R107 Ignition Switch: Key Won’t Turn or No Crank

An R107 that will not crank, or a key that will not turn, usually comes down to one of two parts: the mechanical ignition lock cylinder or the electrical ignition switch behind it. Telling them apart saves money and time.

Mechanical lock vs electrical switch

• The lock cylinder is what the key goes into. It holds the tumblers and the steering lock mechanism.
• The electrical switch is a separate part at the back of the lock housing. It sends power to the ignition, the accessories and the starter.

If the key will not turn, the problem is mechanical. If the key turns but nothing happens, or something works only in certain positions, it is probably electrical.

Symptoms

• Key will not turn: worn key, worn tumblers, or the steering lock under load. Turn the steering wheel slightly while turning the key.
• Key turns but no crank: the electrical switch, the starter circuit, the neutral safety switch or the battery. See starter.
• Car dies when the key moves slightly: a worn electrical switch.
• Radio or accessories work only in certain positions: a worn electrical switch.
• Key comes out while running: a worn lock cylinder; replace it promptly.

Quick tests

1. Try a spare key. A worn key is a common cause of sticking.
2. Wiggle the steering wheel while turning the key.
3. With the key in Start, check for voltage at the starter’s small terminal. If there is no voltage, check the switch and the neutral safety switch.
4. Check the battery voltage and the connections.

Removal

The lock cylinder can usually be removed with the key in a specific position and a release pin pushed with a thin wire. The electrical switch is held by small screws at the back of the lock housing. It is often easier to reach after removing the lower dash cover. Procedures vary by year, so follow the factory manual.

Replacement parts

The electrical switch is available new from Mercedes and aftermarket suppliers and is inexpensive. Lock cylinders are available from Mercedes, often coded to your key. Specialists can also rebuild your existing cylinder so it matches your other locks.

Key options

Many R107s used a single master key for the ignition, doors and trunk, plus a valet key on some cars. New keys can be cut by Mercedes or by specialists using the key number. A worn original key will not work well in a new cylinder. See replacing R107 keys.

Quick answers

• Can I lubricate the lock? Use dry graphite or a lock lubricant made for automotive locks; avoid heavy oil.
• Why does the key turn but nothing happens? Usually the electrical switch, starter circuit or battery.
• Does a new lock mean different keys? Yes, unless you have it coded to your existing key.
• Can a bad switch drain the battery? Rarely, but a worn switch that does not fully turn off can leave circuits powered.