1928 Plymouth 1937 Plymouth Plymouth: The First Decade

Disclaimer: While an effort has been made to assure a reasonable repair procedure, no guarantees are made. We are not responsible for any damage or injuries that may occur as a result of following these instructions. The only vehicle these procedures has been tried on is a 1933 Plymouth PD. Applicability to any other vehicle is for you to decide.

Horn Tune Up


With the exception of a horn labeled “N.E.” for use on the 30-U, the master parts book published in 1934 [Chrysler 1934a] shows that all horns for 1928 through 1934 Plymouths were Klaxon manufactured by Delco. They came in a variety of different shapes but all used the same general design.

When energized, a metal diaphragm is pulled into a electro magnet. When it is pulled in a stud on the diaphragm pushes a tab that opens a switch. The opening of the switch de-energizes the electro magnet releasing the diaphragm. The process repeats many times per second vibrating the diaphragm. The vibrating diaphragm generates sound.

Dual horns use a horn relay to provide power to the horns. The standard single horn has power continuously applied to one terminal with the second terminal grounded through the horn button.

A single horn draws between 5 and 8 amps with dual horn setups drawing twice that amount. This can be a bigger current draw than the headlights and a low voltage at the horn(s) will cause performance problems. For this reason all the wires, connections, switches and relay (if so equipped) must be in good condition.

Conditions Affecting Horn Performance

  1. Low voltage at horn. Voltage should be no less than 5.25v. Low voltage could be caused by:
    • Low Battery
    • Loose or corroded connections
    • Undersized wires in horn circuit. Wire should be at least 14 gauge for the Klaxon 16, 26 and 31 models. For the Model 33 Klaxons, 10 gauge wire is needed.
    • Dirty relay contacts or broken relay.
  2. Loose or broken parts.
    • Internally grounded horns need to have a secure mount that provides a good ground.
    • A loose back shell can affect the tone.
    • Open, shorted or grounded circuits in the horn will keep the horn from properly functioning.

My Experience

Standard equipment on the 1933 Plymouth DeLuxe is a Klaxon 31 which should have an air gap of either 0.015 to 0.017" (for high note) or 0.020 to 0.022" (for low note). It is also supposed to draw between 5 and 7.5 amps of current. Apparently these horns can be used in pairs generating matched tones. It is not clear to me if the single horn setup should use the high or the low tone air gap.

The “air gap” is the space between a large adjusting stud on the diaphragm and the electro-magnet. I was unable to see how to adjust this dimension without removing the “projector” (cone shaped tube that the sound comes out of). Even there, the diaphragm closes off the housing where the air gap is. The only thing I could figure was to install the diaphragm without the projector and use a dial indicator on the diaphragm while pushing it in and out against the electro-magnet. Being uncertain that my measurements were accurate, I did not play with that adjustment.

As long as the horn is opened up, take a look at the conditions of the contacts on the switch inside. They can be lightly dressed with a thin contact point file. My contacts were in good shape and needed no attention.

Apparently some models of horns have a capacitor or resister in them. These must, of course, be in good shape. The model 31 Klaxon does not have these components.

With the horn re-assembled and mounted on the car, it is time to make the current adjustment. I don't have a current meter that can cover the 5 to 8 amp range described in the specifications, so I adjusted by sound.

The current adjustment is a 10-32 set screw with a locking nut located on the back of the Model 31 (it may be under the cover of other models). This screw adjusts the force needed by the diaphragm to open the switch contacts described above. It is a very sensitive adjustment, the 1937 service bulletin says:

Loosen the lock nut and turn current adjusting screw to the left to increase the current, to the right to decrease current. Increasing the current increases the volume. Too much current will cause the horn to have a sputtering sound. This adjustment is very sensitive. Move adjusting screw 1/10 turn and lock in position each time before trying the horn. If ammeter is not available, adjust according to sound.

The current adjustment did make a tremendous improvement on the sound and volume of my horn