Network diagnosis is actually fairly easy if you proceed in a structured manner. There are usually many clues to follow. Just don't jump ahead of your diagnostic path.

Today, a 2014 Silverado was dropped off with a complaint of ABS and brake light on, Sir light on, Service Tire Pressure System and Service Brake System messages displayed and IPC gauges inop. Clue 1: Seeing problems with several different systems at the same time usually means a communication failure. Step 1, Which modules are presently communicating/

[2014 Chevrolet Silverado 1500 LT, Scan Data]

Clue 2: All the non-communicating modules are using terminal 1 which is the low speed LAN. Step 2, What codes are set?

[2014 Chevrolet Silverado 1500 LT, Scan Data]

Several codes set by high speed modules. No low speed modules have coded. The problem appears to be restricted to the low speed LAN. Let's take a fast glance at the high speed LAN.

[2014 Chevrolet Silverado 1500 LT, Scan Data]

[2014 Chevrolet Silverado 1500 LT, Scan Data]

No problems seen here. Let's see what our low speed LAN is doing on terminal 1....

[2014 Chevrolet Silverado 1500 LT, Scan Data]

No low speed modules detected on the LAN. A fast look at LAN voltage will show us why.

[2014 Chevrolet Silverado 1500 LT, Scan Data]

No communication at all. The low speed LAN is a zero to 5 volt system but it's fixed at 2.78 volts. Why? Can it be a short to power? No, the voltage would be higher than 5 volts. How about a short to ground? No, our voltage would be close to zero. Most likely there is a module pulling down the LAN voltage. What's the easiest way to find out which one? Simple resistance testing.

Key off, 2 minute time out, the resistance between terminal 1 and ground is 365 ohms. I would expect to see high kilohm to low megohm readings. Checking the first splice connector, JX200 and pulling the comb, there are 7 component connections. Checking the resistance on each connector, all are very high resistance readings except for terminal G, ckt 5060. This reads 121 ohms. Circuit 5060 leads to the radio.

I pulled the radio and found an aftermarket module wired in behind the radio. I have no idea what it's used for. I unplugged this module and re-installed the splice connector comb. With key on, let's look at the low speed LAN voltage now.

[2014 Chevrolet Silverado 1500 LT, Scan Data]

Looks good now. What modules are communicating?

[2014 Chevrolet Silverado 1500 LT, Scan Data]

OK. All systems are now go, except for the radio which is unplugged. We told the customer that the problem is with the aftermarket module and is his concern now to repair. This entire diagnosis, by proceeding in an orderly fashion, took no more that 20 minutes. Always proceed from A to Z in network diagnosis and you won't get stung.

James from Florida

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