Datalinks General Information. J1939 Datalink. J1587 Datalink. Symptoms of a Malfunctioning J1587 Datalink. Terminology

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1 Datalinks 54.0 General Information J587 Datalink The J587 datalink is a low-speed vehicle datalink that communicates information between the electronic control units (ECU) on the vehicle. The J587 datalink is also referred to as J708. J708 refers to the SAE standard for the physical part of the datalink, such as the wiring and electronic components. J587 refers to the SAE standard for the messaging protocol that communicates on the J708 network. In the context of vehicle repair, the terms J708 and J587 are used interchangeably. The J587 datalink uses a twisted pair of wires to reduce interference from the digital messages being sent on the wires. Wire colors for the J587 datalink are: Orange J587 Dark green J587+ See Fig. for a simplified schematic of the J587 datalink, and to gain a basic understanding of how the J587 datalink is laid out. G is the drawing number of the J587 wiring schematic. The wiring schematics for the J587 datalink are found in module 60. The dash J587 junction block is located behind the center dash panel. See Fig. 2. See Table for diagnostic connector pinouts. Symptoms of a Malfunctioning J587 Datalink Symptoms of a malfunctioning J587 datalink may include: gauges not working ICU displays no J587, no EnG, orno AbS warning lamp(s) such as ABS and CHECK ENGINE on cannot retrieve fault codes from an ECU ServiceLink does not connect to vehicle one or more ECUs do not show up on the ServiceLink ECU list J939 Datalink The J939 datalink is a high-speed vehicle datalink that communicates information between electronic control units (ECU) on the vehicle. The J939 communicates at 250,000 bits per second. Unlike the J587 datalink, the J939 datalink allows an ECU to broadcast requests as well as information. Examples of information that can be communicated on the J939 datalink are: engine rotational speed road speed transmission tailshaft speed engine retarder deactivation request engine torque reduction request communication between the Bulkhead Module, Chassis Module, and the instrumentation control unit See Fig. 3 for a simplified schematic of the J939 datalink, and to get a basic understanding of how the J939 is wired. The backbone of the J939 datalink is the section of the datalink that is between the two terminating resistors. Each ECU is connected to the backbone. The wiring between each ECU and the backbone is referred to as a branch. See Table for diagnostic connector pinouts. Terminology Backbone The main J939 datalink wiring that lies between the two terminating resistors. It does not include the branch circuits to each ECU or to the diagnostic connector. See Fig. 3. Branch Circuit The section of J939 datalink between the backbone and each ECU that has J939, and between the backbone and the diagnostic connector. Diagnostic Connector A 9-pin diagnostic connector is used for troubleshooting the electrical system. ECU The electronic control unit connects to the J939 datalink via a branch circuit. J939 Datalink Wiring Yellow J939+ Business Class M2 Workshop Manual, Supplement 0, September /

2 54.0 Datalinks General Information Green J939 Tee Connector/Splice Junctions where branch circuits and terminating resistors connect to the backbone of the J939 datalink. Terminating Resistors The J939 datalink has two 20 terminating resistors, one at each end of the backbone. The total datalink parallel resistance is 60. Symptoms of a Malfunctioning J939 Symptoms of a malfunctioning J939 include: J587 fault code or blink code indicating a J939 problem. AutoShift does not function correctly. It reverts to AutoSelect mode with a code 35 active. CHECK ENGINE or ENGINE WARNING indicator is lit. TRACTION, WHEEL SPIN, or ATC indicator is lit. Right headlamp and hazard lamps are on. ICU3-M2 displays no Data or no J939. Harsh shifting with Allison 000/2000/2400 transmissions with Allison code U205. This code can only be viewed with Allison software. Since Allison 000/2000/2400 transmissions use J939 and not J587, this fault will not show up in ServiceLink. Allison 000/2000/2400 transmissions generally do well at estimating shift points without J939; therefore this problem often goes unnoticed. Outside of Cab Inside of Cab BHM Dash J587 Junction Block ABS (Frame) CHM TCU (Allison/ AGS) Non MBE Engine AGS Display MBE VCU ICU ABS (Cab) Diag. Conn. 04/2/2006 f NOTE: The solid lines represent required components and wiring. The dashed lines represent optional components and wiring. Fig., Simplified J587 Schematic 050/2 Business Class M2 Workshop Manual, Supplement 0, September 2006

