Noise Suppression Capacitor

By Carl Bilter
March 27, 2024

Often overlooked in restorations are the noise suppression capacitors (a.k.a. condensers) used on letter cars equipped with radios. Do not confuse these capacitors with the distributor capacitor used on breaker point ignitions. That one is essential to proper engine operation; the radio noise suppression capacitors are not. Therefore, for the noise suppression capacitors we are more concerned about correct appearance than functionality.

These were used to reduce ignition noise on the AM radio band. We have found some suitable modern replacement components. There are two types: the coil capacitor and the generator/alternator capacitor.

The coil capacitor was used on all letter cars equipped with radios. It was connected between coil positive and ground as seen in this FSM illustration:

Here is a photo of an original Mopar coil capacitor:

The length and style may have varied some during letter car production. A modern version is available from Standard Motor Products, part number RC-15 as seen below. The wire terminal may need to replaced with a ring terminal. While this capacitor has the correct appearance, it is quite expensive for a capacitor (more than $40). It might be possible to find a suitable NOS aftermarket or used capacitor on ebay for less money. The NOS Mopar units can also be pricey.

A capacitor was also used on the 1955-1960 generator equipped cars that had radios, as seen here:

When the wide-fin alternator was introduced in 1960 (used on the 1960 300F 4 speed cars) and 1961 (used on all 300Gs), a noise suppression capacitor was also utilized as seen below. The 1962 FSM indicates that it was also used at least initially on the new for 1962 narrow-fin alternator. The alternator capacitor was discontinued by the 1963 model year.

A modern, reasonably priced version of the generator and alternator capacitor is available from Standard Motor Products, part number RC-1. This product costs less than $9 at Rock Auto.