73 roadrunner mystery blower function

[73Beep]

Trying to see if anyone else has run across this issue. Factory a/c control. Blower functions on all speeds with knob in max a/c, a/c, and vent. As soon as you click to heat or defrost, the blower shuts off. I did check the resistor, even though all the speeds are present on the a/c & vent positions. The resistor is ok. I believe the issue is in the control but I do not have a diagram available. It just shows the low, med, & high circuits. Any guidance would be appreciated.

 

[pnora]

Sounds like the master switch to me.

 

[Nacho-RT74]

The blower for the heat and def comes directly from control switch, not from the blower speed selector. Is a single low speed AND is the brown wire coming out from the control main switch up to one of the resistors block.

 

[Nacho-RT74]

Here is a 71/72 diagram. Pretty much the same to the 73/74 except for the blower speed switch, althought wiring functions and colors are the same. Just the switch itself along with the plug prongs disposition changes

 

[73Beep]

If I'm correct, I should be checking terminal #3 or #4 on the control switch for voltage when heat or defog is selected? The a/c compressor has been removed from this car but I would like to at least have defog if caught in bad weather. As far as I can tell, the modes are switching in the housing as they should.

 

[pnora]

If I'm correct, I should be checking terminal #3 or #4 on the control switch for voltage when heat or defog is selected? The a/c compressor has been removed from this car but I would like to at least have defog if caught in bad weather. As far as I can tell, the modes are switching in the housing as they should.

You obviously have power to 4 and power out of 3 to the blower switch as you claim all the blower speeds work on the other modes. That only leaves one possibility the master push button switch.

 

[Nacho-RT74]

If I'm correct, I should be checking terminal #3 or #4 on the control switch for voltage when heat or defog is selected? The a/c compressor has been removed from this car but I would like to at least have defog if caught in bad weather. As far as I can tell, the modes are switching in the housing as they should.

Per diagram posted power coming out from 4 (brown wire) on Heat and Def. Be just sure is the brown wire no matter the cavity. I can’t recall if 73/74 control switch meets EXACTLY to the 71/72 diagram, since I recall the 71/72 gets the OFF position in diff position than 73/74. So just in case, you must be sure is the BROWN wire. Can be checked at resistor block wire end.

 

[73Beep]

Sorry for the delay in updates. I got the car fired and tested the functions. Not sure what I repaired but the blower is functioning in all modes at this time. It does not seem to have the speed on heat & defog as it does on the other positions. It's definitely working enough. Thanks to everyone for the input & direction

 

[SureGrip]

The a/c compressor has been removed from this car but I would like to at least have defog if caught in bad weather.

I got some bad news for you.....defog works by having the compressor run with the heat, drying the air.

 

[Nacho-RT74]

I got some bad news for you.....defog works by having the compressor run with the heat, drying the air.

I can’t remember on my car having the compressor on while in deffog. Just the vacuum actuators making flow the air throught the windshield outlets