3 Datalinks 54.0 General Information J587 and J939 Diagnostic Connector Pinouts Pin Description Diagnostic Connector A Ground B Battery +2V D C J939+ E D J939 C E J939 Shield (only if heavy cable is used) F A B F J587+ J G G J587 H H Not Used J Not Used 05/0/2006 f5036b NOTE: The connector is viewed looking toward the diagnostic connector. Table, J587 and J939 Diagnostic Connector Pinouts Terminating Resistors The J939 datalink consists of twisted yellow and green wires. The yellow wire is J939+; the green wire is J939. The J939 datalink has two terminating resistors, one at each end of the backbone. The purpose of the terminating resistors is to minimize the reflection of data on the datalink. Collision of reflected data can cause J939 messages to become partially or completely lost. Data collision can also cause the data to be erratic. Terminating resistors prevent this from occurring. Although the J939 datalink may function with a missing or failed terminating resistor, data collision can occur and cause problems. Each terminating resistor is 20, but the equivalent of two 20 resistors in parallel is 60. Since the resistors are in parallel with one another, their total resistance equals 60. If a terminating resistor is removed, the circuit resistance should be 20. However, with both resistors installed in the circuit there should be 60 measured at any two points between J939+ and J939 in the circuit, such as between pins C and D of the diagnostic connector. IMPORTANT: It is essential that two terminating resistors are installed in the J939 datalink. Numerous J939 problems have been attributed to missing terminating resistors. See Fig. 4 for an example of a wiring diagram that can be used when troubleshooting a J939 datalink problem. One terminating resistor is located in a tee along the left frame rail, usually behind the cab. See Fig. 5. A second terminating resistor is located in the dash. See Fig. 6. Each ECU is generally connected to the J939 backbone using a tee connector or splice. See Fig. 7. Business Class M2 Workshop Manual, Supplement 0, September /3

4 54.0 Datalinks General Information Making the Pinout Measurements Easier to See The pins on the diagnostic connector may be difficult to see when testing. If the pins are difficult to see, use a Y-cable as an extension to the diagnostic connector to make test measurements easier. See Fig. 8 for a drawing of the connector at the end of the Y-cable and the corresponding 9-pin diagnostic connector pins /26/2006. Junction Block Fig. 2, Dashboard J587 Junction Block f Locating J939 Wiring Diagrams The J939 wiring diagram can be found in module 60. Other modules may also contain datalink wiring information. See Table 2. Use PartsPro to obtain drawing numbers for installation drawings, harness assembly drawings, and wiring diagrams applicable to the vehicle being worked on. ABS TRANS ICU ENG 20 Ohm Terminating Resistor Branch Backbone 20 Ohm Terminating Resistor CHM BHM DIAG 04/20/2006 f NOTE: Some ECUs shown in this schematic may not be standard on all vehicles. The schematic does not show optional ECUs such as an Expansion Module or Switch Expansion Module. Fig. 3, Simplified J939 Schematic 050/4 Business Class M2 Workshop Manual, Supplement 0, September 2006

5 Datalinks 54.0 General Information 04/24/2006 Fig. 4, Example of a J939 Datalink Wiring Schematic Ref. Dia. G Chg. Ltr. G f /8/2006 f Terminating Resistor Fig. 6, Terminating Resistor Located in the Dash 04/26/2006 f Connector Tee 2. Terminating Resistor Fig. 5, Terminating Resistor Located in the Chassis Business Class M2 Workshop Manual, Supplement 0, September /5

6 54.0 Datalinks General Information Component Module Locations Component Module Number General J939 harness drawings, schematics, and installation drawings 60 Engine harness and installation drawings 283 and 286 Transmission harness and installation drawings 34A and 343 ABS harness and installation drawings 330, 332, and 333 Table 2, Component Module Locations 2 04/9/2006 f Branch to Backbone 2. Branch to ECU Fig. 7, J939 Tee Connector /9/200 f Pin A 5. Pin G 2. Pin E 6. Pin C (J939+) 3. Pin B 7. Pin D (J939 ) 4. Pin F Fig. 8, Y-Cable Pinouts 4 050/6 Business Class M2 Workshop Manual, Supplement 0, September 2006

