1990 Aerostar: All-wheel drive always engaged.. help please.

Hello,
My 1990 Ford Aerostar van decided last week to continually engage the all-wheel drive... no warning light, no chattering, it's just always engaged. I thought it might work it's way out, but to no avail.
I am not driving on the freeway while this is occuring, and am almost ready to bite the bullet and take it in to a dealer, but was hoping that maybe someone in this newsgroup may have had a similar experience or know the answer to the problem.
Any and all input would be most appreciated.
Thanks.
Bob Hawkins
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OK, be nice to me now... lol
The aerostar AWD system actually has a warning light when the AWD is activated? Which makes it sound like it's disengaged some times.
How does the system turn the AWD on and off?

ready
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The all transfer-case has a speed sensor watching the front axle and a speed sensor watching the rear axle. When traction is good, both should show the same speed. When there is a speed difference between the two ( like the rear wheels spinning ), the module locks the clutch giving you four wheel drive and turns on the light telling you that the front axle is engaged.. So what can cause a different speed reading from the two sensors? Incorrect tire size on one axle ( on a vehicle with four wheel drive, tires should always be changed in sets of four ). Flat tire. Open or shorted sensor.

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That's helpful information... thank you.
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Indeed! Thank you!
I should read up on this system. I have an AWD caravan (which has no electronic sensing necessary for engaging the 4WD). But the AWD caravan is a primarily front wheel drive, with secondary rear wheel drive vehicle.
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