OBD II Not Connecting

[Big T]

i mentioned this in my ECM Reflash Thread on the 6.5 Performance Forum. I need to Smog Check my '99 Suburban in California and cannot because the test centers are unable to connect and communicate with my OBD II or ECM. Today I went to check the VIN # using my Centech code reader and it too was unable to connect. When I first bought this code reader a couple years ago, it always connected and was able to pull information. Have not needed it since. Now it is not connecting.

I checked and cleaned the engine grounds which are installed on the rear of the intake manifold as they should be. I cleaned the grounds to the battery posts (post mod done here). I tested and confirmed power at #16 pin on the DLC plug receptacle. Also confirmed grounds at #4 and 5 pins. If there was no power and grounds at the DLC, my code reader would not power up. I know this because I did burn the #7 fuse while testing at the dash cigarette lighters and the code reader would not power up.

So, my code reader is showing the same issues that the Smog testing centers are showing: no communication with the OBD II. In speaking with Bill Heath, he said it had to be a ground or a fault at the DLC plug, as the connections never fail at the ECM. He also said that the engine would not run if the ECM were faulty, yet the engine runs fine. Just to satisfy myself, I pulled the plugs on the ECM and reinstalled. No change.

I'm at a loss as to what to do. My code reader had previously worked on this truck. I haven't thrown codes in years. Now it won't communicate much the same as the Smog testing station equipment.

 

[ak diesel driver]

I'd try your new ECM when you get it.

 

[Big T]

I'd try your new ECM when you get it.

Have not ordered it....yet. I was checking for the VIN # with the code reader to make sure it matched what's under the windshield and on my registration. Thought was that if they differed, then that was causing the fail at the Smog Teat Stations. Now I have confirmed that it's just not connecting to the ECM.

Also, I tried to order that ECM fro '99 Suburban located in Austin, Texas, but their website said it must be picked up in person. Phill Heath said all ECMs from '96 on were the same and he could program it with my VIN# and the stock tune that went with it. I can buy an ECM locally for less than the one in TX.

 

[THEFERMANATOR]

Any abs issues or anything else?

 

[Big T]

Any abs issues or anything else?

ABS Light has been on for years. Tried many times to resolve with repeated posts here and visits to the dealer to pull code and could never resolve the issue. Code showed fault interruptus on RF. Replaced that sensor several times and, along with LF and even triend new harness to ABS pump and no go. So I just gave up on it.

 

[JayTheCPA]

IIRC, during the last smog check, the State reps were not able to connect to the ECM. If so, point is that this issue probably has existed for some time.

Anyway, aside from the ABS issue, a break in the wiring from the OBD-II port to the ECM might explain why the truck runs yet nobody can read the ECM. Naturally, this presumes that the OBD-II port is a dead-end connection to the ECM.

 

[Big T]

IIRC, during the last smog check, the State reps were not able to connect to the ECM. If so, point is that this issue probably has existed for some time.

Anyway, aside from the ABS issue, a break in the wiring from the OBD-II port to the ECM might explain why the truck runs yet nobody can read the ECM. Naturally, this presumes that the OBD-II port is a dead-end connection to the ECM.

There aren't that many wires to the DLC connector, like 7 or 8 and I've confirmed 3 are good. Highly improbable that any of the other wires have been cut or worn through as they run though a sheath and are located behind the dash.

 

[JayTheCPA]

Here is a thought. Is it possible to move the ECM over to another truck just to see if is the ECM or your truck?

If so, prior to moving the ECM, verify that the reader can in fact read the other truck. This way it will verify that the reader is good and help isolate whether it is your ECM and / or truck.

If your ECM tests good in the other truck, no need to buy the other ECM unless you want a backup.

 

[Big T]

Here is a thought. Is it possible to move the ECM over to another truck just to see if is the ECM or your truck?

If so, prior to moving the ECM, verify that the reader can in fact read the other truck. This way it will verify that the reader is good and help isolate whether it is your ECM and / or truck.

If your ECM tests good in the other truck, no need to buy the other ECM unless you want a backup.

Good idea, but I don't have access to another OBD II truck. My son's is a '94 and thus OBD I.

I talked with Heath this AM and he's lost too. The only way the ECM could be damaged is from a voltage spike. He's wondering if my boat trailer and poor grounds at the back might have something to do with this.

 

[Paveltolz]

Have not ordered it....yet.
Also, I tried to order that ECM fro '99 Suburban located in Austin, Texas, but their website said it must be picked up in person.

Let me know if you want me to grab the one in Texas for you. I'm flying into Austin on Monday and could grab it in the morning Tuesday as Browning Auto is right around the corner from the Airport.

