06 CTS, No Crank, No Communication with ECM, U1000

I am working on an 06 Cadillac CTS, 2.8. The car has a no crank, key stuck in ignition. The scan tool will not communicate with the ECM. I have a U1000 in the other modules. I have checked the powers and grounds at the ECM and they are good. I have under 2 volts on the class 2 serial data at the ECM. I have continuity on the class 2 from the EBCM to the ECM. The ohms look good. What am I missing? Do I have a wiring issue? The car has mouse feces in the engine bay, possible rodent damage? Is there a bad module on the class 2?.

Hi P320.

So you have communication with other modules through CAN lines (but no with ECM) and the other modules are telling you they don´t have communication with ECM through class 2?

Yes

I have continuity between the DLC and ECM on the class 2.

I would go straight for ECM (before checking powers and grounds as you did) and by checking no activity on CAN lines.

what is resistance between can lines at ECM? I mean, CAN-H and CAN-L unplugged from the other modules? If you have 120 ohms, you need an additional 120 ohms resistor, so that you can simulate total 60 ohms in CAN network.

If you have no activity, i would replace ECM.

Is there any CAN expert with a second opinion?

I know there is a 120 ohm resistor in the ECM. I thought I should have 60 ohm between the High speed can + and High speed can -. So I should check from the high speed can to the serial data?

Only way to see clear pic of class 2 serial data is with a scope. It should be 0-7 volt square wave. Your meter isn’t fast enough. also have you checked for 5 volt reference at the maf sensor? You need to check the resistance at pin 6 and 14 at the dlc because the pcm could be using both networks to communicate on.

I had read that with a meter i should see 4 volts since it is not fast like a scope.

When I check pins 6 and 14 for ohms it is open.

I removed the connector at the ecm and checked the ohms for the ECM. GM says it should be 100ohms. I got infinite.

I counted the pins on the connector the wrong way. I have 119ohms.

Ok. Let me know if i´m wrong:
1. As Cheryl said, ECM is using two different protocols.
2. Class 2 (J1850 VPW). All modules (except TCM) are connected to ECM through this network). Class 2 is assigned to (pin 2 at DLC).
3. CAN (ISO XXXX). ECM & TCM are directly connected, sharing data each other (pins 23 & 55 in ECM and 32 & 33 in TCM). There is a second CAN network shared with DLC (pins 33 & 53 in ECM and 20 & 21 in TCM). CAN is assigned to (pin 6 & 14) at DLC
4. CAN network total resistance is 60 ohms (so resistance between pin 6 and 14, should be 60 ohms) with ECM & TCM plugged
5. If you unplug TCM, resistance between pin 6 and 14 at DLC should be 120 ohms (resistance inside ECM). if you can stablish com with scanner, TCM is likely the problem
6. If you unplug ECM, resistance between pin 6 and 14 at DLC should be 120 ohms (resistance inside TCM). if you can stablish com with scanner, ECM is likely the problem
7. As Cheryl said, scope is needed to check voltage and waveform (with multimeter you cannot verify lower and higher voltages, neither square signals)

Bearing in mind that class 2 network is also having issues and the fact TCM is not connected to class 2 network, I would say ECM is the problem.

BTW. Are you having communication with TCM?

Thanks for the help.

I did not realize when I checked the ohms on 6 & 14 yesterday that my dad had the TCM disconnected.

I have 7 mega ohms on 6 & 14 everything connected.

With the TCM disconnected I have infinite ohms.

With the ecm disconnected I have infinite ohms.

Yes, I can communicate with the TCM.

Post wiring diagram.

A couple things to consider here

1. can key be turned to acc, off, run, and crank positions? Another wire to check would be (if it applies) module wake up signal wire to ECM so if key can't be turned it may be in power down mode. Though doubtful

2. Have you tried to run a separate pair of wires to dlc from ecm can high and low to eliminate a almost completely cut wire?

3. Using wiring diagrams look at the communication network diagrams and see where the wires from the ECM tie in for each module that can't communicate with ecm. Then load test those circuits with them unplugged from those modules to see if you get a dim light on one or more sections. Load test with a 4 amp bulb and a jumper wire to battery negative and the load test bulb to positive. Compare brightness to when it's hooked to the battery itself. Just remember continuity/ 0 ohms can happen with just one strand of wire intact.

Here is the diagram of the high speed can bus.

i.imgur.com/6CS6u3j.jpg

i.imgur.com/unkVf7W.jpg

 

1. Can I rule out the module wakeup wire since I have infinite resistance on the pins 6 and 4 of the DLC.

2. No I have not. I have a feeling I have a wire that has been chewed in two.

3. I'll do that next.

With the 15 amp ECU fuse (in the under hood fuse box) remove, I have power on both sides in the fuse box.

If you have infinite resistance on 6 and 14 then yes you can rule it out if all modules where connected when the test was done.

Power to both sides of fuse slots with fuse removed is a good find now to locate where power is being fed. also what is the voltage reading on both sides of fuse slots?

It was 11.8v and 12.2 volts on the fuse.

Will the headlight test work on the can bus? Is the can bus too low of voltage to illuminate a headlight?