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Discussion Starter · #1 ·
So for some reason my heater fan only comes on at high speed. I have a 69 so I believe it should have three speeds - low, medium and high. Any idea what might be wrong? I just rebuilt the box with all new seals and I tested the motor with my battery - worked fine.

David
 

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The heater fan motor speed is controlled by a series of resistors mounted on the heater plenum. Depending on what fan speed you choose with the switch will determine which resistor(s) are placed in the circuit between the motor and ground. When HI fan is selected the circuit bypasses all the resistors.

Are you saying that the motor operates at only one speed regardless of switch position or only works in HI with LO and MED having no motor operation?

Paul
 

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Discussion Starter · #3 ·
Only works when high speed is selected. Sounds like my resistors might have too much resistance or maybe the circuit is open altogether?

david
 

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Only works when high speed is selected. Sounds like my resistors might have too much resistance or maybe the circuit is open altogether?

david
David,
It could be the resistors or the switch. Did everything work correctly before you rebuilt the heater box? Did you look at the resistor network for damage to the resistor coils, etc?

Paul
 

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Discussion Starter · #5 ·
The resistors (coiled wires) inside the box looked fine but I think they were rusty? I could use my ohm meter to check the resistance. Does the electrical diagram show what the resistances should be? Is there anything in the heater control on the dash that I should check?

david
 

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Pic of resistor network:



There are two resistors (coiled wire). The way the system works is when LO is selected the circuit has both resistors connected in series with the motor ground and voltage supply is reduced. When MED is selected the circuit has one resistor connected in series with the motor ground and voltage supply is less reduced. When HI is selected the circuit has no resistors connected in series with the motor ground suppling full battery voltage to the motor (no voltage) reduction. You probably have one open resistor coil (MED) which caused both LO and MED to be open circuits.

Paul
 

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The resistors (coiled wires) inside the box looked fine but I think they were rusty? I could use my ohm meter to check the resistance. Does the electrical diagram show what the resistances should be? Is there anything in the heater control on the dash that I should check?

david
I do not believe Ford ever published the actual resistance (ohms) specs (its not in my 68 shop manual). I think others have measured them so you may want to look around on the net for those resistances. I would at least use you ohm meter to verify that neither of the resistors is 'open'.

Paul
 

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Discussion Starter · #9 ·
So I did a lot of troubleshooting last night but couldn't find my problem. I tested the resistance and found approximately 1 ohm on the low setting and 3 ohms on the medium setting. I just recently rebuilt the box and I didn't remember seeing anything wrong with the coils of wire so I'm not surprised they are okay. I then tested my controls for the fan and found that they are switching okay - approximately 12 volts opens up when I switch between the different speeds. At this point I'm beginning to wonder if my fan motor is bad? Although it works fine at high speed, is it possible the brushes are too wore out for it to run at lower speeds?

david
 

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If you have proper voltage at the resistor connections at various speeds by measuring voltage at the connection to the resistor, you know that the switch and the wiring to the resistor is good. You also have resistance through the resistor, which indicates no break in the coils when no voltage is passing through it.

Have you checked the voltage output to the fan motor in the different switch positions? In other words, keep the resistor wiring connected, but use a meter to check for voltage coming out of the bottom tab of the resistor when the switch is in low and med settings. Maybe the resistor has a break in it when the coils heat up from the current passing through it.

Also, make sure you have a good ground connection for the blower motor.
 

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With the motor connected to the resistors thru the switch run the motor in HI. With it running in HI switch to MED. Does the motor then stay running in MED? Or does is slowly slow down and stop? Or does it just shut off?

Paul
 

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Discussion Starter · #12 ·
I'll check the voltage to the fan with all the wiring connected and try switching from High to Medium to see if the fan stops or slows down. Both good ideas, thanks guys!
 

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Discussion Starter · #14 ·
The ground from the resistor panel must be connected.
Where is that? Currently there is a one pin connector that goes to the motor and a three pin connector that comes from the controller in the dash. I'm an electronics novice but wouldn't the ground from the motor be all that I need?

david
 

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I don't know about other years, but for 69 there is no additional connection at the resistor, aside from the 4 pins you mention above. The blower motor has a ground coming off of it to complete the circuit.
 

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I'd run some emery or sandpaper over those terminals, then a point file or cut-down ladies emery board to clean the ends of the females in the plug, squeeze 'em a bit to make sure they're snug and see whatca got.
 

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Discussion Starter · #17 ·
If you switch from high to medium, the fan stops. If I measure the voltage going to the heater (after the resistors) I have right at 12 volts on all speeds. I did notice that the interior lights dim slightly when I switch the fan on to medium and low. Also when on high speed the fan will surge a little like I'm varying the current. I think I might buy a new resistor board and see if that helps.

david
 
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