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Fixing Communication Problem Also Solves Other ComplaintsPosted 2/2/2007By Mark Owens
Recently, I was diagnosing a communication malfunction on a 2004 Chevrolet Silverado. This led to a wiring problem that, when repaired, corrected multiple intermittent complaints and instrument cluster problems. For example, the cruise control would cancel when the turn signals were activated, but only at night (we later discovered that this occurred when the headlights were on). When diagnosing with a scan tool, multiple U-codes (communication trouble codes) were stored in several modules. The electronic brake control module (among others) would not communicate at random. Bulletin No. 04-05-25-002C (11/23/2005) was related to a U1041 (loss of EBCM data) code. The ground (G110) in question related to the bulletin, which is on the left frame below the driver's door. After the ground was disassembled, cleaned and reinstalled following the bulletin procedure, a communication problem still persisted. A useful tactic in troubleshooting communication problems is to disconnect any related module on the communication circuit. Some vehicles use a data line splice pack, including this vehicle (SP205). This is a connector with one wire from each module connected together with a metal comb creating a central location to isolate communication circuits. On this vehicle, since the antilock brake system (ABS) module was not communicating, it was disconnected. The communication with the rest of the vehicle was now restored (except for the ABS module). I got lucky on this vehicle because the first module disconnected, which solved the problem. The same procedure would apply at the splice pack connector. Instead of disconnecting each module, simply remove the metal comb insert from the splice pack connector and jump each module to the data link connector using a jumper wire and recheck communication. The powers and grounds were checked prior to replacing the ABS module. The ABS module power supply tested good but a voltage drop test from the battery negative to the module ground revealed more than three volts. This was unexpected because the ABS module ground (G110) was just cleaned and tightened. The left frame at G110 was then tested for voltage and more than three volts existed on the frame itself. Now the problem started to make sense. With the frame having voltage, this indicated a bad ground connection to the battery and would explain the complaint of the cruise control cancelling when the exterior lamps were on. The exterior lamps were also grounded on the frame, which aggravated the problem. Voltage drop tests were then performed on the body, engine and other chassis/suspension components. After a close visual inspection of the battery cable routing and reviewing the voltage drop test results, the problem was found. The negative battery cable had been relocated during a previous repair and installed on the radiator support, not the frame in the left front corner. This was causing a bad frame ground (G100). A repair of the ground and the negative battery cable were performed. All the codes were cleared and communications with the entire vehicle were re-established. If a communication problem is an obvious symptom, don't hesitate to diagnose that problem in an attempt to repair a wide variety of complaints.
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