An update for those interested - both aux fan motor and pulse module were defective...

* Checked resistance of aux fan motor - > 250K ohms, with engine at operating temperature. Resistance dropped subtantially as the fan motor cooled but was still higher than that of a new motor. Did not measure amperage draw, in light of elevated resistance.

* Installed new fan motor, in conjunction with new pulse module - fan operational.

* Reinstalled old pulse module, in conjunction with new fan motor - fan non-operational. No current found at pins D1/D2 of module.


So.... the pulse module does have a feedback circuit, where it monitors aux fan draw. The brief voltage spike observed every 6 seconds during the diagnostics was the new module attempting to activate the fan... and the brief 1/2 second run of the old motor that occurred when the new module was installed was due to the pulse module doing what it was designed to do... shut down the motor due to excessive current draw.

Thanks to all for the assistance.