I have a Ford Explorer that I have been having problems with the All Wheel Drive (AWD).
In AWD, a Generic Electronic Module (GEM) monitors the speeds of the front drive shaft and rear drive shaft and when it senses the rear wheels slipping, it increases the duty cycle to a electromagnetic clutch in the transfer case which increases the power to the front wheels.
The drive shaft speeds are measured using Hall effect sensors. I suspect one or both of the sensors may be bad and I would like to know if there is a way to test them. The sensors are 3 lead devices, Vcc, ground and output.
Using a ohm-meter, I checked the resistance across Vcc and ground. On the front sensor, I measured about 1.0 ohm. The rear sensor behaved differently, if I connected the red lead to Vcc and the black lead to ground, I measured 11.66 M ohm. If I reversed the leads, I measured a open circuit.
Does anybody have any ideals as to which sensor is bad or do I need to test them using a different method. The sensors cost about 50 USD each so it would be nice to identify the bad sensor rather than replacing both.
I take it you have read both the Chilton and Haynes manual for your model year, right? You have also pulled any codes from the computer? If you haven't, do so now; this forum probably isn't the best forum to answer your question.
I am personally suspecting the rear sensor. If I were you, I wouldn't bother with further testing, and just replace both (assuming code or other tests don't indicate something else); if it is the sensor, chances are that if one is going bad, the other isn't too far behind.
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.
Any help regarding how the Hall Effect sensors work
The voltage you get from a sensor is quite small and so would be difficulty to measure. However, I suspect that this small signal is not communicated directly in an engine management system, rather the sensor will be embedded with an amplifier and bus management system. It could be that the sensor does not talk unless it is asked to and that is why you are not seeing any signal.
I ordered new factory sensors through Amazon at a fraction of the price from the dealer. I checked them with a ohm meter and they both had a similiar response to the rear sensor so it looked like the front sensor was shorted out. I replaced the front sensor and it has a crack on the face of it. After replacing the sensor everything returned to normal and I was able to read the front/rear driveshaft speeds with the OBD-II reader.
I went ahead and hooked up the JYEtech oscilloscope to one of the speed sensors and also checked the duty cycle being sent to the electromagnetic clutch. The speed sensor outputs a nice 5 volt square wave signal proportional to the speed. It outputs about 840 Hz at 40mph and would be simple to measure with an Arduino. It outputs both front/rear driveshaft speeds whether it is in 4wd or 2wd.
The electromagnetic clutch signal from the GEM runs to a solid state relay and is inversely proportion to the output meaning it is high most of the time under no load and goes low more of the time when wheel slippage occurs.
Following are a few pictures of the ocsilloscope and the sensor.
Hall Effect sensor at 40 mph and photo of sensor. GEM with probe attached.
Duty cycle at idle, duty cycle in 1st gear gunning engine on packed snow. Duty cycle with bad speed sensor.
Once I get some time I might write a script for the Arduino to monitor and record the driveshaft speeds and the clutch duty cycle. I assume I would need to use a MegaArduino for the extra interrupts to measure three frequencies.
