HIGH SPEED LAN Diagnostics / Remembering Ohms Law

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Starting in model year 2004 several general motors vehicles will use a new means of communication between electronic modules via serial data circuits.

To assist in diagnosing this new serial data circuit, the following is being provided as a starting point.

The following conditions may cause a total loss of high speed GMLAN data communication:

1. A short between high speed GMLAN (+) and high speed GMLAN (-) circuits
2. Any of the high speed GMLAN (+ or -) serial data circuits shorted to ground or voltage
3. A module internal malfunction that causes a short to voltage or ground on the high speed GMLAN circuits
4. An open in either/both GMLAN serial data circuits

GM High speed (only) LAN diagnostic/communication checks

1. Key in the off position
2. Use a dvom while reading resistance, put one lead with the correct probes on pin 6 of the aldl diagnostic connector and put the other lead on pin 14 at the dlc.
3. Measure the resistance results

If the resistance is out of limits (ol), look for a open in circuits 2500 or 2501 that runs from the dlc to the bcm on the LAN bus as per the wiring schematic. Repair as necessary.

4. The resistance value should be approximately 60 ohms +/- 10%.

•	If the resistance is @ 128 ohms, look at the wiring schematic for the LAN bus for the vehicle you are working on. Using the schematic, disconnect one module from its LAN connector in approximately the center of the complete LAN circuit. You have now effectively created two LAN circuits.

Procedure for checking an open in one of the gm LAN wires going to the LAN module you disconnected:

•

Probe the LAN circuits 2500 and 2501 from the disconnected module back to the dlc. The resistance should be approximately 128 ohms. If an open is encountered, your LAN circuit 2500 or 2501 is open/grounded (could be to a voltage source which is not present due to the key being off) and/or a module is open/grounded internally between the module and the connector circuit you are probing.

Probe each circuit individually between the dlc and your current location to determine if 2500 or 2501 is open/grounded. Remove one module at a time, working your way back to the dlc to determine which module and/or connection in the circuit between modules is open/grounded while probing the lan circuit back toward the dlc.

•

If the resistance above is approximately 128 ohms, probe the LAN circuit at this module connector toward the pcm - away from the dlc. The resistance value should be approximately 128 ohms. If the resistance value shows open/grounded, probe along the LAN line to the next module and/or skip ahead a couple modules and probe for the open and/or ground. If readily accessible, remove the pcm LAN connector and probe circuits 2500 and 2501 individually to determine which is the culprit. Repair/replace components/wiring as necessary.

5. Reconnect all modules. Measure from the dlc as per step 3 above. The resistance value should be approximately 60 ohms +/- 10%.
6. Connect the tech 2 and candi module to the dlc. Check whether the tech 2 is running software version 28.007 or later. Update as required.
7. Observe the led indicator on the candi module. If the led is flashing slowly (1 time per 3 seconds) there is a problem between the tech 2 and candi module and they may not be able to communicate with vehicle. Correct the tool concern. The cable on the tech 2 is suspect. If the led indicator is flashing fast (1 time per second) it is more likely that they are having a vehicle problem with gmlan.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.