The Chilton and Haynes manuals do not cover the AWD system on my model year so they have not been any help. I have looked into this problem on a number of Ford forums but while this is a common problem, it is beyond what most users can work on themselves. Most people end up taking the vehicle to the Ford dealer for diagnosis and repair of the AWD system. I am new to the Arduino but see that there are a lot of people on this forum that have a good understanding of electronics and have used Hall Effect sensors and understand how they work. That is why I am asking what type of behavior I should expect.
I did buy a Factory OEM manual on DVD which includes schematics, wiring diagrams and some troubleshooting steps for the AWD system. I also have a OBD-II scanner that allows me to read the enhanced Ford diagnostics. I occasionally come up with a problem code for the Hall Effect sensor circuit. The scanner also has the ability to read the PIDs for the front and rear driveshaft speeds but they are reading zero. The problem is that the vehicle is a 1996 model which is the first year the OBD-II protocol was adopted and the scanner mfg does not know if it is possible to read the driveshaft speeds even though the PIDs are listed.
Our library system has a online subscription to the Auto Repair Reference Center (ARRC) which has some good info about the AWD including factory Technical Services Bulletins.
I just bought a JYEtech oscilloscope and a set of insulation piercing probes and they have been more helpjull that anything. The GEM is in the dash behind the radio so I am able to access the signals coming from the Hall Effect sensors and the duty cycle for the electromagnetic clutch. Right now, I cannot see any signal from the sensors and am observing a clutch duty cycle of about 98%. The Vcc is supposed to be about 12vdc but is reading less than 5vdc. There is a wire connector on the back of the transfer case which connects the two Hall sensors, the 4wd motor, and the electronic clutch. When I disconnect this connector, the Vcc goes up to 12vdc so it appears that there is a short in one or both sensors.
One thought I had for testing the sensors would be to disconnect the Vcc wire one sensor at a time and see if the other sensor starts working correctly. I agree with you that it is probably time to replace both sensors and see if that corrects the problem. There is a possibility that the GEM is bad and might need to be replaced.
I have been thinking that it might be interesting to tap into the Hall sensors and clutch duty cycle wires and connect them to a Ardunio so that the driver can see what they are doing on a realtime basis. Many users on the Ford forums have been rewiring the clutch so that they get full-time 4wd which is what I am getting now.
One of my goals is to put together a better troubleshooting process for the AWD and post it on one of the Ford forums so that others can more easily locate their problem. Any help regarding how the Hall Effect sensors work and how to identify whether they are working properly is appreciated.
Thanks 