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I have a 1940 120 Club Coupe. I have been running through electric components on the car. While operating the heater, I discovered the fan forward and reverse function for floor or defrost. I'm lucky that the switch does operate both ways and lights up too. However when you switch to the farthest position either way, the switch light not illuminate nor will the fan run. So is it the switch or the fan that's the problem?

Ask your voltmeter 😀

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It sounds as if you have the rotary style deluxe 2 speed reversing switch which I believe came along in 39 and was probably used with a different mounting in later models too. From the symptom you describe, chances are problem is in the switch.

The far position in either direction is for slow speed and that is controlled by having the voltage for those positions go thru a resistor which is selected and placed in the circuit by the switch. The resistor goes in series with the voltage being fed to the motor so the voltage is lowered. Since motor works at hi speed which gets direct battery voltage but not in the low position which needs the resistors, there is a good possibility the resistors are damaged, missing, or for some reason the wiper inside the switch is not able to connect and bring them in the circuit. I believe the light bulb is wired to be in parallel with the motor voltage feed so it would be bright at hi speed and dimmer in the low speed position. Since it too is acting up I doubt the switch is passing any voltage when at the second positions.

For starters, you might take a look at the rear of your switch and check for the presence of and condition of the resistors. I have one of the switches where a resistor for one direction is completely missing and here is a blurry photo of another forum members switch. His switch still works but the resistors (wire coiled objects in red circles) appear to be in poor condition.




EDIT: I just found a slightly less blurry photo of the deluxe switch on a Max Merritt deluxe heater and switch listing currently on ebay. The resistors on that second switch are not in great shape either with one completely missing the ceramic inner support and the other one looking almost ready to fall out. Make sure you have resistors and their wire ends are securely connected to the switch at each end. If that looks good then chances are the problem is internal or possibly something in the wiring is hooked up incorrectly. The field and armature are separately wired and brought out to the switch which places them in series so motor can run. The switch keeps direction of current flow thru one item the same in both directions but changes direction of current flow thru the other depending on side of switch the knob selects. That flow change in only one item alters the magnetic relationship between the field and armature to get the reverse direction.