TRANSFER CASE STUDIES: DIAGNOSING GM’s 4WD SYSTEM - …

1 32 June 2004 TRANSFER CASESTUDIES: DIAGNOSING GM s4WD SYSTEMBYRANDYBERNKLAUA ccess to DTCs can simplify any systemdiagnosis. But as two separate caseson GM s 4WD SYSTEM reveal, thesecodes are not always 100% has used many different four-wheel-drive control systems in its vehicles overthe years, ranging from pure mechanicalto electronically controlled. Each has its own di-agnostic procedures, and the best source for un-derstanding how these systems work will alwaysbe your information 2004 Photos & screen capture: Randy BernklauSome later model 4WD systemshave scan tool capabilities along withdiagnostic trouble codes (DTCs) tohelp locate and diagnose many of theproblems associated with looked at two GM K 1500trucks that have the NVG 246 two-speed TRANSFER case.

2 One is a 1999model, the other a 2000. Both use thesame controls and happen to havestored the same DTC. The basic sys-tem consists of a TRANSFER case motorand encoder, front differential actua-tor, TRANSFER case module, speed sen-sors, switches and park input and out-put to the PCM. It seems logical touse case studies to show how thesesystems can be TRANSFER case module data canbe accessed with a scan tool such asthe Tech 2, which is GM s officialscan tool. To get to the properscreens, you must choose the Power-train selection, then navigate yourway to the ATC selections. ATC is theGM acronym for active TRANSFER with any module/computer-baseddiagnostic strategy, it s always a goodidea to make checking DTCs one ofthe first steps in your two trucks have 19 possibleDTCs, including those for input, mo-tor, encoder and even VIN F1 on the menu got uswhere we needed to be.

3 Both truckshad one DTC stored: B2725 (activetransfer case mode switch malfunc-tion). A Tech 2 information screenwill also let you know whether certain4WD systems do not allow scan dash-mounted TRANSFER case34 June 2004 TRANSFERCASESTUDIES: DIAGNOSINGGM S4WD SYSTEMLeft:ATC information can be found under the Powertrain menu on the Tech 2 scan tool. Other scan tools may allowaccess to these features as well. Right:A scan tool is also required to access DTCs. History and current DTCs aredisplayed on different menus (History menu shown here). DTCs will stay in memory for 100 key dash buttons to the right of the air vent identify this SYSTEM as an NVG 246automatic two-speed, which means it has DTCs and scan tool control is a set of normallyopen switches that vary the voltagesupplied by the TRANSFER case modulewhen closed.

4 The TRANSFER case mod-ule sends 8 volts to the switches, thenmonitors the return circuit to deter-mine whether any of the switches isdepressed. It can determine this be-cause each switch has a different re-sistor in series with the circuit. Sinceboth trucks have mode switch troublecodes, it seems logical to start the 2000 GM truck, the com-plaint was no four-wheel drive, andthe 4WD light in the IPC (instru-ment panel cluster) came on at times.(This warning light, located in theleft-side LED indicator, can displayvarious messages, from Security to Low Fuel and even 4WD/AWD. )Any ATC SYSTEM malfunction willturn on this light to alert the driver ofa problem.

5 A trouble code is alsostored at that with the complaint andDTC B2725, I was ready to continuemy diagnosis. I simply pressed thebuttons while watching the scan I pressed various buttons, the scandata would not indicate the properinput. Pressing 4HI or 4LO would not always result in the datastream reflecting that condition. Also,the LEDs on the switch would ran-domly go on and easiest way to confirm thisproblem is to tap on the button andwatch the scan data along with theswitch LEDs. When we did this, itcaused all kinds of random lights anddata readings. At one point, it eventurned on the 4WD warning indica-tor.

6 It was obvious the switch had amind of its own and needed s always nice to have such aneasy diagnosis, but should we assumethat all B2725 DTCs indicate theneed for a switch replacement? Afterall, General Motors TSB No. 02-04-21-004 indicates a known problemwith this switch we look at the other truck,it might be useful to see what GM sESI website ( )information says about B2725, to geta better understanding of the SYSTEM sdesign characteristics. The followingdescription comes from document IDNo. 375434: The TRANSFER case shift controlmodule constantly monitors this sig-nal voltage to determine the condi-tion of the mode switch circuit.

7 If nobuttons are pressed, and the transfercase shift control module detects avoltage level outside the possiblerange (approx. volts) forlonger than 5 minutes, the transfercase shift control module will set thisDTC if a button is held down orsticks for a period longer than 5 min-utes. When each of the switches isdepressed they will complete a circuitthrough their own specific TRANSFER case shift control mod-ule continuously monitors the switchinput to determine whether the 4HI,AUTO 4WD, 2HI, and 4LO buttonselections are made by the driver. Now we move on to the 1999 mod-el K 1500 truck. The customer com-plained of no 4WD operation.

8 He didnot mention any warning lights, but itseemed like a good idea to checkDTCs anyway. As mentioned earlier,this one had the same DTC B2725stored. A quick check of the switchesdid not indicate any problem with theinputs from the switch to the modulewhile monitoring scan data. However,36 June 2004 TRANSFERCASESTUDIES: DIAGNOSINGGM S4WD SYSTEMA ccess to datastream information for GM s NVG 246 SYSTEM makes mostproblems simple to locate and diagnose, as our case studies systems also allow bidirectional control for many encoder motor (arrow) is mounted on the front of the TRANSFER case and isused to lock it into 4WD. The scan tool data will show motor position via the en-coder circuit PIDs.

9 There s no need to scrape the mud off the front wheels would not engagewhen the 4WD button was problem was obviously occurring,but the DTC did not seem to matchthe SYSTEM could hear the encoder motor atthe TRANSFER case working, which con-firmed my scan data and helped mefind a diagnostic direction. Since itseemed like the problem was in thefront axle area, I decided to check thefront axle motor actuator next. Using avoltmeter, I determined 0 voltage waspresent on terminal C, (circuit 241,brown wire). I chose this wire based onthe wiring diagram, which showed fuse24 in the IP fuse box feeding voltage tothe front axle motor .

10 Going this farwithout first checking the fuse couldturn out to be pretty embarrassing if theproblem was simply a blown fuse. How-ever, I knew the fuse also powered thetransfer case control module. Since Iwas getting scan data and encoder mo-tor operation, the fuse had to be only culprit left was the wiringharness between the fuse and frontaxle motor . It s always best to startchecking for wiring problems by in-specting the harness in areas whereit s most likely to be damaged by mov-ing engine components, rotating partsor road objects striking the undersideof the vehicle. I located the problemwithin minutes. The wiring harnessnear the right front axle where it en-ters the differential housing was dam-aged.