Mercedes-Benz W124 ignition fault symptoms and typical faults
What is the W124 spark plug gap and which spark plug suits it?
The spark plug gap on the M102, M103 and M104 is 0.8 mm. Resistor (R-marked) plugs cause misfires in engines they are not intended for; for example, in the M104 the plugs must be non-resistor. The original M104 plug is Bosch BCP5ES, and NGK BCP5ES was mentioned as non-resistor.
The car will not start when cold or stalls and gets no spark: is the distributor the cause?
The distributor cap and rotor are a typical fault of M10x engines, and the symptom is hard cold starting and plugs wet with petrol. The fault concerns all but the 280 and 320 engines. Moisture can also condense in the distributor or it can oxidise. The distributor resistance should be about 500–1000 ohms.
Where is the fault if the W124 has no spark?
If only one cylinder has no spark, the fault is often in the plug or the plug lead. If the spark disappears completely, the cause can be the ignition box or the ignition coil; the coil is measured and replaced if the value is below the specification. Comparing with another car's box reveals a faulty box.
Why do M104 ignition coils fail repeatedly?
The wires going to the coils corrode over time and cause breaks. Renewing the whole wiring harness from the ignition unit was mentioned as the solution. The original coil is Bosch 0 221 506 444; the resistance values of coils from other manufacturers may not match the original ones.