I purchased a 94 Aerostar E-AWD. I have never owned any AWD/4WD vehicle before any advice will help. There were no owner's manual or service records. The AWD was disconected (unplugged at transfer case)at purchase. After plugging it in everything seemed fine until 15 minutes at highway speed. Vehicle started a pretty severe vibration, almost like hitting grooved pavement. Stopped after 5 seconds. When it stopped it felt sort of like there was a torqueing tension released from the frame. I could actually feel a kick from the right side of my seat. This repeated again after 2 or 3 more miles until I exited expressway and unplugged TC again. The vehicle has mis-matched tires -- 205/75/14 in front and 195/75/14 in rear. Could this be the cause?

 

Yes, you definitely have a problem with inappropriate lock up of the transfer case solenoid. The master controller for the AWD system is found under the driver's seat, a metal box approximately 3x5 inches. You can unplug the connector for this instead of crawling under the van all the time. For the time being, just leave one or the other unplugged until you sort out the problem. The most common reason for the van to behave like this is incongruity of signals from the various sensors montoring the AWD system. Mismatched tires or tires with varying air pressures supposedly can cause this problem - according to the owners manual and Ford service manual. I used to believe this but then discovered using the tiny emergency spare tire did not seem elicit any problems from my AWD system when pressed into service one day.

There are three sensor feeding data to the AWD controller. Two sensors on the transfer case and one on the rear differential. First thing to check it whether all three are connected and free of corrosion or grounded via excessive grease/grime build up. The sensors are nothing more than strong magnets sitting over a slotted tone wheel inside the TC and differential. As the slotted wheel turns past the magnet, it generates a magnetic pulse signal that is relayed to the controller. The controller monitors the various speed inputs and decides when to engage the TC solenoid if wheel slip is detected. If there has been any appreciable wear to internal components, small metallic particles can build up on the sensor magnets rendering them ineffective. All of the sensors are held in by a single bolt. You could pull them and check for fuzzy build up. These are the easiest things to begin your trouble shooting. The rest of the system including the controller and solenoid and usually pretty bulletproof. Most times, the problem turns out to electrical connections or bad sensors.

 

Thanks for the info! Hopefully this can be fixed relatively inexpensively and won't be too difficult either. My Chilton's book was delivered today and it really has minimal info on any of this. Since then I pulled the fuse to disable the blinking light. Will this accomplish the same thing as unplugging the controller?

 

If the sensors were at fault, then the computer under the driver seat should flash a code on the display right of the speedometer. When everything was connected did the "Electronic 4 Wheel Drive" indicator flash any code? Does the display briefly light up when the engine is started. This indicates the system is functioning properly. You mentioned in your reply the "blinking light". How many times did it flash? Pulling the fuse is not the same as disconnecting the module under the seat.

 

Tire size could be part of the problem. With the smaller tires on the rear, the control system sees this as a rear spinout and tries to engage. There is a threshold that must be exceeded for this to happen and this threshold is lowest for rear wheel spinouts. The van should really have equal tire sizes all the way around.

Even without the electonic module connected, you still have AWD. One third torque to the front and two thirds to the rear. The only time the electronic control is needed is when a wheel completlely loses traction. Then the transfercase locks briefly, sending torque to the opposite axle.

 

see above....

 

Thanks for all the info.

The light was blinking 8 times originally. I've read this indicates lack of continuity between sensors, control module, transfer case, which would be the case when disconected. After I plugged it in at the transfer case the light acted normal -- came on when I started and then went off. After unplugging it again the light returned to the 8 blink code. It's still unplugged (at transfer case) and went out when I unplugged the fuse (less anoying at least).

This weekend I plan on getting underneath it and getting a good look, cleaning the sensors, connections; checking fluid levels etc. If that doesn't help I'll try rotating tires front to back. The next step I guess would be biting the bullet for 4 new matching tires.

The van runs real good for its age (178,000 mi.) and I only have $900 invested so far. It may be a good thing that the problem is my AWD locks up when it shouldn't -- I would be more worried if it didn't work at all. The AWD feature was real appealing to me as I have a small cabin in western NY, south of Buffalo. You know, where those reporters for the Weather Channel broadcast from every time the heavy snow comes off Lake Erie. I should have about 5 weeks to get it done if I want to feel secure going there.

 

Just a follow up and thanks for all the good advice.

Last weekend I got under the van and located the speed sensors on the transfer case. Removal was fairly easy. They looked very clean with no particles on them at all. The connections were oily so I cleaned them with carb cleaner. The bolt holding the sensor in the rear differential didn't break loose easily so I sprayed it (twice now) with some WD40 rather than risk stripping or breaking the bolt. How does this sensor interact with the AWD?

The system seems to be operating but I only drove it 12 miles at highway speed. It was cold and windy so I decided to rotate the tires another time later this week, plus it was time to watch my beloved Bills finally get a victory. My appologies to Miami fans, I know they will be back with a vengeance.

In another post Aerostar1 mentioned it was possible to install a switch to engage the lockup on an as-needed basis. Although my preference is to troubleshoot it properly, this idea occured to me as an alternative after I've exhausted money and patience. Where can I get a wiring diagram? Could it be done at the fuse? My manual has nothing on this system.

 

The dealer service manual has a complete section on trouble-shooting the electronics. This will give you some testing ideas and the wiring diagram. "Pinpoint test F4" tells you which wires will lock the clutch to test its operation. Maybe your local Ford dealer would let you look at this section.