Shown below is 2003 Silverado diagnostic link connector (DLC) diagram describing how it works. As you can see here, only four (4) DLC pin terminals are used. Now you can fix NO COMUNICATION problem on this model of vehicle ( even 2004-2006 models) using a cheap voltmeter and scanner. Please review the details below.

2003 gm bus wiring communication diagram

Note: Green lines above are all class 2 bus wires connected between DLC terminals and splice bar modules. You can get this exact wiring diagram from your local parts stores or get it online from ATS.

2003 gm dlc wiring diagram

Precaution before using this diagram:

1.) I made this diagram myself to show how GM computer circuit is wired together with the DLC. This is only an approximate diagram as I see it. Some modules and fuses were omitted to simplify the diagram. I don't profess that all of these information are correct but in case you find some errors, please contact me at my ATS homepage.

2.) From the conception of OBD2 in 1995 upto 2002, most electronic circuits were controlled mainly by engine control module (ECM) alongside the body control module (BCM). This method required a lot of wiring harness between those two modules including the sensors and other body parts. SOLUTON? Use numerous modules or multiplex grouped together by two splices as shown above. This design NOT only save costs but it allows cheap upgrades by merely adding new modules to the circuit.

2003 Chevy Silverado DLC Diagram Description:

Pin 2 is the circuit main data bus wire link. It's like the communication "messenger" between all the modules used in the vehicle. Both ECM and BCM are connected together by red wire making these 2 modules a two-wire connection. This means both modules are inter dependent on each other so if BCM is at fault, ECM would not run. All the rest of the modules like transmission control module (TCM), (HVAC) for heating and AC use a single wire for communication. Please note the Transfer case module gets it's signal from Electronic brake module EBCM.

Regarding the BCM 2 wire connection to the ECM, the BCM is the hub or gateway module for all the modules. This means BCM feeds bus data signal not only to the ECM but all the rest of the modules gets it's feedback from BCM before giving them to the ECM.

Communication is also rated by speed and this model year is called Class 2 serial data bus protocol, it is thought to be slow when compared to 2007-2014 models. To make this diagram simple to understand, please concentrate on the wiring flow rather than the complex computer terms here. Take note also about the two (2) splices that are used here: on the left (splice 1) you have the more dominant modules whereas on the right (splice 2) are the modules that follow only. The follower modules also have to pass thru a relay (shown in green box).

Recommended DLC pin terminal voltage readings at key on engine off (KOEO):

Pin 2 (data bus signal wiring) should read 5-12 volts depending on the speed of the data bus used. I normally use 5 volts at KOEO for slower class 2 serial data bus voltage.

Pin 4 (ground circuit) should read zero volt or no higher than 0.250 volt.

Pin 5 (ground circuit) should read zero volt or no higher than 0.250 volt.

Pin 16 is power supply and should read 12 volts all the time.

Above test is done using a digital voltmeter with negative lead connected to battery negative and positive lead probing the above terminal pins. You can also do the same test by probing Pin 5 against Pin 2 when testing the data bus voltage or probe Pin 5 against Pin 16 to test the power supply voltage in the data bus circuit.

Troubleshooting NO COMMUNICATION for 2003 GM vehicle DLC during KOEO:

1.) If NO COMMUNICATION shows up in your scanner, it could lead to major problems like no starts, cutting off, intermittent stalling, etc. One of the most common symptom is no communication between BCM and ECM modules. Begin by probing the voltage of Pin 2 against Pin 5 to see if you get 5 volts at KOEO. If you get 7 volts, it is being pulled up by a short to power. Now transfer your voltmeter positive lead to pin M red dot at Splice 1. If you have 7 volts there, unplug the BCM to see if voltage changes. Do this also one at a time with ECM at pin B red dot then followed by S2, S3 and S4 red dots at splice 1 until the voltage goes back to normal (5 volts).

What is the 2003 serial data bus speed NORMAL voltage reading?

There are 3 possible serial data bus speed voltage used in 2003-2006 vehicles: 5 volts, 7 volts and 12 volts. These are the voltage readings at KOEO. With ignition key off, voltage should drop to zero. Since we are talking GM Class 2 serial data bus protocol here, it is normal to find 1.2 to 1.8 volts to be the engine idling voltage of Pin 2 when measured against Pin 5 using a voltmeter. However at KOEO for 2003-2006 GM vehicles, expect a normal voltage of 5 volts.

Using Isolating 2003 Class 2 serial data bus modules:

Let's say the vehicle has NO COMMUNICATION and you suspect the modules involved are in splice 1 and splice 2, this can be tested by unhooking pin S1 red dot at splice 1. If you only suspect it is splice 2 modules only, disconnect pin blue dot (top) X of splice 1 and all modules and relay are isolated in splice 2. The exact location of the splice bars splice 1 and 2 including the colors of the bus data wires are shown in detail if you have the specific wiring diagram of your vehicle.

To test each modules before replacing them:

Check the tightness and integrity of each wire connections in the plugin terminals of the culprit module. If loose or not plugged in properly can cause intermittent problem. Using the power and ground distribution boxes wiring diagram, you can find which module has shorted wires.

Note for module isolation:

Disconnecting B red dot in splice 1 will isolate the ECM from the data bus circuit. If you disconnect S2 red dot will isolate HVAC module whereas disconnecting S5 red dot will isolate VCIM module from the data bus circuit.

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