic as the rotary switch of the XK150. The annotations added in red show the equivalent XK150 switch terminals. Using several 5+amp test leads, it is possible to bench-test a DR3 motor in all three switch position without having to remove the dash rotary switch from the car.)

DR3 two speed internal wiring diagram in pdf form.

Because the DR3 uses a powered field coil, rather than a permanent magnet arrangement, the DR3 armature rotation is independent of the polarity of the car in which it is fitted. This because the magnetic field around the armature is reversed in polarity at the same time as the current direction is reversed in the armature windings.

Because the offset of the wiper rack in its tube is changed by 8mm (1/3") by the direction of rotation of the wiper motor wheel in the normal and parking modes, then the normal-mode rotation of the armature in a LHD car has to be the opposite of a RHD car. Thus to convert a LHD DR3 motor to RHD service, the pair of connections to the armature or the field coil have to be reversed - but not both.

It is easiest to change over the armature connections. Note that there is a pair of wires connected to each armature tag. Both wires of each pair are moved to the other tag. One pair is the GRN and GRN-BLU, the other pair is GRN-RED and the wire to the park switch.

Use a fine point soldering iron, preferably 80W or higher. There is a lot of copper to heat up. In some models, the incoming GRN wire (12V supply) may be routed through a high temperature switch built into the wiper motor heavy wall case.

Description of the DR3 circuit in operation

In a LHD car with the wipers in use, the wiper rack is normally operating further out of the rack tube. When the wiper motor switch is turned to PARK/OFF, there is still power supplied to the motor. The switch logic reverses the current flow through the field coil, and thus the motor armature reverses its rotational direction, and in doing so, a cam arrangement on the wheel's connecting link pin shifts the rack position about 8mm (1/3") into the rack tube during the very next rotation of the wheel. This means that in parking phase, the crosshead and the park switch actuating pin on the end of the rack is operating closer to the parking switch by 8mm. The park switch actuating pin then hits the parking switch, opens the grounding circuit, and the motor then stops immediately. The wheel in the motor only turns about 270 when in parking mode.

To restart the LHD wiper motor, the wiper switch is turned first to NORMAL. As soon as that happens, the fact that the parking switch is already open-circuit is irrelevant as the wiper switch then grounds the power supply to the armature via the dash switch, reverses the current through the field coil, and the motor starts turning in the normal direction, and the cam on the pin on the wheel goes back to normal, then the rack is immediately pulled 8mm out of the rack tube, and the park switch actuating pin no longer touches the parking switch.

In a RHD car, the rack is operating further into the rack tube under normal operation, but when in a RHD car the wiper switch is turned PARK/OFF, the rack then shifts 8mm further out of the rack, the park switch actuating pin then activates the parking switch, and the wiper arms park on the RH side of the windscreen.

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