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Fleetwatch FMS · Vehicle Modules
JX Vehicle Module Troubleshooting Checklist
The JX vehicle module is a data logger that plugs into the bus’s J1939 or J1708 diagnostic connector through a Y-harness. It reads the odometer and other data from the vehicle and passes it to a TX55 transceiver, which sends it by radio to the FR55/FR200 reader in the fuel lane and to the MR55 handheld.
JX vehicle module with Y-harness and TX55 transmitter
Find Your Symptom
| What you’re seeing | Go to |
|---|---|
| MR55 can’t read the module, or reports show it as “no longer responding to queries” | Section 1 |
| Odometer is stuck or no longer counting | Section 2 |
| MR55 shows only the vehicle number, with no odometer or hours | Section 3 |
| Vehicle is flagged with a low read rate | Section 4 |
| Replacing the JX board or the TX55’s X-Bee chip | Sections 5 and 6 |
Section 1: MR55 Can’t Read the Module
| 1 | Move the bus out of the fuel lane and into a maintenance bay or parking area. The FR55/FR200 in the service lane interferes with the MR55. |
| 2 | Turn the bus ignition on and hold the MR55 within 2 feet of the TX55 to read it. If it still won’t read, try a different MR55 (and make sure the MR55 battery is charged), or continue to step 3. |
| 3 | Check the Y-harness is plugged into the bus’s J1708 or J1939 connector. The JX gets both power and data from this connector. |
Typical Y-harness connection to the bus.
| 4 | Check the connector between the JX and the TX55. If it’s unplugged, connect it and try reading again. |
JX-to-TX55 connector: disconnected (left) and plugged in (right).
| 5 | Quick swap test. Plug a known-good JX module into the bus connector and try reading it. If a second JX can’t be read either (with a known-good MR55), the problem is the bus: check the J1708/J1939 connector for proper wire termination, blown fuses, or a disconnect between the engine ECU and the connector. Otherwise, remove the JX cover and continue. |
| 6 | Check the LEDs on the JX board. If any LED is on, the module has power: try a new TX55, or check the TX55 wiring (step 8). If no LEDs are lit, go to step 7. |
A lit LED means the JX is receiving power from the bus.
| 7 |
No LEDs? Check the power at the green connector.
a. Make sure the wiring is correct: black ground wire on the right, red +12 VDC wire just to its left.
b. Set your meter to DC volts. Put the black lead on the black wire’s screw and the red lead on the red wire’s screw.
c. You should read 12–32 VDC.
d. No voltage? Unplug the connector from the board and measure again. If you now see voltage, replace the JX board. If it’s still under 12 VDC, the problem is the bus connector: the pinout is wrong or something on the bus is disconnected.
|
Power and data wires at the green connector. Check the order and correct any deviations.
| 8 | Power is good but the TX55 still won’t read? Check the wiring from the JX to the TX55 (see the TX55 plug wiring in the reference section), or connect another TX55 and try again. |
| 9 | Multiple JX and MR55 combinations fail? Verify the bus is actually supplying power to the JX at the diagnostic connector. |
Section 2: Odometer Stuck or Not Counting
Step 1. Read the module with the MR55 and compare the screen to these examples:
|
BUS> 0009 / 000123
ODO> 12345.6 EHRS> 7890.1 IHRS> 2345.6 |
Normal The TX55 and the JX are both working. |
|
BUS> 0009 / 000123
|
JX not responding The TX55 answers, but the JX data logger doesn’t. Check the JX wiring terminations, or replace the JX board (Section 5) or the whole JX system (box, data/power pigtail, and TX55 pigtail). |
Step 2. Check the odometer source (configuration). The configuration tells the JX which network message to read the odometer from. Current recommendations:
| Network | Recommended (read-only) | If the odometer must be set to match the dash |
|---|---|---|
| J1939 | 017 | 929 (161 and 673 are legacy) |
| J1708 | 00 | 97 |
See JX Odometer Configurations for the full list, including electric bus and fleet-specific configurations.
To view or set the odometer source on the MR55:
1. From the Main Menu, press 1 (READ). The module must be read first.
2. On the Read Results screen, press CLEAR to open the options menu.
3. Press 3 (INITIAL VALUES), then 2 (ODO SOURCE).
4. Enter the configuration value and press SEND.
5. Press CLEAR to return to the options menu, or RESET to return to the Main Menu.
Step 3. Check the wire order on the JX’s green connectors. The colored wires must be in the correct order from right to left (see the wiring reference at the end). The third wire in the 3-pin connector is either gray or bare.
Step 4. Check the wiring at the bus connector. The data lines must be correct for the JX to work. Backwards data lines on the bus connector can cause stuck odometer problems.