7 Datalinks 54.0 Repair Procedures Background Information Although the J939 datalink backbone and branch circuits can be repaired, it is often preferable to replace the faulty section of datalink. The technician must determine whether the datalink should be repaired or replaced. If a repair to the backbone or a branch circuit will take more than 20 minutes to complete, Freightliner recommends that the faulty section of the datalink be replaced. The length of the branch circuits can be as short as several inches to as long as 0 feet. The technician must decide whether to repair, replace, or overlay the faulty section of the datalink. Repair Procedures CAUTION Use care when soldering connections inside the dash. Do not allow a hot soldering iron to contact other wires or components, as damage could occur. If you are unable to safely solder the connection, overlay the damaged datalink with a new datalink. IMPORTANT: The following procedure is the only approved method of repairing broken wires on a Freightliner vehicle. This procedure (solder splicing) is done using solder repair kit ESYES and is for 4- or 6-gauge wire. Do not repair wire that is 2 gauge or larger. Replace it instead.. Strip the ends of the wire to be repaired. Make sure the stripped ends are 3/8- to /2-inch (0- to 3-mm) long. 2. If repairing an exterior wire, slip a 3-inch (75-mm) long piece of heat shrink over one end of the wire. See Fig.. 3. Slip a solder sleeve from kit ESYES over one end of the wire. See Fig.. 4. Using a suitable crimp tool and a crimp splice from the kit, crimp the ends of the wire as follows: 4. Insert a stripped wire end into the crimp splice until it touches the wire stop in the middle of the crimp splice. See Fig Center the crimping tool between the wire stop and the end of the crimp splice, then crimp the wire. 4.3 Repeat the previous substeps for the other end of the wire. 5. Check the crimp, making sure the crimping tool impression is on both ends of the crimp splice. 6. Slide the solder sleeve over the crimp splice so that the solder ring is over the center of the crimp splice. See Fig. 3. Then apply 250 F (2 C) heat until the solder flows into the splice crimp and the solder sleeve has shrunk completely against the wire. 7. Slide the heat shrink over the splice, then apply 250 F (2 C) heat to it until it is completely shrunk against the wire insulation. Some of the sealant material should be bubbling out of the ends of the heat shrink. Business Class M2 Workshop Manual, Supplement 0, September /

8 54.0 Datalinks Repair Procedures 4 3 A A 2 /04/94 A. 3/8 to /2 Inch (0 to 3 mm). Wire End 2. Crimp Splice 3. Solder Sleeve 4. Heat Shrink Fig., Exterior Wire Repair f540392a A A /04/94 A. Center crimping tool here.. Wire Stop 2. Wire 3. Crimp Splice (shown crimped) Fig. 2, Example of a Wire Crimp f540395a /04/94 3 f540394a. Solder 2. Crimp Splice (shown crimped) 3. Wire 4. Solder Sleeve Fig. 3, Wire Ready for Soldering 00/2 Business Class M2 Workshop Manual, Supplement 0, September 2006

9 Datalinks 54.0 Troubleshooting the J587 Datalink Diagnosing the J587 Datalink Test No. Test Description Test Action Determine if ServiceLink connects to the vehicle J587 datalink. Attempt to connect ServiceLink and refresh the ECU list. If ServiceLink connects to the J587 datalink, the ECU list displays J Detected and some or all ECUs display in white under the J708 ECU list. Does ServiceLink connect to the J587 datalink? 2 Repeat test no. on another vehicle with a known good J587 datalink. Does ServiceLink connect to the vehicle? 3 Make sure the park brake is set. Turn the ignition on. Does the ICU display no J587? 4 Is the power light illuminated on the vehicle interface adaptor? Yes Go to test no. 9. Go to test no. 2. No Yes Go to test no. 3. Check cables between PC and vehicle; check vehicle interface adaptor; check PC No settings. Repair as necessary. Yes Go to test no. 5. No Yes Go to test no. 4. Check the J587 datalink wiring between the diagnostic connector and the dash junction block for shorts. 5 Disconnect BHM connector B. Attempt to connect ServiceLink and refresh the ECU list. If ServiceLink connects to the J587 datalink, the ECU displays J Detected and the ICU, BHM, and other ECUs (depending on vehicle configuration) display in white under the J708 ECU list. No Yes No Repair as necessary. Check power and ground supply to diagnostic connector. If OK, check PC cable and interface device. Repair as necessary. There is a problem in the J587 wiring outside of the cab between the BHM connector B and the ABS and CHM. Locate source of trouble and repair as necessary. Go to test no. 6. Does ServiceLink connect to the J587 datalink? 6 Check if the vehicle is equipped with any of the following: non-mbe engine Allison transmission AGS transmission Does the vehicle have any of the above options? Yes Go to test no. 7. Go to test no. 8. No Business Class M2 Workshop Manual, Supplement 0, September /

