Motorola 51/55 amp Alternator Field Excite / Engine Harness Spoiler Alert

[KWKloeber]

All M-25, XP, 35 owners (maybe others) -  Just an FYI if you have been having charging issues.

In doing a panel/harness rebuild, I ran across a Universal OEM engine harness that had the wrong alternator field excite configuration.  The alternator field excite circuit is simply a solid 12vdc applied from the cockpit panel to wake up the alternator so that it starts charging at low RPM.

The cockpit harness had been done correctly from Seaward/Bristol/CTY factory, but the engine end of the harness (engine side of the gummy bear plug, i.e., what is connected to the engine components) was WRONG.

The 51-amp / 55-amp 8MRxxxx model Motorola alternators that were used on most above engines have a YELLOW field excite wire terminating in a BLACK plastic slide-on terminal cover (technically called a Delphi Packard 56 connector -- see lower center of pic below, your wire/connector may be painted over as shown.) The PURPLE harness wire (the excite wire originates at the ignition key switch on the panel) mates with the Packard 56 connector.

This OEM Universal engine harness had its field excite wire terminated in a ring terminal.  So on the assembly line, when the harness was connected to the engine there was no Packard terminal to plug into the alternator connector -- consequently the harness wire was "mistakenly" put onto the tachometer (A/C output) terminal post (or maybe the give-a-crap ratio was very low that day, and it was done knowingly rather than switching the terminals. I mean really, techs know if there's a yellow excite wire, it gets hooked up.)  I could tell that the harness wire and ring terminal were OEM, so the SNAFU was at the Universal factory, not by a P.O.

The effect is that, at low RPM, the alternator will not excite and charge -- I also don't see how the tachometer could register correctly with a solid 12v DC applied to the 12v AC signal.  If you are having charge and/or tach issues, you might first check to see if your harness excite wire is properly connected.

Some of the alternators that Universal used (i.e., 72 amp) had post-type excite terminals on the back of the regulator, so I suspect that this was simply a mix up of using the wrong engine harness (for 72 amp) on an engine that had an (51/55 amp) alternator looking for a Packard-type connector.

kk