06 Cobalt LS, No Crank No Start and Weird Electrical Issues

Itjustruns

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It's been a long time since I posted. I just wanted to let anyone who has this issue know how to fix it.

First of all going down the line. Check fuses, in the passenger front, on the left near the floorboard there is a panel that goes to your BCM, and it has fuses and connectors. Check them all. Under the hood check your fuse box, and make sure to check all of your relays as well. Moving on, make sure your battery is actually good. Have it load tested and make sure it has sufficient charge, weak batteries can make these cars act all sorts of messed up. Check to see if your car has the ignition recall. The ignition cylinders/switches for these cars are notoriously junk and can cause many weird issues. Make sure your car doesn't have remote start and if it does check the wiring, remote starts can short out weird things, mostly aftermarket we are talking here. These are the basics you should check before diving in deeper.

The ground wires from the ECM and TCM share the same ground G105(picture of location below). If you google this ground you will find it grounds MANY important electrical components. It's on the front of the transmission housing, very accessible other than a thick coolant hose that can get in the way of taking it off, I just man handled it to the side. There are two terminals that are connected to it, one is very thick and goes to the body to act as a secondary ground, and the other has 5 small wires connected to it, that feed back into the main harness. Two of these grounds go to the ECM and TCM. they will be black and white for both. The ground is in 90% of the cases the issue. Make sure the wire isn't brittle and breaking, corroded, and has good connection. If this doesn't fix your issue, trace the grounds back to their sources and make sure they aren't broken along the way. If this still doesn't fix your issue, there is a white/red wire that goes to both the ECM and TCM. This is the positive power wire for both. Splice them together/check it isn't broken along it's path.

Physical Picture: https://onedrive.live.com/?authkey=!ABf9Tf2qYjaAzJ0&cid=26AFF792CB22F291&id=26AFF792CB22F291!21342&parId=26AFF792CB22F291!16657&o=OneUp
Diagram: https://onedrive.live.com/?authkey=!AE8_3dJdwZ0WL8Y&cid=26AFF792CB22F291&id=26AFF792CB22F291!21340&parId=26AFF792CB22F291!16657&o=OneUp

If you have to ground your TCM through the body it is a symptom of a growing problem, not the permanent solution. If you decide to ground your TCM to a known good ground or the strut tower like most people on youtube show(video link below), and it fixes your issue, G105 is probably not getting great connection, in rare cases the internal ground in the TCM goes bad and adding a secondary ground is the only solution.

Link:


If this does NOT fix your problem, there are a few places to look going forward. If the car cranks but doesn't start, G105 is your issue. The car has to have at least a connection to the TCM to crank, and adding a secondary ground let's it complete it's circuit. The car has to have a working ECM to start, so the idea is G105 is getting horrible connection to both of these vital components, however the added ground to the TCM is allowing the car to crank.

If you add the secondary ground and it doesn't even crank.....triple check all fuses and relays, check the connectors for the TCM,ECM,BCM and make sure the pins aren't broken and are clean, make sure your neutral safety switch isn't bad, make sure ignition switch/cylinder isn't bad, triple check G105, check to make sure the bolt that holds the terminals on the fuse block isn't corroded and those wires are getting good connection/aren't broken, make sure the connections on the bottom of the fuse block are good, and the block itself isn't cracked/broken, on rare occasions the ground on the strut tower doesn't get good connection so try a better ground location, there is one under the driver side headlight that goes to, you guessed it, G105. However this is a lot thicker of a wire and terminal so doesn't lose connection nearly as often. Voltmeters are your friends here, check EVERYTHING to make sure you have a complete circuit. If all else fails and you still have no result, it's time to take it to an electrical shop, because a computer is more than likely fried. TCM's and ECM's are VIN matched so random ones will not work, they have to be programmed for the correct VIN, there is a great company called Garrett Tuning that will do this for a hair over $100 currently.

In my case, G105 was corroded to the point the wires broke off taking it off, however it already shorted the TCM and fried it. got a programmed TCM from Garrett. Had a relay that shorted out on the fuse box as well, and also had a burnt radio fuse on the BCM, top left third fuse down, 15 AMPs. This fuse also has to do with the power locks for the doors and the car completely spurted out with the added TCM ground doing this so I figured it just was overloaded from shorting. to fix the ground, I took the terminal to a wheel brush and cleaned it up, grabbed some thick copper wire and crimped it down to the terminal, soldered it for good measure, stripped wire off of all 5 of the grounds it connected to, then soldered the thick wire to them and insulated the exposed wire.