 

[JayTheCPA]

How about the rolled carcass? Hook a battery to it and see what happens? But then again I do not remember whether the carcass is OBD-I or II.

 

[Paveltolz]

How about the rolled carcass? Hook a battery to it and see what happens? But then again I do not remember whether the carcass is OBD-I or II.

That was a 1995, OBD-1

 

[Big T]

That was a 1995, OBD-1

And per vociferous request of my wife, the wreck is gone .

Paul, they're looking for the ECM for me now. I think the ECM is still in the truck. If there is a problem with shipping it, I will take you up on your kind offer.

I don't know why, but I seem to be hit with smog tech challenges on this truck. This non-communication issue seems to have tested all the experts.

 

[Paveltolz]

Steve, if it will help, I have the schematics for the 1997... it won't have the pass-lock circuit but it should, key word "should" have the power and signal circuits, wire colors, splice points etc. I can look them up, scan and email them to you to go over.
I'd have bet money (and lost) that I had another ECM laying around here from my 1996 but, alas, I can't find it. Probably sent it to Heath to help with his backlog of cores.
Let me know on the Texas unit. I'm scheduled to get in very late Monday but will be staying just around the corner from Browning the first night so I can grab it easy on my way out of town Tuesday morn.

 

[Big T]

Yes, schematic would help.

Still waiting on Browning. I've called back and they're working on locating it. Meanwhile April the dogs and I took off geocaching.

 

[WarWagon]

Maybe cut to the chase and use the electrical shop that found the window short taking the speedo out on the 94.

First swap the ECM as it's "easy" and they do fail in 'truth stranger than fiction' ways. Say oil on a knock sensor connector causing the ECM to declare the ignition control module bad because it can't advance the timing enough to hear a knock. Oh wait the ECM's memory is bad so it won't store a code after limp mode with the SES light on. So after ECM replacement the random code finally shows up and the ignition coil pack module doesn't fix it... Took a shop all summer to track that fing sensor and connector down.

Next I would check the two positive voltages at the ECM pins making sure the ECM isn't browning out from bad ignition switch, etc. One is normal power and the other "memory" power. (Fusible links on memory and main power go out with age.) Then make sure there isn't any voltage at the ground pin.

The OBD-II port and related wiring IS NOT INNOCENT! It will have to be tested with a flowchart as well.

Next you are looking at the engine harness for shorts and disconnecting every single sensor and ECM output looking for a short causing the ECM to go nuts.

Wires do fail with vibration breaking them or internal corrosion. Not fun to find. <-- That's a period.

One multi day adventure had several corroded electrical butt connectors on a ECM ground causing high resistance preventing it from going into diag mode and communicating with the scanner. We found this as the voltage at the OBDI connector was out of spec per the flowchart as were checking it's pins for voltages. My point here: If you reference the frame, door jamb, any other ground than the pin at the OBD-II port you can miss the ground voltage issue.

 

[Paveltolz]

Schematics sent. Too large to post up here though. Check for connectors, particularly #2, for damage, looseness. That's the one that connects directly to the ECM via a purple wire, circuit 1807. Several splices along the run of that line between DLC and ECM. It would help explain the reason why the truck is running but not able to communicate with the Scanners.

 

[THEFERMANATOR]

There could be an issue with the class 2 data bus. OBD2 trucks use a common data com wire for all OBD2 communications. If any module on the bus shorts out, it takes down the whole databus. For your 99 I believe it's the ECM, ABS, airbag module, possibly the cluster, maybe the passlock module, and probably the central timer module(door lock RKE control). If any one of them shorts out, it takes out the whole datab us which would stop all communications via the OBD2 port. So it could be a vehicle problem, or a failed ECM.

 

[Big T]

OK, I'm working through the diagnostics per the instructions Paul sent (see attached, hope you can read). In the case of the UART Data Line, I get down to Step 4 and the Voltage is 4.4 which points to a short to voltage in Tan (800). In the case of the CLASS-2 Data Line, I get down to Step 6 and the Voltage is 2.3 which points to a short to voltage in PPL (Purple 1807) wire.

However, in Step 3 I initially got 7 ohms, then 77 ohms resistance. I don't know if I have the meter set up right as I barely know how to use it. This reading could indicate an issue with the Electronic Brake Control Module (EBCM). They say the EBCM is located near the Master Cylinder, but I don't know what to look for.

That's the best I can come up with for now.

Taking off for a hike with the wife and the dogs.

Happy New Year!

 

[Paveltolz]

Figures, the 'won't connect to Brake Control Module' trouble shooting page was the one I figured you wouldn't need.
Sorry. E-mailed it and it is attached here as well.
Happy New Year's all,

Paul