'73 Charger Blower fan not working on low speed

[Charger Fan]

Fan wasn't working at all on any speed so I trace the problem to loose terminals on the switch. Replaced the switch with a NOS part and now Med & Hi fan positions work but Lo speed doesn't. Any ideas what I should check next?.

 

[Jerry Hall]

Blower Resistor?

 

[Nacho-RT74]

That’s weird, because low speed is the DEFAULT speed. *** soon you turn on the AC, still with the speed switch disconected it should work. So as mentioned, everything points out to the resistor.

 

[Master Gunner]

Backwards, High speed is default, you slow it down by using resistors (lowering voltage), but yes your problem is most likely The resistor.

Mark

 

[Nacho-RT74]

No… SLOW speed is the default speed. Try it!

Check the diagram. The AC blower source coming from control switch is spliced into the blower lever speed as an input for the speed selector AND the feed for the low speed resistor.

just try it on your own car disconecting the blower speed switch and turning on the AC

the blower speed switch reroutes the power to the next speed resistor, and by ohms law STILL with the low speed getting power since is coming straight from control switch but being at the same time the input for the speed selector, the power will run through the less resistance circuit once is set.

 

[Charger Fan]

I did some checking tonight and found that the fan will run at high speed with the resistor completely disconnected from the circuit. I unplugged all the wire terminals and laid the resistor on the floor. With the A/C switch set to A/C, turning the ignition on turned the fan on at high speed. So my experiment told me the resistor only controls med and low speed. I did rule out my new switch as the cause because with the reisistor in place and the fan switch set to low I could hear the fan motor running but not a speed that you could feel any air coming from the vents. In med speed it blows just fine as well as at high speed. I concluded that there is to much resistance through the resistor at low speed hence the barely turning fan. The resistor coils gets extremely hot as you would expect but I can't figure out what would go bad on them if the wires are still intact. Furthermore, I cleaned any loose corrosion and/or surface debris off the coils to see it that would have an impact but it didn't. I obviously have to much resistance at low speed but can't figure out what goes bad on the resistor. Its a piece of coiled wire at a certain length to get whatever resistance is required to get the speed required. The resistor I have is a three coil resistor P/N 3502570. I assume its the right one for '73 Charger with a 3 speed fan. Looking on-line its challenge to find a replacement for a decent price if you can find one at all. What goes bas on these things? If I have to buy another one I will, just wish I understood why the one I have is bad because visually there is nothing apparently wrong with it.

 

[Charger Fan]

Then again maybe it is the fan motor. Its likely 50 yrs old and maybe the voltage at low speed isn't quite enough to get it going?

 

[Master Gunner]

Electric motors only have one speed, the only way to get lower speeds out of them is to limit the voltage, thus the resistor, which limits voltage giving "speeds". This is easily tested by running 12v directly to the motor, it will run at high speed. The only way to get slower speeds is to limit voltage.

Mark

 

[Nacho-RT74]

Yes, resistor only controls low, mid and heater speed which is a diff speed from the A/C speeds

when you set the high speed on speed selector, the dark green wire coming out from the switch runs straight to the bulkhead.

the resistor block gets also a dark green wire because that wire is also the output from the resistor block after get any of the low/mid/heater speeds selection to also run to the bulkhead. Both dark green wires are spliced into the terminal on bulkhead. The power runs through the less resistance network no matter if some other wire is also hot.

 

[Nacho-RT74]

Here is the 71/72 diagram. The same for 73/74 with just the speed switch design, but wired exactly the same

also diff location on bulkhead cavities.

 

[RJRENTON]

Electric motors only have one speed, the only way to get lower speeds out of them is to limit the voltage, thus the resistor, which limits voltage giving "speeds". This is easily tested by running 12v directly to the motor, it will run at high speed. The only way to get slower speeds is to limit voltage.

Mark

ABSOLUTELY CORRECT....the resistor element that reduces the voltage is usually located in ductwork, where it is cooled be the air stream. Its switched value is controlled by the fan switch; low speed equals max resistance and slow speed; high speed equals no resistance and the motor receives full voltage and high speed. Very likely that the resistor element is open or the switch has a bad contact. The blower motor is a simple DC compound wound motor with carbon brushes that are subject to wear. If the brushes wear out, the motor will not operate. Very easy to replace the brushes. Just my opinion of course.
BOB RENTON

 

[Nacho-RT74]

ONCE AGAIN… The blower speed switch is not working on low speed since is the default speed coming straight from control switch and also serves as input for the blower speed switch when selecting the next higher speed which by the reduced resistance, get almost inoperative the lower speed circuit still being feeded.

still unplugging the blower speed switch, the blower will work… on low speed.

everything points out straight to the resistor on resistors block