Windshield Wipers Only Work on High Speed

Switch

That the motor operates at one speed confirms power to the switch is good, but the switch itself could have failed. Use a multimeter bridged between the upstream supply and each of the contacts while operating the switch. If a reading above 10 volts is made on each contact, the switch is fine and the problem is downstream. If the reading is made only on the high-speed terminal, the switch has failed.

Inside the switch are a number of terminals that are sprung to make contact as the control is turned or slid like a lever; these contacts can lose their “springiness” and fail to make contact. Further, they can become clogged with atmospheric contaminants that act as insulation. That the high speed -- the least frequently used -- selector still works suggests the tension has gone from the low and intermediate speed contacts. If they can be accessed and bent to create more tension this is a temporary fix at best; replace the switch.

Wiring Harness

Having ascertained that the switch is good, test the wiring harness. If the wiring harness is connected at either or both ends with a multiblock, consult a workshop manual -- many manufacturers and enthusiast websites publish schematics online -- to learn which wires and which terminals convey which wiper speed signals; draw a plan on a piece of paper showing the wire colors, terminal positions and their functions.

Use your multimeter to bridge between the low speed terminals on the switch and the wiper motor; with the switch moved to “Low” you should get at least a 10-volt reading. If you do, the wiring is fine; if you do not, the wiring has failed. Repeat the exercise with the intermediate speed. If both circuits are sound, the problem is in the motor; if they are not, replace the harness.

Wiper Motor

The wiper motor could have failed; it could be receiving the correct signals from the switch via the wires, but be unable to act accordingly. The circuits and windings inside wiper motor housings are seldom user-serviceable; obtain a replacement from a dealer or junk yard.

Alternate Solutions

Faulty grounds are renowned for causing a multitude of seemingly unrelated problems. Most wiper motors have a dedicated ground wire in their harness, however some utilize the bolts securing them to the body as a ground. Trace the ground wire from the motor to its mounting point, then scour both the underside of the connector and the mounting point on the chassis with very course sandpaper -- 60-grit or heavier is recommended -- then replace, ensuring the fastener is securely tightened. Following the instructions on the packaging, use dielectric grease at the contact points for an even better repair.

If working on a vehicle which does ground through its fasteners, remove the bolts and lift the motor away from its mount, then relate the procedure detailed above to the contact surfaces.

Some modern vehicles use a relay or control module between the switch and the motor. These components are seldom user-serviceable, and even diagnosis can be difficult for the untrained technician. If all other tests show good function, take the vehicle to a certified auto electrician for final diagnosis.