The game slam tilts every game, and other switches (such as the Striker hole) register on their own. The slam tilt switch itself tests open.

Slam tilt every game and random switch hits: bent switch diode

Cause: The diode on the left ramp switch had been bent, probably by an airborne ball, and was shorting the switch's wires together.

Checks

• Inspect the switch diodes along the ball's flight paths for leads bent into each other or onto metal.

Fix

• Straighten or replace the diode so nothing touches.

Systems

• WPC DCS

Games

• World Cup Soccer

Related tips

• Phantom switch closures: one switch triggers something unrelated
• Mini playfield switches misread and one switch shows as another: bad diode and miswire
• Testing a switch diode out of circuit
• A dedicated (direct) switch does not work
• Random coil firing or slam tilt from weak 12 V (notably Theatre of Magic)
• One switch does not work
• Switch diode failures and what they look like
• Testing a leaf switch diode in place

Source: Pinball Repair help group!! (Facebook group)