Blower fan and high rev's

Weather is turning here in Colorado and I need to start using my heater to keep the windows from fogging. I have noticed it in the past but I am wondering if something is wrong. When I put the blower fan (Hot or Cold) on I notice the truck adds about 10,000 RPM to whatever it was running at. Is this normal?

Thanks
Paul

The engine rpms increase by 1,000?

I think there is a throttle kicker solenoid (shown here highlighted in orange from the '81 EVTM)
But that wouldn't explain why it affects the heater.



Perhaps that's because the weak alternator otherwise wouldn't keep up with the draw of the blower motor.

Note the last paragraph under How The Circuit Works...

Vacuum related maybe? (vacuum motors being actuated, etc)

That was my first thought Chris, but AFAIK the vacuum switching is entirely mechanical behind the controls.
Fan on or off and speed doesn't effect the vacuum and where it's routed.

According to the picture from the EVTM the kicker operates in all positions except vent and floor.

Maybe I misinterpreted the OP, and this jump in idle happens when they shift from vent to heat, or panel to defrost?
If that's the case it could well be a cracked/broken line or a damper motor with a blown diaphragm.

Edit: does the idle kick up if you have it set in 'floor' or 'vent'?

To add to the description.

When the engine is in gear and fan is on low or high and any of the other settings (Floor vent etc.) it revs at about 1,000, without the fan on its usually around 700-800 rpm.

With the fan on and when I put it in park the rev's go up to 2,000 -2,200 rpm's.

I think you must have a bad vacuum leak.
Vent or floor shouldn't effect the throttle kicker according to the diagram, but moving the control from 'OFF' to any position would engage the fan.

Maybe Chris has some insight, since that was his first suggestion?

Not exactly, but I agree with Jim.  It's a vacuum leak.  Selecting any mode other than "Off" forces the fan to be powered and spinning.  

I don't think this was asked - the engine RPM isn't correlated to the speed of the fan, right?  The engine RPM increase stays the same regardless of the fan speed (low vs high... but still on (not in Off mode)), right?

I'd first check the vacuum lines in the engine compartment, near the heater box.  It looks like this:



That is fed by a larger line attached to a vacuum tee on the firewall - try disconnecting the line going to the climate system, and cap it.  See if the RPM increase goes away, and we will go from there.  See the circled line near the top of this picture:

Tried each of the vacuum lines (Mine has three) and nothing changed. However I did notice that when I switched from defrost & floor to the AC option the revs went back to normal. Also the I went from Vent to AC the revs also went back to normal. My AC is not working but still blows.

I can physically see the accelerator jump when the rev's drop from 2,500 to 1,000.

Whether it is set to cold or hot (top slider) makes not difference.

So the throttle positioner is s working (everything but vent & floor)

And yet the rpms jump whenever you move the lever from 'off'
Very strange that the rpms drop when the kicker opens the throttle plate..

That's a big jump for a vacuum leak that you should be able to hear in the cab.

Definitely confusing