Mercedes-Benz W126 no spark: ignition module and pulse sensor
W126 cranks but does not start, how is the spark tested?
Hold an ignition lead to a spark plug with the plug body against the chassis, and have someone else crank the engine. The spark is hard to see in daylight, so do the test in the dark. If there is no spark, the fault is usually in the distributor pulse sensor, the ignition module or the ignition switch. If there is a spark, check the distributor cap and rotor.
How are the pulse sensor and the ignition module ground checked?
The pulse sensor is inside the distributor, and its green wire goes to the ignition module. On the M103 the crankshaft position sensor's resistance is measured with an ohmmeter, and its permitted range is 680-1200 ohms. Also check the ignition module's ground and mounting bolts, and the distributor cap terminals, which can burn or oxidize.
Can the W126 ignition module be replaced with any spare part?
Not always. The units of the K and KE systems are of different construction, and units used before 1986 are not interchangeable with each other. Some of the replacement units have been found faulty: in one comparison only the original Bosch unit, part number 0227100023, worked.
What other than a lack of spark can prevent starting even though the starter turns?
If the starter turns but the engine does not turn with it, the cause is usually the starter's bendix gear: its fork or pinion shaft is worn or stuck. The ignition is then not at fault, and the starter should be checked before replacing ignition parts.