Blower fan will not blow when heat is switched to defrost.

[rstanley70]

79 C20 A/C equipped.
Blower fan works on all speeds when selector is on vent or heat.
Blower runs only on low when switching to defrost.
Vacuum lines are good. Blower switch is new. I'm not a 100% sure the AC compressor is good but all the wires are hooked up.

 

[83Burban]

Like... the blower only comes on when in the low position? Or the blower just stays low regardless of the position?

 

[rstanley70]

The blower fan blows at all speeds when on heat/vent.
If the fan is on high and I switch to defrost, it slows to low.
It's weird.

 

[75gmck25]

The controls are set up to turn the A/C compressor on when its in defrost, so you can dry the moisture off the windows, but all speeds should work normally. What happens to the blower speed when you are in the A/C position on the slider?

 

[rstanley70]

When on the AC portion it blows at all speeds.

 

[AuroraGirl]

is there maybe a restriction that switching to defrost doesnt switch the whole thing and the low speed is actually just inefifcient sending of air ? like maybe the cable hits something, the door isnt barely allowing that direction, then the cable just flexes in the sheath upstream where you dont see? I have my non ac truck with a on-off switch on the blower switch for speed and the lever for temp as a hand slider.(Broke the unit on first attempt to move from cool to warm) and I just never went and made it all proper. But on ac, that circuitry for the defrost and such, you could be looking at a short or something pulling voltage down when it does that. does your compressor wiring all work right and your fuses good, all the firewall wiring for the AC stuff good, the connector for fan all looks fine no corrosion or burnt wires

and to test the issue, could you isolate the blower from ignition swithc. doesnt the wiring carry the full amps through ignition switch or am I thinking of a ford rn? If it does, you could use a relay with the ignition only as a trigger and wire straight to the appropiate wire for the fan speed switch or however it sends from there. eliminate that wiring before all that

 

[Ellie Niner]

The only time I remember this happening was on a control that automatically set the fan to high speed when you selected defrost, and the high speed fan relay was bad... but this ain't it, as you have high speed in other positions. Mebbe call an exorcist?

...unless it's using a different relay for high speed in the defrost position. Not too likely, but possible.

Edit: The vehicles I'm remember with the automatic High/Defrost mode were the ones with Comfortron automatic climate control systems like you'd find in a 1970's Cadillac.

 

[wanderinthru]

When on the AC portion it blows at all speeds.

Your defrost/a/c selector has to be hitting a wire when you slide it over to defrost. Fan speed and what holes blow are not related. Fan is electric selector is vacume. Or, that's all I've ever seen.

 

[AuroraGirl]

Your defrost/a/c selector has to be hitting a wire when you slide it over to defrost. Fan speed and what holes blow are not related. Fan is electric selector is vacume. Or, that's all I've ever seen.

i think the electronics for it to kick on compressor when in defrost or ac means there is a board or something that trips when slid into them weather it be vac signal or the lever itself, which means, to me, that the part that does that specifically or some weird physics stuffs like diodes or draws from AC clutch are acting weird on it. IF not a mechanical blockage type thing.

but if it happened only in defrost or was always not full speed i think we would be looking at a different for sure. its like jets, no one knows how they fly. they just do.

 

[wanderinthru]

i think the electronics for it to kick on compressor when in defrost or ac means there is a board or something that trips when slid into them weather it be vac signal or the lever itself, which means, to me, that the part that does that specifically or some weird physics stuffs like diodes or draws from AC clutch are acting weird on it. IF not a mechanical blockage type thing.

but if it happened only in defrost or was always not full speed i think we would be looking at a different for sure. its like jets, no one knows how they fly. they just do.

I've never seen them connected. Defrost should work fine weather a/c compressor does or does not even exist or not.

 

[Ellie Niner]

I've never seen them connected. Defrost should work fine weather a/c compressor does or does not even exist or not.

All of the older GM vehicles I've had have engaged the A/C compressor with defrost (whether you want it or not), though it won't kick on if the outside temperature is lower than about 40 degrees. A/C does dehumidify the air, so does help quite a bit with defogging windows when it's damp outside.

@AuroraGirl IIRC, there are some spring contacts in the heater control panel that complete a connection to the compressor by physically grounding against the control lever in certain positions (everything but HEAT, VENT, and OFF). Can't remember 100%, as I've never really had a problem outside of a broken vacuum line, which is always in the engine compartment.

 

[Turbo4whl]

Is there maybe a restriction that switching to defrost doesnt switch the whole thing and the low speed is actually just inefifcient sending of air ? like maybe the cable hits something, the door isnt barely allowing that direction, then the cable just flexes in the sheath upstream where you dont see? .....

I agree with Taylor. Sounds like the defroster door in the heater box is not moving. With the other doors closed, no place for the fan air to go.

 

[rstanley70]

Thank you everyone.. I ended up pulling the selector out and took the face plate off.
The nubbin where the defrost spot was filthy and gummed over with grease and years of farm dust.
Once clean it worked fine.
When the face plate was back on it restricted the full movement of the selector to defrost.
Made a little more room with my dremel and now we have a fully functional heater and defrost.
Rookie poster mistake... I figured it had to be something super complicated but alas my bad.
42 years on the farm collected a ton of dirt and grime on all the controls.

 

[AuroraGirl]

you and catbox would get along, nice truck !