OBD2 won't connect to scan tool. Fuses good. Need ideas.

[JessMan]

Hey gang. Had a CEL pop up on me while I was driving. Car is running fine, no noticeable changes. Went to Oriellys to get the code read, they used a Bosch scan tool. It just read connection error. Then went to Auto Zone, they had a small little dinky scan tool. It powered up when he connected it but read an error. So it seems to have power but isn't communicating. I went ahead and checked the fuses that I could find that may be a part of it(there isn't a specific OBD2 fuse). I checked the Comm, AUX, both lighters, and a couple others. I checked all of the ones in the kick panel even though they may not be a part of it. They all looked good. Both Cigarette lighter sockets work. Has anyone had any type of similar problem? I'm just not sure what to look for at this point. Thanks.

[smwalker]

How do the pins on the connector look? Any damage?

Should see 6 terminals in there.

[smwalker]

The fuse you want is F5 in the rear (trunk) fuse block. although it seems like it would be ok or many other items would not be working: cluster and DIC



And here is a description of the system from the Service manual. THere are some resistors mentioned there, they are up near the radio Not sure those could cause problems.


Data Link Communications Description and Operation

Circuit Description

The communication among control modules is performed through the High speed GMLAN serial data circuits and the Low speed GMLAN serial data circuit. The modules that need real time communication are attached to the High Speed GMLAN network. The body control module (BCM) is the gateway between the high and low speed networks. Refer to Body Control System Description and Operation for more information about the gateway.

Signal supervision is the process of determining whether an expected signal is being received or not. Some messages are sent on a periodic basis and are interpreted as a heartbeat of a device. If such a signal is lost, the signal supervision part of the software will set a no communication DTC (U code) against the missing device. This code is mapped on the scan tool screen as a code against the physical device. A lost communication DTC typically is set in modules other than the module with a communication failure.

GMLAN High Speed Circuit Description

The data link connector (DLC) allows a scan tool to communicate with the high speed GMLAN serial data circuit. The serial data is transmitted on two twisted wires that allow speed up to 500 Kb/s. The twisted pair is terminated with two 120 ohms resistors, the location of these resistors is listed in the Master Electrical Component List. The high speed GMLAN is a differential bus. The high speed GMLAN serial data bus (+) and high speed GMLAN serial data (-) are driven to opposite extremes from a rest or idle level. The idle level, which is approximately 2.5 volts, is considered recessive transmitted data and is interpreted as a logic 1. Driving the lines to their extremes, adds one volt to the high speed GMLAN serial data bus (+) and subtracts one volt from the high speed GMLAN serial data bus (-) wire. If a communication signal is lost, the application will set a no communication code against the respective control module. This code is mapped on the scan tool screen as a code against the physical device. Note: a loss of serial data DTC does not represent a failure of the module that the code is set in.

The high speed GMLAN serial data allows communication between the BCM, engine control module (ECM), transmission control module (TCM), steering angle sensor (SAS), electronic brake control module (EBCM) and the OnStar® vehicle communication interface module (VCIM)

GMLAN Low Speed Circuit Description

The data link connector (DLC) allows a scan tool to communicate with the low speed GMLAN serial data circuit. The serial data is transmitted over a single wire to the appropriate control modules. Under normal vehicle operating conditions, the speed of the buss is 33.33 Kb/s. This protocol produces a simple pulse train sent out over the GMLAN low speed serial data bus. When a module pulls the buss high, 5 volts, this creates a logic state of 0 on the buss. When the buss is pulled low, 0 volts, it is translated as a logic state of 1. To wake the control modules connected to the GMLAN low speed serial data buss, a high voltage wake up pulse is sent out over the buss, the voltage level of the pulse is +10 volts. Modules connected to the GMLAN low speed buss can be part of a virtual network as described in GMLAN High Speed Circuit Description above. The modules on the GMLAN low speed serial data buss are connected to the buss using several splice or “star” connectors separating groups of modules. Refer to Data Communication Schematics for specific schematics information.

