1991 Mercedes-Benz 420SEL Distributor Assembly Replacement Cost

Upon diagnosis of distributor failure, the distributor will need to be replaced immediately. Not only will the vehicle likely fail to start, run, or produce power, but it may also create debris inside of the engine crankcase if there is physical damage.

Some makes and models have constant issues with distributors, and require replacement before the first 100,000 miles is reached. However, in most cases, distributor replacement will likely occur after 150,000 miles. The distributor assembly does require frequent maintenance to ensure proper operation, so the components within the distributor are much more likely to fail than the distributor assembly as a whole.

Diagnostics will take one of two paths for a distributor, mechanical or electrical. If the distributor is suspected of mechanical failure, including failure of the vacuum canister, mechanical advance, bearing, or drive shaft, the distributor may be inspected in or out of the engine. The vacuum advance canister can be tested with a vacuum gauge to ensure there are no leaks in the diaphragm. If damage to the ignition distributor drive shaft is suspected, it must be removed and inspected. Electrical diagnosis begins by testing the ignition coil in most cases, and if the ignition coil and primary ignition wire are working correctly, the distributor cap and rotor will be inspected, as well the ignition module. Finally, the pickup coil will be tested for proper resistance across the coil, or proper voltage production when signaling the ignition module to fire. Note, the majority of these components can be replaced individually, but when multiple components are found to be faulty, it may be more economical to replace the ignition distributor assembly.

The ignition distributor will likely be left in the vehicle during inspection and testing, so it must be removed for replacement. In preparation for removal, the engine is turned by hand to top dead center on cylinder one on the power stroke. This will point the distributor rotor towards the electrode on the distributor cap corresponding to cylinder one. At that point, the retaining bolt(s) is removed, and the distributor will twist out of the engine block or cylinder head. Installing the new ignition distributor will involve rotating the distributor backwards to ensure that the forward rotation of the distributor will result in the distributor rotor pointing to the same location as before it was removed. All electrical connections are replaced, including the spark plug wires, and, if equipped, the vacuum advance canister is connected to engine vacuum.

Installing aftermarket distributors should be done after much research. Some of the aftermarket parts available, especially for American classic cars and trucks, are not applicable to the vehicles without modification of the engine wiring harness. Any time a modification is done to an engine, research and consultation should be conducted prior.

The ignition coil transforms battery voltage to thousands of volts, and many times is housed by the distributor assembly. If the coil is not housed in the assembly, the center spark plug wire is for the ignition coil and can cause electrical shock if handled improperly.

Replacing the distributor assembly requires knowledge of engine timing. If the ignition distributor is replaced in an incorrect position, the result could be a non-running engine, or burnt cylinder head valves. It is imperative to understand how to time a distributor prior to beginning this job, but otherwise the well-skilled DIYer can handle the task.