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post #1 of 16 (permalink) Old 08-08-2013, 11:37 AM Thread Starter
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OBD Powered but No Connection

Ok guys. I am looking for some help. I have an 05 Cobalt LS that has the MIL light on. I hooked up a OBD scan tool that is can compliant and has worked on the vehicle before. It receives power but will not link. I'm already aware of fuse issues with the OBD connector so I pulled all ECM related fuses in both fuse boxes and checked them all with no change. I went to 2 local shops (not GM Dealers) who also could not connect. They both told me to have fun at the Dealership. Now I am trying to find out what else I can do about this before Chevy takes me for a new $900 ECM that I probably don't need. I'm assuming that this is either a ground or wiring problem but don't know where to start on either issue. The only issue that I've found in this process is the ECM fuse in the kick box has a 20A in place of what is marked as a 10A. Anyone know how to test ECM or ECM connections? Also, is there a way to test the wiring between the OBD port and the ECM?

Vehicle is almost entirely stock with no electrical or engine modifications.
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post #2 of 16 (permalink) Old 08-08-2013, 01:32 PM
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Just for your info....2005-06 have a "PCM" with a separate ignition module. 07-up have an "ECM".

Have you tried disconnecting battery and unplugging the "PCM" connectors? Spray contact cleaner in plugs then re-insert and remove them 3 or 4 times in their socket to clean both contact surfaces. Then plug back everything together and try your OBDII.

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post #3 of 16 (permalink) Old 08-08-2013, 01:57 PM Thread Starter
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That is one of my issues. I can't seem to locate my PCM. I was looking for the ECM which I couldn't find, now I know why I guess. Is the PCM the box directly in front of the jump post under the hood? Where are the connectors for it?

---------- Post added at 01:57 PM ---------- Previous post was at 01:50 PM ----------

OK, I found a picture of it now that I know what I am looking for. I also found the ground issue associated with it which would not surprise me if that was the issue. I will take a look at both and get back to you guys with an update. Thanks again. Amazing how much info you can find when you know what the heck you're looking for!
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post #4 of 16 (permalink) Old 08-08-2013, 02:00 PM
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If it was a ground you'd have a truck load of problems like your cluster flickering, intermittent steering, hard shift if you have an automatic.

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post #5 of 16 (permalink) Old 08-08-2013, 02:07 PM Thread Starter
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If it was a ground you'd have a truck load of problems like your cluster flickering, intermittent steering, hard shift if ypu have an automatic.
So it has to be a connector issue, right? The dealership is telling me that the PCM has to be replaced or reprogrammed which makes no sense to me, especially since they haven't even seen the vehicle yet. They also say only GM dealerships can diagnose and work on PCM issues.

If the PCM stopped working then the vehicle wouldn't run or would at least have a huge host of issues instead of just this, right?
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post #6 of 16 (permalink) Old 08-08-2013, 02:13 PM
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I would imagine, mind you it could be the OBDII translator circuit only that's defective in the PCM, meaning everything else would work okay. Usually when a "PCM" fails your while driving it will keep your engine alive until you shut it down but will not restart. This is why I believe you just have a dirty contact somewhere. OBDII runs on same circuitry as GMLan and your GMLan obviously works because your car is not in limp mode.

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post #7 of 16 (permalink) Old 08-08-2013, 02:23 PM Thread Starter
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So where are the actual connectors for the PCM at? It looks like the cover is just a cover and not part of the PCM but I don't want to go ripping it apart without being sure. Also, is there a way to check the OBD port itself to make sure it is not just the wiring for the port. I have power to the port but don't know which wire is which so it may be the wiring for the port that is the problem.
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post #8 of 16 (permalink) Old 08-08-2013, 02:31 PM
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---------- Post added at 06:31 PM ---------- Previous post was at 06:30 PM ----------

Your looking for Can buss pin 6 and 14

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post #9 of 16 (permalink) Old 08-08-2013, 02:38 PM Thread Starter
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---------- Post added at 06:31 PM ---------- Previous post was at 06:30 PM ----------

Your looking for Can buss pin 6 and 14
Electrical is not my strong point. I can hook a voltometer from there to ground and check for signal with the key in the on position but not started, correct?
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post #10 of 16 (permalink) Old 08-08-2013, 02:50 PM
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Scan Tool Does Not Communicate with High Speed GMLAN Device
Modules connected to the high speed GMLAN serial data circuits monitor for serial data communications on the high speed GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module detects a bus-off condition a DTC U0001 or U2100 will be set. These DTCs can be retrieved as history only.

Diagnostic Aids
The high speed GMLAN serial data buss uses two 120 ohms terminating resistors that are in parallel with the high speed GMLAN (+) and (-) circuits. One of the resistors is connected at the data link connector (DLC) end of the link and the other is at the engine management module, engine control module (ECM) or powertrain control module (PCM), end of the link. The DLC terminating resistor resides internally in the electronic power steering (EPS) module. When testing for a short between high speed GMLAN (+) and (-) with the engine management module removed, a reading off 120 ohms is normal. However, to completely analyze the buss the BCM should be temporally removed when testing for shorts between the high speed GMLAN (+) and (-) while also making sure that all modules and the scan tool are disconnected from the buss. A normal reading across the high speed GMLAN buss with the terminating resistor at the DLC remove and all module and scan tool disconnected is infinite ohms. Make certain to reconnect the BCM after testing, it is necessary for proper vehicle powermoding.

The engine will not start when there is a total malfunction of the high speed GMLAN serial data circuits while the engine is not running. The following conditions may cause a total loss of high speed GMLAN data communication:

• A short between high speed GMLAN (+) and high speed GMLAN (-) circuits

• Any of the high speed GMLAN serial data circuits shorted to ground or voltage

• A module internal malfunction that causes a short to voltage or ground on the high speed GMLAN circuits

Test Description
The numbers below refer to the step numbers on the diagnostic table.

A partial malfunction in the high speed GMLAN serial data circuits uses a different procedure from a total malfunction of the high speed GMLAN data circuits.

The DTC U2100 may be retrieved with a history status, but is not the cause of the present condition.

Data link connector terminals 6 and 14 provide the connection to the GMLAN serial data high circuit and the GMLAN serial data low circuit respectively.

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