Four-Wheel Drive. A Clue Discovered

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2 Is Your Ford Four-Wheel Drive System Crazy? KEEP THOSE TRANNYS ROLLING Is Your Ford SPEAKER Four-Wheel Drive by Pete Huscher System Crazy? While working on the ATRA HotLine, I received a call from a technician named Gary. Gary was working on a 2004 Ford Explorer that was experiencing erratic four-wheel drive system operation. According to Gary, the four-wheel drive lights were flashing, and he was getting unrequested shifts to four-wheel drive Low range at times. He d already checked the fourwheel drive control module (integrated with the PCM) for codes and each time he checked the system, different codes would appear. The first time he checked for codes, he found P1812 and P1815 (mode switch circuit codes), and P1824 (4WD clutch circuit) in the system. The second time he found encoder circuit codes P1846 (A circuit) and P1850 (B circuit). The third time, the remaining encoder circuit codes P1854 (C circuit) and P1858 (D circuit) were present. Gary said the four-wheel drive system was acting crazy, and the codes kept alternating between the mode switch circuit, 4WD clutch circuit, and the encoder circuits. Needless to say, he was stumped as to where to go from here. The Diagnosis Gary assured me that he d followed the diagnostic routine for each of the codes and they hadn t led him anywhere. He d already monitored the signals coming into the four-wheel drive control module/pcm from the mode switch circuits and the signals didn t change during the unrequested shift to 4WD Low range. He then monitored the signals coming from the encoder circuits; he found an erratic signal from each of the circuits. After checking the wiring harness from the four-wheel drive control module to the encoder/motor assembly and finding no problems, Gary was confident that he had a bad encoder/ motor assembly, so he replaced it. After replacing the encoder/motor assembly, Gary rechecked the encoder circuits and found the signals from the encoder were still erratic. That s when he decided to call the ATRA HotLine. I listened to Gary as he explained his problem, and then told him that I was working on a seminar for this year s Powertrain Expo on Ford s 4WD systems. Gary was more than happy to work with me to perform some quick checks and see if we could locate the problem. A Clue Discovered While we went over the diagnostic routine he d performed so far, Gary mentioned that this Explorer didn t have a four-wheel drive system problem until they d removed and repaired the transmission. That s when the four-wheel drive system started acting crazy, he said. To be sure, Gary contacted the customer and verified that they weren t having any problems with the fourwheel drive system prior to the transmission repair. By now we were confident that the problem was created at some point during the removal, repair and installation of the transmission. I questioned Gary about the removal and installation process. He said everything went together without any problems; he d coated both sides of the gasket that fits between the transfer case and transmission with silicone, and then installed the transfer case. He checked the 4WD clutch circuit and found high resistance to ground. He then rechecked the connections to the encoder/motor assembly and everything looked good. Gary couldn t understand why he was still getting high resistance to ground on the 4WD clutch circuit and an erratic signal on the encoder circuits. The Path to Ground According to the wiring schematic for the four-wheel drive system, the 4WD clutch and encoder/motor assembly used the transfer case as a path to ground. The first step was to check the 4WD clutch ground circuit. Gary checked the continuity between transfer case connector pin 12 and the transfer case itself. The continuity was good, which indicated that the clutch circuit was capable of grounding through the transfer case. The next step was to check the transfer case ground. Gary used a voltage drop test to verify that the transfer case had a good path to ground. To perform this test, he: Turned the key on, engine off, to energize the solenoids and power the circuit. Set his DVOM to 20 volts DC. Connected the positive lead to the positive battery post. 26 GEARS October/November 2008

