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Discussion Starter · #1 ·
07 Cobalt 2.2 ecotec auto


Completely unnecessary back story.

Just bought a beat up 07 Cobalt since our family needs a cheap car to get around town with. My wife traveled to North Carolina on vacation and decided to have a heart attack. She's stuck out here while she heals but she is doing relatively well. I flew out to help her recover and we have been borrowing our friends car for a while and we need our own.



Real info here

I can do pretty much any mechanical work on normal vehicles but am at a loss with electrical diagnosis.

The Cobalt won't crank, the cel never illuminates, power steering message, no power to pcm/ecm fuse under hood. No door chime.

Radio and interior lights work fine (except front speakers of course)

I have verified 12v and 5v at the ignition switch.

I've tried adding grounds to the ecm. I've tried wiggling connectors at the bcm ecm and fuse block.

I have read a lot of posts pointing to bad connections at the fuse block, bcm issues, ground and grounding the ecu, gmlan etc

Also on my searches it seems that the ecm and bcm are programmed together with the cars mileage so you can't just swap them around all Willynilly.

I would really appreciate some guidance on what to check next.

Thank you! If anyone needs pics of anything let me know
 

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Discussion Starter · #2 · (Edited)
Cleaning grounds under the coolant bottle now. Seems the ecm relay isnt getting triggered. I'm down to diagnosing the bcm or gm high speed lan issues. I tried a junkyard ecm and still no cel with key on.


If someone would be so kind to walk me through diagnosis or know of a good shop or individual in North Carolina area that can diagnose this? I'm getting my butt kicked!!
 

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Hmmm, okay have you connected a scan tool to the obd2 port? If so is it getting communications with the BCM? Because so far it sounds like the exact same issue I had. But I replaced my entire fuse box (make sure it's compatible with your year model and engine bay connectors at the fuse box) and plugged in my abs module to have communication with the ecm and tcm.
 

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Discussion Starter · #4 · (Edited)
I have power at the OBD2 but no communication. I'm using one of those cheap bluetooth OBD2 scanners but it's worked on every other car. Check engine light never illuminates when turning the key on.

I'll check to make sure the abs module is plugged in, look at the wiring, test it or grab one at the junkyard from a totaled car today. Same with the fuse box.
 

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I have power at the OBD2 but no communication. I'm using one of those cheap bluetooth OBD2 scanners but it's worked on every other car. Check engine light never illuminates when turning the key on.

Even you say you plugged in your abs module you mean an obd2 scanner?
Did you check ports 6 and 14 ohms? It should look like this
Rectangle Font Slope Line Parallel


And I connected the actual abs module for the brake system and finally got communication with the ecm and tcm. Now if you're getting communications from your tcm then there's a possibility that you might have a bad ecm. But you would have to do further testing for that.
 

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Discussion Starter · #6 ·
I edited my original reply once I re read it. I'm starting with getting the battery terminals and jump posts super clean and adding spark plug grease to them all. Then onto probing the OBD.

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Discussion Starter · #7 · (Edited)
I wasn't sure which way to check the pins do I did both. I'm getting 5.12k ohms on 6 and 14 separately with battery disconnected and key off grounding to the body. I checked other threads and saw people probing directly between the two pins 6 and 14 with the battery disconnected. I tried that and got 0ohms resistance between the two and also 0ohms resistance between those pins individually and the ground pins. I unplugged the small connector towards the top of the power steering module and got no connection to ground. I plugged it back in and am now getting 63.1ohms between 6 and 14.

Checking voltage on 6 and 14 I get .98v and .88v. Is it supposed to be 2.5v?

ECM still isn't powering up as expected.
Looking like a canbus problem?

The two grounds and the power pin on the OBD2 show 0ohms resistance to ground and battery voltage on positive.

When I turn the key on also when trying to start the bcm relays seem to click properly if that helps. Radio, windows, etc seem to be working normally.

I don't seem to have abs and I don't see a disconnected connector from someone swapping a non abs module in.

I also get about 7 to ten clunks from under the dash which sounds like a blend door trying to move if that could be related.

Automotive tire Motor vehicle Automotive design Vehicle Automotive exterior

Rectangle Font Gadget Pattern Electric blue
 

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FYI mileage/ odometer & vin# is stored in the BCM. The BCM talks to the other modules via the GM high speed LAN (canbus) with key off measure the resistance from each lead to ground and from lead to lead (GM LAN hi side and low side). To verify no shorts or opens.
You will need a little better ODB diagnostic tool to scan all of the monitors better.
As for no start, there are schematics here on the forum that layout the logic.

Good luck. 98% of the time all modules are good and swapping out modules will hose your ride without reprogramming them for your ride.
 

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Discussion Starter · #10 · (Edited)
Battery hooked up and key off I get 60.8ohms between 6 and 14.

With battery disconnected key off i'm getting 5.12k on the canbus wires separately checking between the wires and the ground separately.

That's how I checked them before, let me know if this isn't right. I'm learning as I go and trying not to get distracted jumping to other possibilities.
 

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Discussion Starter · #12 · (Edited)
I would be thrilled and disappointed with myself at the same time if it was just the starter. I already took it off and tested it with battery cables, it engaged and spun strong. Also the previous owner threw a starter at it and it didn't help. I've been cleaning and greasing all the grounds I've found so far. Should I try another battery even though I'm getting 13v? I know these cars are particularly temperamental about power and grounds.

I'm stuck on finding good diagrams and wrapping my head around what happens to trigger the ecm on as I have no power at the main ecm. I have power and grounds at the fuse box under the hood. I'm pretty much looking at the area between the bcm and ecm. I verified I have good ignition switch signal coming into the bcm after that it gets foggy.


I get battery voltage at bcm pin 56 white which doesn't sound right from what I see.
No key is zero, key in not turned is 13v, acc is 22.5mv, run is 4.3v,crank is 3.09v.

From what I read 56 is the power from the ignition and what's strange is I don't see that voltage spike at the ignition wires.

Pin 62 has perfect 5v From the ignition. I guess coby7 has a good schematic that I have to find.

i ohmed out the gmlan and it seems ok unless I'm checking it wrong.

Could these solder blobs from the previous owner be messing with the bcm? I read good power after and before them.
Electrical wiring Circuit component Electronic engineering Computer hardware Cable


The ground wire for the fuse block is completely corroded which seems like a good place to start but I still read 0 resistance on it after cleaning. I'll be cutting it and renewing it soon.

Does anyone know of any Bluetooth obd2 phone apps that can communicate with the multiple modules? I'd like to see which modules I can communicate with or not.

I'm willing to trailer this thing to someone's house if you're willing to go over it with me. I can provide a little $$ and imbibment of choice.
 
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