I've made the wireing like described today (single contector at the motor). Could not hear any significant difference in speed between the two speeds. Can anybody confirm that there should be/should not be much difference?
Yes, there is not as much of a fan speed difference with the single wire defogger motor as compared to the resistor mounted on the plenum housing.
Attached is the only occurrence that is shown in a 70 Plymouth service manual that details the wiring for the single wire version. The wiring gauge sizes are correct. When testing the impedance difference on the single wire motor end harness, setting an analog meter to read 0 Ohms between black and the motor end will show a -3 Ohm loss reading between the white wire and the motor end.
With the "improved" version defogger using the resistor mounted on the plenum housing, the circuit board shows a -10 Ohm difference for the low speed compared to 0 Ohms for the high speed. It is much more apparent that there is a speed change. This same resistor cicuit board is what was used for the two speed fan positions on non A/C E body heater boxes.
Jim