10 54.0 Datalinks Troubleshooting the J587 Datalink Diagnosing the J587 Datalink Test No. Test Description Test Action 7 Disconnect the BHM connector C2. Attempt to connect ServiceLink and refresh the ECU list. If ServiceLink connects to the J587 datalink, the ECU list displays J Detected and the ICU, BHM, CHM, and other ECUs (depending on vehicle configuration) display in white under the J708 ECU list. Yes No There is a problem between the BHM and non-mbe engine or between the BHM and the transmission control unit (Allison and AGS only). Repair as necessary. Go to test no. 8. Does ServiceLink connect to the J587 datalink? 8 Locate the J587 junction block behind the center dash panel. Follow these steps:. Disconnect one of the J587 connectors from the junction block. 2. Attempt to connect ServiceLink and refresh the ECU list. Does either the ICU or the BHM now display in white in the ECU list? 2. If yes, stop. Enter "yes" in the Test column to the right and follow the Action given. 2.2 If no, continue to step Plug the connector into the junction block. 4. Disconnect the next connector on the J587 junction block that has not yet been disconnected. 5. Refresh the ServiceLink ECU list. Does either the ICU or the BHM now display in white in the ECU list? 5. If yes, stop. Enter "yes" in the Test column to the right and follow the Action given. 5.2 If no, continue to step Are there any remaining connectors on the junction block that have not yet been disconnected? 6. If yes, go to step If no, enter "no" in the Test column and follow the Action given. Yes No The J587 circuit that was just disconnected from the junction block is shorted or the ECU on the circuit is faulty. Repair as necessary. There is a short between the J587 junction block and the diagnostic connector. 300/2 Business Class M2 Workshop Manual, Supplement 0, September 2006

11 Datalinks 54.0 Troubleshooting the J587 Datalink Diagnosing the J587 Datalink Test No. Test Description Test Action 9 With ServiceLink connected, look at the ECU list and determine which ECU(s) do not display in white. Case : None (All ECUs display.) Case 2: The BHM, CHM, and all other outside-of-cab ECUs Case 3: The ICU or one of the other inside-of-cab ECUs Case 4: One of the outside-of-cab ECUs Case 5: All ECUs except the CHM Case Case 2 Case 3 Case 4 Case 5 Table, Diagnosing the J587 Datalink There is no problem found. Check the J587 wiring between the BHM and the J587 junction block for an open. Repair as necessary. Check the J587 wiring between the ECU that does not display and the J587 junction block for an open. Check the power and ground circuits to the missing ECU and the ECU itself. Repair as necessary. Check the J587 wiring between the ECU that does not display and the BHM for an open. Check the power and ground circuits to the missing ECU and the ECU itself. Check power and ground supply to BHM. Check BHM itself. Business Class M2 Workshop Manual, Supplement 0, September /3

12 Datalinks 54.0 Troubleshooting the J939 Datalink Use the following test procedures in the order given to successfully locate J939 datalink problems. Do not skip test procedures or steps unless directed to do so. Test : J939 Resistance Test Test 2: ECU Communication Test Using Datalink Monitor Template Test 3: Testing J939 for Circuit Faults (shorts to power and ground) Test 4: Pinpointing Short Circuits on the J939 Datalink Test : J939 Resistance Test The J939 resistance test checks whether or not both terminating resistors are installed and ensures that there is at least a complete circuit from the diagnostic connector through the backbone loop. It does not ensure that branch circuits to each ECU are okay. IMPORTANT: The batteries must be disconnected prior to any J939 resistance tests being performed. Failure to do so may result in inconclusive resistance measurements.. Turn the ignition off. 2. Disconnect the negative leads from the batteries. 3. Connect the leads of a digital multimeter (DMM) to pins C and D of the 9-pin diagnostic connector. See Table. Set the DMM to ohms. 4. Measure the resistance. 5. After the test is complete, connect the batteries. 6. See Table 2 for test results and possible causes. J939 Diagnostic Connector Pinouts Pin Description Diagnostic Connector A Ground B Battery +2V D C J939+ E D J939 C E J939 Shield (only if heavy cable is used) F A B F J587+ J G G J587 H H Not Used J Not Used 05/0/2006 f5036b NOTE: The connector is viewed looking toward the diagnostic connector. Table, J939 Diagnostic Connector Pinouts 60 ±6 20 ±2 40 ±4 0 to 5 Test : J939 Resistance Test The J939 datalink backbone is intact and both terminating resistors are installed. Perform the procedures in Test 2. Either of the following: One of the terminating resistors is missing. One of the terminating resistors is open. Three terminating resistors have been installed; one must be removed. There must be one terminating resistor at each end of the backbone for a total of two. J939+ and J939 have shorted together somewhere in the datalink. Business Class M2 Workshop Manual, Supplement 0, September /