Section 3: MR55 Shows Only the Vehicle Number
| 1 | Check the connector between the JX and TX55 boxes for loose or incorrectly pinned wires. A loose or crossed wire can cause this. |
| 2 | Connection is good? Try a new JX board or a new JX data logger (Section 5). The TX55 can usually be reused. |
Section 4: Low Read Rate
| 1 |
Check how the TX55 is mounted.
• Mount it on fiberglass, plastic, or glass only, never metal or anything else that blocks radio waves.
• There should be no metal between the TX55 and the lane reader during servicing.
• The flat (largest) side of the TX55 should face the reader, parallel with the side of the vehicle. The signal is strongest from the largest sides.
|
Correct TX55 orientation and placement. It’s typically mounted in the driver’s window, not behind or touching metal.
| 2 | Check where the bus stops. The TX55 must pass by or stop in front of the lane reader. Make sure the module doesn’t stop short of it. |
Make sure the module doesn’t stop short of the receiver.
| 3 |
Check the TX55 tuning with the MR55.
a. Read the module (Main Menu 1), then press CLEAR, then 1 (RF PARAMETERS).
b. Hold the MR55 very still, about 2 feet from the TX55, and press 1 (AUTO TUNE).
c. The MR55 shows the current value and asks Do you want to re-tune?
The result looks like AX@05R03F4. The two digits after AX@ are the tune result: 01–62 is valid, 00 or 3F (63) is not. Try again if the result has more than two zeros. The digits after R are signal strength; well above 0100 is normally good. Values may be in hex.
|
Caution: Don’t use the TUNE or SENSITIVITY options unless Fleetwatch personnel tell you to. Doing so can have adverse effects and, in some cases, void warranties. AUTO TUNE is fine to use.
Section 5: Replacing the JX Board
| 1 | Remove the four screws holding the JX cover in place. |
| 2 | Unplug the J1939 network connector (bottom left of the board) and the TX55 connector (bottom right). |
| 3 | Remove the Phillips-head screw in the upper left corner, then squeeze the plastic stand-offs (right center and bottom left) to release the board. Needle-nose pliers or a stand-off tool may help. |
| 4 | Snap a known-good JX board onto the stand-offs, replace the screw, and reconnect the J1939 and TX55 connectors. |
A replacement JX board defaults to bus number 000000. Program the correct bus number and odometer source with the MR55 after installing it.
Section 6: Diagnosing the TX55’s X-Bee Chip
If the MR55 won’t read the vehicle, open the TX55 box and look at the board. The “IL” light lights or flashes when the TX55 senses its red inductive antenna. If the IL light is on but the MR55 still can’t read, the X-Bee chip has failed.
The X-Bee chip (blue) on the TX55 board.
| 1 | Remove the X-Bee chip by gently pulling it evenly out of the board. |
| 2 | Insert a new X-Bee chip and make sure it’s seated firmly against the board. |
| 3 | Re-test with the MR55. |
Still won’t read? Replace the whole TX55 box, or replace the TX55 board:
• Remove power by disconnecting all wired connections.
• Remove all screws. One screw sits under the X-Bee chip, so remove the chip first.
• Unsolder the red antenna leads from the board.
• Install the new board, replace the screw under the X-Bee, install the X-Bee, reconnect the wires, and solder the red antenna to the board.
Reference: Connector Pinouts and Wire Colors
J1708 6-pin connector (Deutsch HD16-6-12S-B010, mates to HD10-6-12P)
| Function | Color | Pin |
|---|---|---|
| Data A (+) | Brown | A |
| Data B (−) | White | B |
| V+ (9–32 V) | Red | C |
| No standard function | Green | D |
| Ground | Black | E |
| No standard function | Blue | F |
J1939 9-pin connector (Deutsch HD16-9-1939S, mates to HD10-9-1939P)
| Function | Color | Pin |
|---|---|---|
| Battery (−) GND | Black | 1 / A |
| Battery (+) | Red | 2 / B |
| CAN_H | Green | 3 / C |
| CAN_L | Blue | 4 / D |
| CAN_SHLD | Gray or bare | 5 / E |
| J1708 (+) Data (A) | Brown | 6 / F |
| J1708 (−) Data (B) | White | 7 / G |
| Proprietary | N/C | 8 / H |
| Proprietary | N/C | 9 / J |
TX55 plug wiring
| Function | Wire color |
|---|---|
| RESET | White |
| READY | Orange |
| POWER | Brown |
| ON | Green |
| GROUND | Black |
| MOSI | Blue |
| CMD | Yellow |
| MISO | Red |
• The sum of J1708 Data A (+) and Data B (−) is designed not to exceed 5.0 VDC. Voltages are nominal and can vary.
• When the CAN bus is idle, CAN_H and CAN_L both carry about 2.5 V. While data is transmitting, CAN_H can rise to about 3.75 V and CAN_L can drop to about 1.25 V.
Still stuck after these checks? Contact Fleetwatch Support.