Problem solved- update! Found a main vacuum line, (white one), broken right above where the oil filler tube is. I guess if the plenum door is not in the proper position, it screws with the fan? Works like a champ now even with 442,000 miles on this van! --- 1997 aerostar, 3.0. Doesn't matter what I'm using, vent, heater, or AC. When I have the fan switch "on", The fan will shut off, then turn back on whenever it feels like. I've tried turning the controls on and off in any and all ways, and still random fan goes on-off when it's turned on. Is there a sensor somewhere that could cause this? Or what? Thanks in advance for all replies

 

There is no sensor for the heater. My first guess would be a loose/broken wire at the switch or blower motor resistor. I would check those spots first. Then my second guess would be the switch is on its way out. Switch is still available from Ford, looking at around $28.

 

The brushes wore down in the blower motor on my 97 Aerostar and I had replace it after it started acting up like yours. Try tapping on the blower motor when it stops to see if that restarts it. If it does, replace the motor.

 

please read original post. Thanks; Tim

 

Glad you found it and thanks for the heads up about the vacuum line! don-ohio (:^)

 

The usual symptom with a leaking vacuum line like that is when the engine is under load, and vacuum drops, the heater doors default to blowing to the defrosters. It may seem like the fan had cut out, as all other outputs stop blowing, and the air blows weakly against the windshield.

So replacing the leaky vacuum line will fix the apparent fan failure problem. Well done.

 

"problem solved! 97 aerostar heater/AC blower turns off and on randomly"

So that no one is confused, there is no vacuum switch that turns the blower motor ON/OFF. The problem "solved" here was never the motor being turned off and on - it was the air flow from the various outlets being redirected to the defrost vents because of a loss in vacuum. With no vacuum (whatever the cause), air flow is redirected to the windshield defrost vents in a "fail-safe" mode. Defrosting the windshield is the most important task the heater can do - everything else is for comfort.

Vacuum frequently drops to nothing while going up steep hills if the check valve fails to work properly as it often does. Air stops coming out of the dash vents in air conditioning mode or to the floor in heating mode and starts coming out of the defrost vents till you get over the hill and the load on the engine allows it to produce a vacuum again.