How to Test a Pinball Coil (Solenoid) with a Multimeter

SAFETY FIRST: always test a coil with the machine switched OFF and unplugged. A coil measured under power can give false readings and expose you to live voltage.

A weak, dead or buzzing coil (solenoid) is one of the most common faults on classic solid-state pinball machines. The good news is that a coil is easy to check with an inexpensive multimeter. Here's how to do it properly, and how to tell whether the coil itself, its diode, or the driver transistor is the real problem.

Checking coil resistance

1. Set your multimeter to its lowest resistance (ohms) range.
2. Place the red and black probes on the two lugs of the coil. A healthy coil generally reads a couple of ohms or more. Exact values vary by coil type — flipper, pop bumper, kicker and knocker coils all differ — so the most reliable check is to compare against a known-good coil of the same part number or the value listed in your machine's manual. A reading near zero ohms points to a shorted coil, and a reading of infinity (open circuit) means a broken winding.
3. If the resistance reads suspiciously low, disconnect the ground wire from the coil and measure again. If the reading is still low, the coil (or its diode) is faulty. If the resistance now reads normal, the coil is fine and the driver transistor on the board has likely shorted and needs replacing.
4. The coil's 1N4004 diode can also fail and drag the reading down. Lift or cut one leg of the diode away from the coil and measure the coil on its own to rule the diode in or out.
5. When you find a genuinely shorted coil, treat it as a warning about the rest of the circuit. A coil that has shorted will very often have taken out the driver transistor on the driver board (or the transistor mounted under the playfield, depending on the game). In that case, plan on replacing not just the coil and its diode, but the silicon in its drive circuit as well.

How a coil circuit works

A pinball coil is switched to ground by a driver transistor. When the CPU tells the driver to fire, the transistor pulls the coil's ground side low and the coil energizes. The diode across the coil protects that transistor from the voltage spike the coil produces when it switches off. That's why these three parts — coil, diode and driver transistor — fail together so often, and why you test them as a group.

When the driver board is the real problem

If you keep finding blown driver transistors — or several coils misbehaving at once — the fault is on the driver board, not the coils. On classic Gottlieb System 80/80A/80B and Williams System 3-7 machines, chasing individual blown transistors on a corroded 40-year-old board can turn into an endless loop. That's where a modern all-in-one replacement board makes sense: it replaces the MPU and driver stage with fresh, reliable electronics, needs no original battery (so no future battery leakage or corrosion), and comes with schematics available online for easier repairs down the road.

Browse replacement boards by system in our Gottlieb System 80 and Williams collections, or read our companion guide on diagnosing a pinball that won't turn on.