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4 Is Your Ford Four-Wheel Drive System Crazy? Figure 1: updated gasket, part # 4L2Z 7086 AA Connected the negative lead to the negative battery post to get a good system base voltage reading. The DVOM read 12.5 volts, which indicated that the battery had a good charge. Next, he: Moved his negative lead to the engine block to verify the voltage drop between the engine block and the battery negative terminal. The voltage drop to the engine block ground path was less than 0.1 volts; well within specs. Then he: Moved the negative lead to the transmission case to check the voltage drop there. He found that the transmission case also had a drop of less than 0.1 volts; again well within specifications. For the next step, Gary was going to have to check the transfer case assembly for a good path to ground. To do so, he: Raised the vehicle on the rack and positioned the DVOM so he could read the display. Connected the negative lead to the transfer case. With the positive lead connected to the positive battery post and the negative lead connected to the transfer case assembly, Gary found only 0.6 volts in the circuit he had a voltage drop of more than 11.9 volts! So the transfer case assembly didn t have a good path to ground. Gary immediately installed a ground strap from the transfer case to the transmission and rechecked the voltage drop. With the ground strap in place, the DVOM now indicated a voltage drop of just 0.1 volts. The transfer case assembly now had a good ground. He reconnected the transfer case connector, lowered the vehicle, and rechecked the signals from the encoder circuits. The encoder circuit signals were no longer erratic and the 4WD clutch circuit indicated a good ground. Preliminary checks indicated the problem was fixed: It was now time to recheck the four-wheel drive system operation. Gary cleared the codes from the PCM and headed out for a test drive. The Test Drive As Gary began his test drive, everything seemed to be working properly. The transmission shifted properly and the transfer case stayed in 2WD range. Each time he requested the transfer case to engage into 4WD Low or 4WD High, the four-wheel drive control system made the shift as requested. And when Gary requested the transfer case to return to Auto 4x4 mode, the transfer case returned to 2WD mode, exactly the way it was supposed to. The fourwheel drive system seemed to be working perfectly. With the four-wheel drive system working properly, Gary returned to the shop to recheck the PCM for codes. There were no engine performance codes, transmission codes or four-wheel drive system codes in the PCM. It looked like Gary had this one whipped. He returned the vehicle to the customer, who s been driving it for a couple of days and everything seems to be working properly. Good job, Gary! 28 GEARS October/November 2008

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6 Is Your Ford Four-Wheel Drive System Crazy? Figure 2: Metal washer embedded in gasket to provide ground path. The Conclusion, with Top Secret Information A couple days later, I had the opportunity to stop by my local Ford Dealership and chat with their transmission and driveability technicians John and Charley. I shared Gary s story with them, and by the time I was done, they each had a great big cat that ate the canary grin on their faces. I knew something was up, so I had to ask. John started walking back to his work area laughing and said that he d seen this problem before. I was intrigued, to say the least, so I followed John back to his work area, where he reached into a drawer and pulled out a package of gaskets (figure 1). He handed them to me and said here s your fix. Puzzled for a second, I asked him, What s this gasket got to do with my transfer case electrical system problem? John laughed again and said look at the gasket. As I looked at the gasket, I noticed it had a metal ring built in around one of the bolt holes (figure 2). While I was examining the gasket, John explained: He d encountered the same problem when the 2004 Ford Explorer first came out. He said every time he removed a transfer case and reinstalled it, he would end up with erratic transfer case operation, the 4WD light flashing, and alternating transfer case system codes would set in the PCM. After several attempts to fix the problem by checking wiring, connections and then replacing the PCM, he finally looked at the old gasket and saw the metal ring. John went on to say that the transfer case housing was made out of magnesium while the transmission housing was aluminum. The contact of the two different metals caused corrosion between the two housings, and on the transfer case attaching bolts. Ford replaced the attaching bolts with specially-coated bolts to prevent corrosion. But this coating interfered with the transfer case ground, so they added the metal ring to the gasket to provide a ground path between the transfer case housing and the transmission case. John went on to explain that coating the gasket with silicone or using an incorrect gasket one without the metal ring would interrupt the ground and cause erratic four-wheel drive system operation. As I was leaving the dealership, I thought to myself, WOW! this is top secret information. If it wasn t for John, we would never have known about the special bolts and gasket needed to allow the transfer case system to operate properly. Just think of the wasted time and money that would be spent checking wiring, connections and possibly replacing the PCM, due to a $19 gasket (Ford P/N 4L2Z-7086-AA). So if you re working on a 2004-ornewer Ford Explorer, Ford Expedition, or Mercury Mountaineer with a fourwheel drive system that s acting crazy, be sure to check the transfer case ground. With this top secret information and the correct gasket, you, too, should be able to keep those trannies and four-wheel drive systems rolling! A Personal Invitation The ATRA 2008 Powertrain Expo is right around the corner, on the last week of October. I d like to invite each and every one of you personally to join us in Las Vegas for the most comprehensive management and technical training seminar program available anywhere in our industry. I ll be hosting the Diagnosing Today s Domestic Four-Wheel Drive Systems presentation, and I look forward to seeing you all there again this year. With you there, we can make this the best Powertrain Expo ever. 30 GEARS October/November 2008