13 54.0 Datalinks Troubleshooting the J939 Datalink Greater than 000 Any other readings Test : J939 Resistance Test Either of the following: An open in the circuit between the diagnostic connector and the J939 backbone. Both terminating resistors are missing or open. Any of the following: Incorrect terminating resistor resistance. Poor or corroded connections. Short circuit to ground or an open circuit somewhere on the datalink. Go to Test 2 to pinpoint the problem. Table 2, Test : J939 Resistance Test Test 2: ECU Communication Test Using Datalink Monitor Template The following test procedures check for communication with each ECU connected to the J939 datalink. If one ECU fails to communicate, pinpoint whether the problem is with the wiring or an ECU. If all ECUs communicate as they should, the J939 datalink is probably not the problem. Test 2.: Check Whether Each ECU Connected to the J939 Datalink Responds. Connect ServiceLink to the vehicle. 2. Open the M2 J939 Test Datalink Monitor template. 3. Following the instructions in the template, check whether each ECU that is supposed to be connected to the datalink responds. 4. See Table 3 for test results and possible causes. Test 2.2: Check the J939 Datalink Wiring to the ECU That Does Not Respond IMPORTANT: The batteries must be disconnected prior to any J939 resistance tests being performed. Failure to do so will result in incorrect resistance measurements.. Disconnect the negative leads from the batteries. 2. Locate the connector at the ECU that did not respond and disconnect it. 3. Locate the pins for J939+ and J939. Refer to the appropriate wiring diagrams if necessary. 4. Make sure that J939+ and J939 polarity is correct at the component before proceeding. If reversed, correct the wiring and verify that this corrected the problem. 5. Using a digital multimeter set to read ohms, measure the resistance across the two J939 datalink pins at the connector to the suspect ECU (harness side). 6. After the test is completed, connect the batteries. 7. See Table 4 for test results and possible causes. Test 2.3: Install a Test ECU to Confirm the Problem. Install a test ECU. 2. Connect ServiceLink to the vehicle. 3. Open the M2 J939 Test Datalink Monitor template. 4. Following the instructions in the template, check whether each ECU that is supposed to be connected to the datalink responds. 5. See Table 5 for test results and possible causes. 30/2 Business Class M2 Workshop Manual, Supplement 0, September 2006

14 Datalinks 54.0 Troubleshooting the J939 Datalink Test 3: Testing J939 for Circuit Faults (shorts to power and ground) The following test procedure checks for shorts to power and shorts to ground on the J939 datalink. NOTE: All test procedures are performed using a digital multimeter (DMM) set to read voltage. Before proceeding, verify that battery voltage (approximately +2 VDC) is available at pin B of the diagnostic connector. With the ignition on, use a DMM to test for voltage at pin B by placing the positive (+) lead on pin B and the negative ( ) lead on a good chassis ground. Test 3.: Test J939+ for Shorts to Power and Ground. Turn the ignition on. 2. Touch the positive (+) lead to pin B (+2 VDC) and the negative ( ) lead to pin C (J939+) of the diagnostic connector. 3. See Table 6 for test results and possible causes. Test 3.2: Test J939 for Shorts to Power and Ground. Turn the ignition on. 2. Touch the positive (+) lead to pin B (+2 VDC) and the negative ( ) lead to pin D (J939 ) of the diagnostic connector. 3. See Table 7 for test results and possible causes. Test 4: Pinpointing Short Circuits on the J939 Datalink The following test procedure pinpoints shorts to power and shorts to ground on the J939 datalink. The tests will indicate which side of the cab/engine interface the problem exists. It will indicate whether the problem is in the wiring outside or inside the cab. NOTE: All tests are performed using a DMM set to read voltage. Before proceeding, verify that battery voltage (approximately +2 VDC) is available at pin B of the diagnostic connector. With the ignition on, use a DMM to test for voltage at pin B by placing the positive (+) lead on pin B and the negative ( ) lead on a good chassis ground. Test 4.: Pinpoint J939+ for Short to Power or Ground Only perform this test procedure if directed here in Test 3... Disconnect the 76-pin cab-to-engine bulkhead connector that contains J939 wiring. 2. Turn the ignition on. 3. Touch the positive (+) lead to pin B (+2 VDC) and the negative ( ) lead to pin C (J939+) of the diagnostic connector. 4. See Table 8 for test results and possible causes. Test 4.2: Pinpoint J939 for Short to Power or Ground Only perform this test procedure if directed here in Test Disconnect the 76-pin cab-to-engine bulkhead connector that contains J939 wiring. 2. Turn the ignition on. 3. Touch the positive (+) lead to pin B (+2 VDC) and the negative ( ) lead to pin D (J939 ) of the diagnostic connector. 4. See Table 9 for test results and possible causes. Business Class M2 Workshop Manual, Supplement 0, September /3

