32 June 2004

TRANSFER CASESTUDIES:

DIAGNOSING GM’s

4WD SYSTEMBY RANDY BERNKLAU

Access to DTCs can simplify any systemdiagnosis. But as two separate cases

on GM’s 4WD system reveal, thesecodes are not always 100% accurate.

GM has used many different four-wheel-drive control systems in its vehicles overthe years, ranging from pure mechanical

to electronically controlled. Each has its own di-agnostic procedures, and the best source for un-derstanding how these systems work will alwaysbe your information system.

33June 2004

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Some later model 4WD systemshave scan tool capabilities along withdiagnostic trouble codes (DTCs) tohelp locate and diagnose many of theproblems associated with them.

We looked at two GM K 1500trucks that have the NVG 246 two-speed transfer case. 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 repaired.

The 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 case.As with any module/computer-baseddiagnostic strategy, it’s always a goodidea to make checking DTCs one ofthe first steps in your diagnosis.These two trucks have 19 possibleDTCs, including those for input, mo-

tor, encoder and even VIN codes.Selecting “F1” on the menu got us

where we needed to be. 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 toolaccess.

The dash-mounted transfer case

34 June 2004

TRANSFER CASE STUDIES: DIAGNOSING GM’S 4WD SYSTEM

Left: 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 cycles.

The 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 capabilities.

mode control is a set of normallyopen switches that vary the voltagesupplied by the transfer case modulewhen closed. 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 there.

On 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. A trouble code is alsostored at that time.

Armed with the complaint andDTC B2725, I was ready to continuemy diagnosis. I simply pressed thebuttons while watching the scan data.As 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 off.

The 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. It was obvious the switch had amind of its own and needed replace-ment.

It’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 assembly.

Before we look at the other truck,it might be useful to see what GM’sESI website (www.acdelcotds.com)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. If nobuttons are pressed, and the transfercase shift control module detects avoltage level outside the possiblerange (approx. 0.5-1.0 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 resistor.The 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. 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

TRANSFER CASE STUDIES: DIAGNOSING GM’S 4WD SYSTEM

Access to datastream information for GM’s NVG 246 system makes mostproblems simple to locate and diagnose, as our case studies demonstrate.These systems also allow bidirectional control for many outputs.

The 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. There’s no need to scrape the mud off the undercarriage.

the front wheels would not engagewhen the 4WD button was selected.A problem was obviously occurring,but the DTC did not seem to matchthe system malfunction.

I 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 the

front 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. Going this farwithout first checking the fuse couldturn out to be pretty embarrassing if the

problem 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 good.

The 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. At least two wires were broken,including the brown wire, which canbe seen in the photo lower left. A har-ness repair was in order.

I had found the problem, but stillcould not explain why the system hadstored code B2527. Was there anoth-er, intermittent problem just waitingfor me to turn my back? I decided tolook closer at the description forB2527 and found something interest-ing. “...the transfer case shift controlmodule will set this DTC if a buttonis held down or sticks for a periodlonger than 5 minutes.”

It seemed very possible that a drivermight hold down a 4WD button forfive minutes if he got stuck and the4WD system wasn’t working. To testmy theory, I held the button down forseveral minutes to create the 4WDwarning light visible in the photo onpage 33. The system also set a codeB2527. Maybe, just maybe, the DTCwas actually set by the driver due tothe broken wire to the front axle. Imay never know for sure.

It’s easy to fall into the habit of re-placing common problematic compo-nents without verifying that they ac-tually are defective. As this secondcase study of GM’s relatively simpletruck 4WD control system proves,this approach can get you into trou-ble. It always pays to test and verifybefore replacing a component.

38 June 2004

TRANSFER CASE STUDIES: DIAGNOSING GM’S 4WD SYSTEM

GM’s 4WD system also uses a motor (arrow) on the front of the housing toengage/disengage the front drive axles. The harness connector is out in theopen, so circuit testing can be done at this location with relative ease.

The wiring on this vehicle was damaged but did not actually set a DTC. Thecode that was stored may or may not have been set by driver operation.

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