Alright I think I've got a winner. This weekend I put two resistors in, one in-line with the field that is controlled by the PS1 board, and one in-line with the armature.
I only bought 1 ohm resistors, even though I was measuring closer to 2 ohms across the field windings. And I couldn't fit more than two of them into the chassis.
In any case, I figured that some additional load for each circuit would more closely mimic the net resistance that the Pullmor AC motor would normally have when wired in series add originally intended.
I measured the draw at the transformer posts prior to adding the resistors. I was getting about 4.9 to 5 amps when pulling 15 MTH premier freight cars. After installing the resistors, it dropped to around 4.2 to 4.5. Not as much reduction as I expected, but maybe enough to allow the field and armature to run cooler for longer and leaves a little extra power on my KW to run the B side train.
So I closed it all up and let 'er rip for a while to see how well she runs. I can tell you, slow speeds and starts under loads are WAY smoother than running this guy on the original E-unit and feeding AC to the motor. It really behaves much more like my dual-can DC PS1 locos in the rest of the fleet now. You wouldn't know it's got an old growling open frame motor from the 70's in there!
After running for more than an hour continuously pulling that big 15 car freight consist, and a significant part of that running up the incline (which it had no problem with, BTW! ) I took it apart to check internal temps. It was warm, bordering on hot but still just cool enough to touch for a couple seconds before it got unconformable. The board itself was very warm, the resistors were hot as expected, but the motor was a pleasant warm without burning my fingers.
I'm calling this project done! Now I have a cheapo Lionel EP5 from the 70's that sounds and acts like an MTH EP5 from the 90's!
Photos attached so you can see how I crammed the resistors in. I also cleaned up my wiring a bit. Video showing a slow start and some running.
Thanks for the help @ADCX Rob and others! I now know how to convert an AC series wound motor to work with full functionality on a PS1 board!