I wanted to make this detailed post because I have seen this issue everywhere on the internet, to the point where it might as well have been a damn recall for this issue, and nobody really has an answer or in depth problem solving guide for it other than "run the second ground and if it doesn't work your TCM is fried." Too many people have dumped hundreds if not thousands into dealership visits and computers for an issue that costs potentially a few wires, at worst a few hundred bucks.

Link to Garret's here: https://www.garretttuning.com/collections/factory-gm-ecm
They have compatibility lists and number matching for each product.

Another person who had this issue had a few codes and hard shifting afterwards, here was his fix:

cobaltfixstuff.PNG
Quick link to the fuse location: https://onedrive.live.com/?authkey=...91!16680&parId=26AFF792CB22F291!16657&o=OneUp
 
Last edited:

Jabber

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It's been a long time since I posted. I just wanted to let anyone who has this issue know how to fix it.

First of all going down the line. Check fuses, in the passenger front, on the left near the floorboard there is a panel that goes to your BCM, and it has fuses and connectors. Check them all. Under the hood check your fuse box, and make sure to check all of your relays as well. Moving on, make sure your battery is actually good. Have it load tested and make sure it has sufficient charge, weak batteries can make these cars act all sorts of messed up. Check to see if your car has the ignition recall. The ignition cylinders/switches for these cars are notoriously junk and can cause many weird issues. Make sure your car doesn't have remote start and if it does check the wiring, remote starts can short out weird things, mostly aftermarket we are talking here. These are the basics you should check before diving in deeper.

The ground wires from the ECM and TCM share the same ground G105(picture of location below). If you google this ground you will find it grounds MANY important electrical components. It's on the front of the transmission housing, very accessible other than a thick coolant hose that can get in the way of taking it off, I just man handled it to the side. There are two terminals that are connected to it, one is very thick and goes to the body to act as a secondary ground, and the other has 5 small wires connected to it, that feed back into the main harness. Two of these grounds go to the ECM and TCM. they will be black and white for both. The ground is in 90% of the cases the issue. Make sure the wire isn't brittle and breaking, corroded, and has good connection. If this doesn't fix your issue, trace the grounds back to their sources and make sure they aren't broken along the way. If this still doesn't fix your issue, there is a white/red wire that goes to both the ECM and TCM. This is the positive power wire for both. Splice them together/check it isn't broken along it's path.

Physical Picture: https://onedrive.live.com/?authkey=!ABf9Tf2qYjaAzJ0&cid=26AFF792CB22F291&id=26AFF792CB22F291!21342&parId=26AFF792CB22F291!16657&o=OneUp
Diagram: https://onedrive.live.com/?authkey=!AE8_3dJdwZ0WL8Y&cid=26AFF792CB22F291&id=26AFF792CB22F291!21340&parId=26AFF792CB22F291!16657&o=OneUp

If you have to ground your TCM through the body it is a symptom of a growing problem, not the permanent solution. If you decide to ground your TCM to a known good ground or the strut tower like most people on youtube show(video link below), and it fixes your issue, G105 is probably not getting great connection, in rare cases the internal ground in the TCM goes bad and adding a secondary ground is the only solution.

Link:


If this does NOT fix your problem, there are a few places to look going forward. If the car cranks but doesn't start, G105 is your issue. The car has to have at least a connection to the TCM to crank, and adding a secondary ground let's it complete it's circuit. The car has to have a working ECM to start, so the idea is G105 is getting horrible connection to both of these vital components, however the added ground to the TCM is allowing the car to crank.

If you add the secondary ground and it doesn't even crank.....triple check all fuses and relays, check the connectors for the TCM,ECM,BCM and make sure the pins aren't broken and are clean, make sure your neutral safety switch isn't bad, make sure ignition switch/cylinder isn't bad, triple check G105, check to make sure the bolt that holds the terminals on the fuse block isn't corroded and those wires are getting good connection/aren't broken, make sure the connections on the bottom of the fuse block are good, and the block itself isn't cracked/broken, on rare occasions the ground on the strut tower doesn't get good connection so try a better ground location, there is one under the driver side headlight that goes to, you guessed it, G105. However this is a lot thicker of a wire and terminal so doesn't lose connection nearly as often. Voltmeters are your friends here, check EVERYTHING to make sure you have a complete circuit. If all else fails and you still have no result, it's time to take it to an electrical shop, because a computer is more than likely fried. TCM's and ECM's are VIN matched so random ones will not work, they have to be programmed for the correct VIN, there is a great company called Garrett Tuning that will do this for a hair over $100 currently.