The following lists state the splices and modules connected to the low speed serial data circuits:

I/P Splice

BCM
Driver information display
Instrument panel cluster (IPC)
Inflatable restraint sensing and diagnostic Module (SDM)
Theft deterrent module (TDM)
radio/HVAC control – integrated radio chassis (IRC)
Heating ventilation and cooling (HVAC) control module
Remote function actuator (RFA)
Active occupant sensing (AOS) module
Heated seat module
OnStar® vehicle communication interface module (VCIM)
Data Link Connector (DLC)

The data link connector (DLC) is a standardized 16-cavity connector. The DLC low speed serial data circuit is connected directly to the I/P splice then to all other splices or modules. Connector design and location is dictated by an industry wide standard, and is required to provide the following:

Pin 1 GMLAN low speed communications terminal
Pin 4 Scan tool power ground terminal
Pin 5 Common signal ground terminal
Pin 6 High speed GMLAN serial data bus (+) terminal
Pin 14 High speed GMLAN serial data bus (-) terminal
Pin 16 Scan tool power, battery positive voltage terminal
Serial Data Reference

The scan tool communicates over the various busses on the vehicle. When a scan tool is installed on a vehicle, the scan tool will try to communicate with every module that could be optioned into the vehicle. If an option is not installed on the vehicle, the scan tool will display No Comm for that options specific control module.

In order to avert misdiagnoses of No Communication with a specific module, refer to Data Link References for a list of modules, the busses they communicate with, and the RPO codes for a specific module.

[JessMan]

Already checked that fuse smwalker. It isn't blown. And if it was, wouldn't that effect more than just the OBD2 port? I'll check how the pins look and shine a light into the port tomorrow.

[TooManyHobbies]

Do you have any aftermarket gauges on the CANBUS? I had a gauge go bad that caused all kinds of weird issues. If you do, unplug it and see if the issue goes away.

[JessMan]

Nothing aftermarket on the interior and no tune. The only mods are a Vararam Intake and mid pipe delete which I did the intake over a year ago and the mid pipes a few months ago.

Also another symptom I noticed today is my DOD is no longer engaging. Is that just a bi-product of having a CEL like the remote start not working or would that possibly be the cause of the CEL?

[JessMan]

Nothing looks out of place on the OBD2 plug. Is there anyway that I can use a multi-meter or test light on something to see what's up? My biggest confusion here is that the scan tools powered up when they were plugged in so power is there. It's just not reading.

[Napalm]

Do you still have your onstar module in the car - might try unplugging it. I thought I had read once that if it goes wonky - the datalink dies. but that might be on a different vehicle

you can't really meter the CANBUS connections -

When you check your fuses - are you just doing visual or did you meter them? I always make sure to ring them with a meter - as it might look ok but still be frail inside.

if you have continuity as an option on your meter use that - if you don't then check resistance and the value should be as near to 0 as possible due to short wire and short connection.

Did you check your battery voltage at the battery? it is over 11v when you're trying to hook up?

Yes your DOD won't activate with some engine codes. so that's not uncommon.


last ditcher - pull the battery cables for some 30 minutes or so - re hook up and then try to read it

[JessMan]

I've never used the on star but I'm assuming it's still in there.

For the fuses my multi meter is broken so I've just been visually checking them.

I haven't checked the volts at the battery(multi meter is dead) but I haven't had an fluctuations in the lights or anything like that. I'll see if I can borrow a multi meter from someone and check those things.

[JessMan]

Alright gang. Turns out the parts stores scan tools are garbage and just don't work properly. Used my buddies Bluetooth OBD2 reader and the Toque app. Code is a P0521 for the oil pressure sensor, which I'm assuming it means the sending unit. We cleared the code and it came right back each time I started the car so it's a problem. Now that I know what it is I'll be able to fix it. Thanks for all of the help gang. Next step is to replace the sending unit and hope that fixes it. I've seen some threads where that is not always the fix.