15 54.0 Datalinks Troubleshooting the J939 Datalink Test 2.: Check Whether Each ECU Connected to the J939 Datalink Responds All ECUs respond. The J939 datalink is probably not the problem. One ECU fails to respond. Perform the procedures in Test 2.2. Any of the following: No ECUs respond. The polarity of J939+ and J939 pinouts at the diagnostic connector may be reversed. There may be a problem with the PC-to-vehicle communications adaptor or cables. The entire datalink may be down due to a short to power or to ground. Go to Test 3 to pinpoint the problem. Table 3, Test 2.: Check Whether Each ECU Connected to the J939 Datalink Responds 60 ±6 Test 2.2: Check the J939 Datalink Wiring to the ECU That Does Not Respond The J939 datalink itself is probably not the problem. Make sure that there is power and ground to the suspect ECU. Go to Test 2.3 after ECU power and ground connections have been confirmed. There is a problem with the J939 wiring between the ECU connector and its connection to the Not 60 ±6 J939 backbone. Repair as necessary. Table 4, Test 2.2: Check the J939 Datalink Wiring to the ECU That Does Not Respond All ECUs respond. The ECU still does not respond. Test 2.3: Install a Test ECU to Confirm the Problem The ECU was faulty and the test ECU confirmed this. Replace the ECU. The problem has not been confirmed. Carefully repeat all the diagnostics. If the ECU still does not respond, contact Freightliner for assistance. Table 5, Test 2.3: Install a Test ECU to Confirm the Problem Test 3.: Test J939+ for Shorts to Power and Ground 0 VDC J939+ is shorted to power. Go to Test VDC (battery voltage) J939+ is shorted to ground. Go to Test 4.. Any other reading J939+ is neither shorted to power or ground. Go to Test 3.2. Table 6, Test 3.: Test J939+ for Shorts to Power and Ground 30/4 Business Class M2 Workshop Manual, Supplement 0, September 2006

16 Datalinks 54.0 Troubleshooting the J939 Datalink Test 3.2: Test J939 for Shorts to Power and Ground 0 VDC J939 is shorted to power. Go to Test VDC (battery voltage) J939 is shorted to ground. Go to Test 4.2. Any other reading J939 is neither shorted to power or ground. There may be a problem with the PC-to-vehicle communications adaptor or cables. The datalink itself appears to be okay. Table 7, Test 3.2: Test J939 for Shorts to Power and Ground Test 4.: Pinpoint J939+ for Short to Power or Ground 0 VDC J939+ is shorted to power inside the cab. Locate and repair the short. 2 VDC (battery voltage) J939+ is shorted to ground inside the cab. Locate and repair the short. Any other reading J939+ is shorted to power or ground outside the cab. Locate and repair the short. Table 8, Test 4.: Pinpoint J939+ for Short to Power or Ground Test 4.2: Pinpoint J939 for Short to Power or Ground 0 VDC J939 is shorted to power inside the cab. Locate and repair the short. 2 VDC (battery voltage) J939 is shorted to ground inside the cab. Locate and repair the short. Any other reading J939 is shorted to power or ground outside the cab. Locate and repair the short. Table 9, Test 4.2: Pinpoint J939 for Short to Power or Ground Business Class M2 Workshop Manual, Supplement 0, September /5