Oh wow—I gotta ponder on this one. I’m busy cooking dinner now. Later.

There are some NOS Prestolite regulators on e-bay.

I dunno, but it sounds like you are trying to use a “Type B” regulator. Your Johnson has a “Type A” generator.

EDIT: Otherwise known as “internally grounded field” (B) and “externally grounded field” (A)

• This reply was modified 4 years, 9 months ago by frankr.

Not knowinig what you have, all I can do is explain how they control the field circuit (and thus voltage).

A type A generator starts producing a voltage in the armature windings as it starts to spin (remember, it retains some residual magnetism in the fields to get things started). That voltage in the armature is fed to the battery if high enough to close the cut-out relay contacts. At the same time, it is also fed to one end of the field coil windings. It passes through the field windings, and goes out the field terminal on the gen. From there, it goes to the field terminal on the regulator. The voltage regulator contacts are closed if calling for gen output, so the field circuit is grounded, completing the field circuit from armature to ground, and a current flows through the field windings. The current flowing through the field windings causes them to become electro-magnets, adding to the residual magnetism already there. The added magnetism causes more voltage to be produced in the armature, till eventually it is high enough to close the cut-out relay and go on to the battery. And so it goes till the voltage reaches it’s set limit, then the voltage regulator contacts open, interrupting the field ground and current and charging more or less stops. Actually, the contacts vibrate, causing a rapid connect/disconnect to ground.

OK, if you waded through all that, the important thing to absorb is the voltage regulator GROUNDS the field circuit when charging is called for (external ground).

Without going through the whole scenario, a type B generator is different in that the inner end of the field coil is grounded to the case instead of going to the armature.. The type B voltage regulator applies battery voltage to the field terminal on the gen to complete the circuit to it’s internal ground, increasing the field magnetism and causing a voltage to be produced in the armature and on to the battery. And so it goes.

Summing up, Type A regulator GROUNDS the field terminal, Type B regulator APPLIES VOLTAGE to the field terminal.

Got it????

One last thing, then I’ll shut up. For years, OMC put the regulator in a junction box in the boat. Then they eliminated the junction box (good riddance) and put the regulator under the motor hood and changed the part number. Supposedly, the under-hood version is sealed against fuel vapors which might be ignited by the vibrating contacts inside. Remember, it is right under the carburetor. Word from here is use the correct part number for under-hood mounting.