Voltage loss to blower motor (1969 with factory A/C)

[MaximRecoil]

I, too, was concerned about what I perceived as low blower speed (especially in HIGH) so I connected the motor directly to the battery and the speed rose to what I considered "correct"....problem is the motor got so hot it blistered the paint.

Draw your own conclusions.

That's interesting. Do you know what the voltage is at your blower motor when on "heat" and the fan is on high? And if you have A/C, what is the voltage when the fan is on high and "A/C" or "Max A/C" is selected?

I still can't imagine how the motor is being slowed down so much when on high. Though I haven't torn my ducting apart yet to look at my blower motor resistor and the wiring to it, the diagram from the FSM that I posted shows the resistor being bypassed when on high. Yes, there is a fairly long run of somewhat thin wire to it (probably 16 gauge), but that should only result in a drop of a volt or two compared to the alternator voltage, not 8 volts (or 5 volts when A/C or Max A/C is selected).

If I remember right, "Pete in NH" is an electrical engineer, so if he reads this, I have a couple of questions:

1. Based on that diagram, can you see anything that would result in an ~8.4 volt drop (from ~14.4 volts to ~6 volts), measured at the blower motor, with "heat" and "high" selected?

2. Can you see anything that would suddenly allow an extra ~3 volts (from ~6 volts to ~9 volts) to make it to the blower motor, thus significantly increasing its speed, when "heat" is deselected and "A/C" or "Max A/C" is selected (fan speed switch not touched, i.e., still on "high")?

[Pete in NH]

Hi,
I have the same diagram Maxim posted in a 69 Plymouth FSM I have. What they are not showing us are the switching functions in either the blower speed switch or A/C controls, so the following are just some educated guesses. Note they are showing three resistors in the blower speed control set up, if you were bypassing all resistors in the high speed setting you would only need two resistors. So, I suspect they are leaving one resistor in the circuit even on high speed. The way the circuit diagram is drawn and without really knowing the switch internal functions I'm guessing at the following- warm heater air is lighter and easier to move than cold A/C air so for the A/C mode they bump up the motor speed even more over the high speed heater position.  I think there is always a resistor in the circuit because as John mentioned the motor is likely something less than a true 12 volt motor. I think in the heater high speed position you have a resistor of a certain value that winds up with another resistor in parallel with it in the high speed A/C position to lower its value a little more. I suspect you could confirm all this by tracing the switch functions through the switches and then drawing the whole thing out.

[MaximRecoil]

Excellent. Your explanation makes sense, thanks. I guess I can leave the blower motor alone, and that's one less part of the HVAC system I have to worry about fixing.

Now I only need to replace the torn flexible ducting that goes to the defrost vents on top of the dash, and do something about the leaky temperature control valve on the firewall (I currently have it bypassed so it is always on hot). The former will be easy; the latter won't.