I know this subject has been posted many times but I never paid attention because it wasnt happening to me.
But now my 93 4.0 awd is doing the thump and bang when I make turns, left or right. 4wd light blinking as well.
Anyone care to comment, or direct me where to search for info?
thanks.

 

one of my dad's was doing this... one day it made a loud BANG and the transfer case stared leaking fluid... he kept on driving it... now it sounds like its ALL broken... constant banging and crunching...

any ways check your cv joints first, if they're worn out w a torn boot it can cause this, but it's most likly one of the sensors has gone bad. if they send a false reading to the 4wd brain it will try to make the front spin faster than it should, causing binding... or atleast i was told

 

Unplug the AWD module under the driver's seat and see if the symptoms go away. If they do, then check the two speed sensors on the Transfer Case.

 

Ditto copper's advice.

The sensors are falsely reporting wheel slippage which causes the electronic 4WD to lock up. Since the wheels are not slipping, torque builds up in the front driveline and what you are hearing/feeling is the sudden release of this excessive torque via the tire's contact with the pavement. Torque is evenly distributed in straight ahead situations but when turning, the outside wheel revolves faster than the inside one. The purpose of the differential is to allow this happen. When the front differential is locked by the controller, there is no way to distribute that excess torque unless one wheel breaks loose. Unhooking the controller under the driver's seat removes the locking differential from the problem.

 

Does pulling the 4wd fuse accomplish the same thing as disconnecting the module?

 

Aha. I pulled the plug on the 3 by 4 inch box that clamps under the driver seat, and the binding problem quit.

Question: Do I still have the 70-30 split from the transfer case.
Will it hurt to drive disconnected. (I'm not really up on this stuff).

Also, seems like I read some time ago that accessing the box required removing the seat and lifting the carpet. There does seem to be a solid square box type thing under the carpet?? Sure glad I didnt have to do that!!

So I will look up sensor info next.
Sure do appreciate the smarts available on this site. Thanks again.
George

 

George,

You now have an open differential between the front and rear axles. You should be able to run like that for as long as you don't drive on very slippery surfaces. The 4wd computer locks up the center diff when it detects a certain amount of speed difference between the front and rear axles. This is how it determines slippage. However, it can be fooled by failing sensors, or if the sizes of the front and rear tires are too different, like from uneven wearing, and the speed differences becomes the greatest when you're making tight turns. Then the computer can then try to engage the center diff lock at the wrong time.

 

Back again.
Spent half the afternoon searching the web, no luck. What is the correct name and part number for the transfer case sensors, does anyone know. Also where to get them.

Incidentally, I saw on ebay someone selling a new in ford box aero transfer case for 350 bucks. Wonder if they come complete with the sensors??.

Also wonder if that is a for real deal??

 

I guess they are all generically called "speed sensors". The two used for sensing the front and rear axle speeds should be the same, both with 2 wires. One is at the top of the case, but I forgot where the other one is. I don't know how to check for their functionality other than by measuring the resistance across the wires. Sometimes, they may meaure with proper resistance, but it's not an assurance that they are functional.

There is a another big one used for the speedometer, and it has 3 wires, used for sensing forward and reverse directions. That one costs around $80, so don't damage it.

 

Dana TC28 transfer case
#21 are the front and rear axle speed sensors



they may carry speed sensors
Dana 28 Transfer Case Aerostar Van's between 1990 and 1997 Parts Illustration!


this company used to carry the transfer case speed sensors
Transfer Case Express, Inc. · New, rebuilt transfer cases ( transfercase ). Lowest prices. In California, world-wide delivery

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Front and Rear Axle Speed Sensors
The Front and Rear Axle Speed Sensors receive power from the Electronic 4 Wheel Drive Module, and send back a square wave signal proportional to the wheel speed. The Front Sensor is located at the lower gear; the Rear Sensor is at the link-ring. These sensors detect gear teeth motion.

Continuous Transfer Case Clutch Engagement

Excessive NVH and wheel hop that occurs while driving straight or during a tight to moderate turn may be caused by the electronic clutch in the transfer case. The control module (located under the driver's seat) disengages the clutch every 1-3 seconds. Normally, the clutch releasing in a turn is what the driver is feeling. If the clutch is not disengaging, then a binding condition occurs that causes tire chirping and an excessive NVH condition. Refer to the following steps to correct this condition.


Initial Checks

1. Rear axle must be 3.73 ratio.

2. Check to see if any aftermarket electrical devices that have been added to the vehicle have caused wiring harness damage. If they have caused harness damage, then disconnect aftermarket component and repair wiring harness damage.

3. Check the following electrical plug connections:

a. Control module 14-pin plug under driver's seat.

b. 8-pin (8-way) plug at the transfer case.

c. Two (2) sensor plug locations (front and rear pickups) on the transfer case.

d. Clutch wiring plug on the transfer case.


Determining and Repairing the Root Cause

1. If wheel hop condition can be made to re-occur, then unplug the control module under the driver's seat. If the wheel hop condition STOPS, the problem is an electrical concern in the wiring, plug connectors or sensors.

2. Re-check wires and plug connections; drive vehicle to test if the problem has been corrected.

3. Replace sensors. Replace front sensor, drive the vehicle to check if the problem has been corrected, if not, then replace the rear sensor. If replacing both sensors does not correct the problem, repeat all of the steps listed above in the initial checks.

4. If wheel hop condition can be made to re-occur, then unplug the control module under the driver's seat. If the wheel hop condition continues, the problem is a grounded/shorted wire from the dash to the control module, a mechanical problem in the front suspension, or an internal transfer case problem. Inspect these areas further.

5. If wheel hop condition can NOT be made to re-occur, then check the wiring and plugs and replace both sensors on the transfer case and return the vehicle to the customer. If the customer returns the vehicle for service with the same complaint, the problem could be an intermittent wire short or plug connection.

6. If the wheel hop occurs after two or more hours of driving, the primary failure possibility is the sensors on the transfer case. Repeat Step 5 above.

If a problem is detected by the control module, a code will be selected and sent to the electronic four-wheel drive indicator lamp located on the dash. The vehicle operator and/or service technician needs to count the flashes that occur between the pauses and refer to the «Diagnostic Code Chart.»




there is an extensive section in the Ford Svc cd on troubleshooting the speed sensor circuits

 

much obliged, 96

George

 

George,

The speed sensors on the T/C has three wires. Measure the three resistances between the pins 1-2, 1-3 and 2-3. They should be about 1000, 2000 and 3000 ohms, but not necessarily in that order since my memory is really groggy.

I bought one sensor from the link that Cliff provided. It's used, but it works fine. $15 if memory serves.

Also, when you unplug that box under the driver's seat, nothing is harmed. You still have 70/30 rear/front. However, if you ever get stuck in snow/mud, the T/C will no longer lock up to get you out.