An R107 that will not start is stressful. Split the problem and you avoid replacing a fuel distributor because a fuse was open. This is the checklist I use on a cranking or silent car.
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Related: OVP, fuel pump, pump relay, hot start.
Step 0 — Safety
Park in neutral/park with the brake set. Have a extinguisher if you will crack fuel fittings. No sparks near fuel.
1. No crank vs crank-no-start
No crank: battery state of charge and connections (trunk grounds!), starter relay/solenoid, neutral safety switch, seized starter, aftermarket alarm interrupt.
Cranks but will not fire: fuel, spark, compression, injection enable.
A digital multimeter at the battery under load tells you if voltage collapses before you blame the starter.
2. Fuel pump path
- Key-on: do you hear pump prime?
- Check fuel pump relay location for your year
- Check related fuses and the OVP relay
- Confirm voltage at the pump connector
3. Fuel pressure
If the pump runs but the engine will not catch, measure CIS pressures. Hot-only failures often mean rest pressure loss (accumulator / check valve / leaks). Use a proper gauge — see K-Jetronic.
4. Spark
Confirm spark at a plug lead with a safe tester. Inspect cap, rotor, plugs, and coil. Moisture under the hardtop after rain can create no-spark mysteries.
5. Hot vs cold no-start
Cold only: cold-start valve, thermo-time switch, WUR control pressure, weak battery.
Hot only: rest pressure path first.
Both: pump, relay, ignition, immobilizer/alarm.
6. 560SL alarms / immobilizers
Aftermarket alarms strand more classics than factory ghosts. Know what was added and where the interrupt relay lives.
7. When to stop and call a tow
If you smell heavy fuel, see drips, or the starter is grinding — stop. Flooded engines and broken starters get worse with ego cranking.
Details that finish this topic
Cranks no start vs no crank
On an R107 this means verifying the condition on your year/engine before ordering parts. When in doubt, match the data card and engine number, then confirm with a specialist who works on 107s regularly.