In my case, G105 was corroded to the point the wires broke off taking it off, however it already shorted the TCM and fried it. got a programmed TCM from Garrett. Had a relay that shorted out on the fuse box as well, and also had a burnt radio fuse on the BCM, top left third fuse down, 15 AMPs. This fuse also has to do with the power locks for the doors and the car completely spurted out with the added TCM ground doing this so I figured it just was overloaded from shorting. to fix the ground, I took the terminal to a wheel brush and cleaned it up, grabbed some thick copper wire and crimped it down to the terminal, soldered it for good measure, stripped wire off of all 5 of the grounds it connected to, then soldered the thick wire to them and insulated the exposed wire.

I wanted to make this detailed post because I have seen this issue everywhere on the internet, to the point where it might as well have been a damn recall for this issue, and nobody really has an answer or in depth problem solving guide for it other than "run the second ground and if it doesn't work your TCM is fried." Too many people have dumped hundreds if not thousands into dealership visits and computers for an issue that costs potentially a few wires, at worst a few hundred bucks.

Link to Garret's here: https://www.garretttuning.com/collections/factory-gm-ecm
They have compatibility lists and number matching for each product.

Another person who had this issue had a few codes and hard shifting afterwards, here was his fix:

View attachment 3396
Quick link to the fuse location: https://onedrive.live.com/?authkey=!ANTBr_uQFLfOLpg&cid=26AFF792CB22F291&id=26AFF792CB22F291!16680&parId=26AFF792CB22F291!16657&o=OneUp
This is an old post, but let me just say thank you for this post. My teenage son has an 09 Cobalt and the check engine light came on, It was throwing ECM and TCM codes, tire sensor light, then it was sometimes not starting or even turning over, just nothing and it finally quit starting all together. I tried the ground the TCM/ECM to the strut bolt hack and nothing.

I found the ground you mentioned on the front of the transmission and sure enough, corroded, some wires barely holding on. Mine actually had 6 wires, but stripped them, new connector and put it on temporarily tonight and fired right up with no codes thrown. Knock on wood, this is the correct fix over trying to ground the TCM. Now to solder and make a heavy duty connection that is waterproof so this doesn't happen again.

Not sure if you'll see this since your post is a few years old, but thank you for posting such a detailed fix.
 

Shaggy

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This is an old post, but let me just say thank you for this post. My teenage son has an 09 Cobalt and the check engine light came on, It was throwing ECM and TCM codes, tire sensor light, then it was sometimes not starting or even turning over, just nothing and it finally quit starting all together. I tried the ground the TCM/ECM to the strut bolt hack and nothing.

I found the ground you mentioned on the front of the transmission and sure enough, corroded, some wires barely holding on. Mine actually had 6 wires, but stripped them, new connector and put it on temporarily tonight and fired right up with no codes thrown. Knock on wood, this is the correct fix over trying to ground the TCM. Now to solder and make a heavy duty connection that is waterproof so this doesn't happen again.

Not sure if you'll see this since your post is a few years old, but thank you for posting such a detailed fix.

This is really awesome to hear. I'm glad this post has helped you out and this is another reason why I will never shut the forum down, as there is just to much good info on here to lose.

Thanks again!
 

CaptBanana

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Massachusetts
I created an account just to chime in here. While I didn't find my answer on this forum, I just wanted to add some color.

Symptoms I experienced:
- Check engine light
- POWER STEERING message, loss of power steering
- Occasional strange shifts / hanging revs
- Automatic lock/unlock when entering/exiting park wasn't working
- Every time I hit the brake while in drive, the shift interlock would click

Codes:
- TCM communication, basically everywhere
- Vehicle speed sensor, power steering module
- TPS performance, engine control module
- One or two others

I took a look at G105 and low and behold, a gentle pull is all it took for the entire bundling of grounds to come off of the grounding lug:
IMG_5590.jpg

Solution:
1. Stripped all grounds back 1/2", twisted together & soldered.
2. Crimped on a new lug I got at the hardware store - closest they had was 6awg, 1/2" hole.
3. Soldered the wire bundle to the lug & heatshrinked the crimped area to discourage water ingress
4. Re-assembled, started it up, and cleared all the codes.

Given it took 15 New England winters and 180k miles to become an issue, this fix should easily last until the car's retirement :)
 

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