A flipper is weak, chatters or vibrates when held, or keeps burning coils.

Weak, chattering or buzzing flipper: end-of-stroke switch

Cause: The end-of-stroke (EOS) switch, which opens at full stroke to switch the coil to its weak hold winding, is dirty, poorly closed or opening too early. If it never opens, the coil can burn when the button is held.

Checks

• Raise the playfield and work the flipper by hand to see the EOS switch beside the coil open as the arm swings.
• Clip a jumper across the EOS switch lugs, power up and flip with quick taps only. If the flipper is suddenly strong, the EOS switch is at fault.

Fix

• Power off, remove the jumper, then clean the EOS contacts (card or alcohol on solid-state, a file on EM).
• Straighten or remove any shifted fish-paper insulator between the blades, making sure the blades cannot touch when open.
• Adjust the gap as described in the EOS adjustment entry.

Never hold the flipper up while the EOS switch is jumpered; the coil will overheat.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7
• Williams System 9
• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C
• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95
• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200
• Bally 6803
• Gottlieb System 1
• Gottlieb System 80
• Gottlieb System 80A
• Gottlieb System 80B
• Gottlieb System 3
• Data East CPU Version 1/2
• Data East CPU Version 3
• Data East/Sega CPU Version 3b
• Capcom
• Electro-mechanical

Related tips

• Weak flippers on an EM: adjust the end-of-stroke switch before buying coils
• Flipper weak because the return spring is too strong
• What the flipper end-of-stroke switch does
• Weak flipper: dirty end-of-stroke switch
• Flipper buzzes when held up
• Replacing a flipper coil
• Flipper weak: worn plunger, coil stop or sleeve
• Flipper buzzes and vibrates when held

Source: Pinball Machine Care & Maintenance, Bernard B. Kamoroff (Bell Springs Pinball Alley)