75 Heater blower

[Tmta]

Hopefully someone can help me out.  I rebuilt the heater box in my daughter's 75 2002.  I purchased a new heater blower motor, so I know the motor works.  Reading a post from hegedus (Jan 2013), he suggest plugging the fan in and testing, prior to installing the box.  The fan does not work.  I am not an electrician, so be gentle, but I checked for power at the plugs on the heater box, and I found 12v.  Can someone give me some guidance on what to test or what the possible problem could be?

 

Thanks

Tom

[Mike Self]

Take a couple of jumper wires and connect the fan directly to a known 12v source, like the car's battery, then see what happens.  

 

As I'm sure you know, there's a resistor pack on the heater's case--that's what gives you three speeds for the fan.  If the fan works when connected directly to +12v, then I'd suspect the resistor as the problem when there's no action after you connect your jumper leads to the heater box terminals instead of the terminals on the fan.

 

cheers

mike 

[Tmta]

Thanks Mike.  I did that and the fan worked fine.  Is there any way to test the resistor, before replacing it?  

[Mike Self]

It's been so long since I've had a heater box apart...but with the heater disconnected from power you should be able to use a ohmmeter to determine whether there's an electrical path through the resistor.  If there is, you'll get a reading in ohms.  If the resistor is broken, you'll get an infinity reading on the ohmmeter.

 

Another thing to check:  your '75 has the heater blower switch as one of the heater control levers (upper left); the switch has open contacts that have been known to get a bit of corrosion on 'em, especially after sitting for awhile, so make sure that you have +12 volts at the wire that plugs into the heater box.  If there's no power there, check to see if you have +12 volts at the wire that feeds the heater blower switch.  If no power there, check the fuse...

 

mike 

[Tmta]

So, when put my test across the acutal coils of the register, it show continuity.  I have power coming into the the circuit.  I have sprayed the switch contacts with a contact cleaner also.  When plugged one of the wires that lead to the fan into the female terminal of the resistor (inside the box) part of the terminal broke.  I am wondering if those terminals are the problem. I have used alligator clips to go from the inside terminals to the connectors for the fan, but still doesnt work.

[ray_]

You do need continuity through that resistor board, to get power to the fan.

 

Good luck,

[732002Malaga]

Was you ground connected to finish the circuit?

[Tmta]

I used a jumper wire from the brown wires on the heater box, to a ground wire and then tired attaching the jumper wire to the screw hole for the box. And nothing.  

[ray_]

Brown is always ground on old BMWs.

 

[jgerock]

19 hours ago, Tmta said:

So, when put my test across the acutal coils of the register, it show continuity.  I have power coming into the the circuit.  I have sprayed the switch contacts with a contact cleaner also.  When plugged one of the wires that lead to the fan into the female terminal of the resistor (inside the box) part of the terminal broke.  I am wondering if those terminals are the problem. I have used alligator clips to go from the inside terminals to the connectors for the fan, but still doesnt work.

Try soldering a connector onto the broken one at the resistor board. Pretty simple fix that I have done before.  The outside of the heater box has the resistor values (molded into the plastic) which correspond to the fan speed.  This is why it is important to connect the wires to the side of the box in the correct locations.  

[Tmta]

jgerock, before disconnecting the wires i wrote down their positions, so I am pretty sure they are connected correctly.  I guess I am going to lean towards the resistor board needs to be replaced.  I was concerned it might be the switch.