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

[Itjustruns]

So yesterday I bought this cobalt for 200 bucks. has 120k. It was stranded at a gas station where the previous owner shut it off to pump gas and it wouldn't turn back on. The previous owner said it had been acting weird for about a week not wanting to start, the speedometer would randomly clunk out, and the transmission wouldn't shift. I jumped the starter posts to get the car started and take it home, so the starter IS good. I checked all the relays, not ALL of the fuses yet but all of the relays on the car are good. The battery fuse is good, the connections to and from the starter are good. The transmission seems healthy and the fluid was at a good level and good color. The car also blinks fast for the turn signals both left and right as if a bulb is out but both sides work front and back fine. I also noticed while the speedo isn't working the RPM's are, and the odometer isn't going up like it's frozen. I'm pretty sure the transmission is in limp mode but I am not familiar enough with Cobalts to know if there should be a dash light or sign other than not shifting. It seems it instantly goes to third gear when you put her in drive. Reverse works, and Low gear puts it in the same gear drive does, which is DEFINITELY not 1st, as I was at about 3-4k around 35-45mph. Key on I get all dash lights and good power, however I notice my fuel pump also doesn't kick in key on. Engine runs AMAZING. I did get a code for a lost connection to TCM, which would explain limp mode.

So anybody with a little Cobalt know how have any idea? Think there is a bad ground completely obliterating all of my electronics? I heard that the fuse board can crack and cause weird problems on these cars. Could a faulty TCM actually cause ALL of these weird problems? I figure it has to be from the fuse board as from my knowledge the only thing that separates the starter from the battery is the fuse board and the relay specifically for it, other than the ignition switch position. I figure my switch is good as they key was on start position when I jumped the posts as I didn't want to risk jumping security.

Any shots in the dark before I tear into it? I was originally planning on scrapping the car, but with so low miles and such a beautiful interior and engine bay, it would be a shame to kill a car with so much life left.

 

[Itjustruns]

Update! I've torn the car apart and checked many things. I checked common grounds, security, and the rest of the fuses on the car. I'm scratching my head here and not making much progress. Multiple relays are clicking pretty bad, still have the code for Lost Connection to TCM. Sometimes when I turn the key to the on position the throttle body opens and closes a few times rapidly, sometimes a few relays click once, sometimes they click half a dozen times, it's random. I highly doubt a bad TCM would cause all of this and I'm about out of grounds to check which leads me to believe I either need to cut open my wiring harness and trace power back to absolutely everything through the harness. The odometer lights and power flickers about as many times as the relays click, it seems to be in sync, checked the battery and even swapped it with a different battery just to be safe. if EVERY single wire in the harness checks out I guess the BCM is bad and I just have to take the hit and scrap the car, as replacement would cost considerably more than I could sell the car for, and it's starting to take to much time. And we all know time is money.

 

[jcaouette207]

Still scratching your head? Because I can tell you that I went through this exact situation about 6 months ago, and a bad TCM can in fact cause every one of your symptoms and is most likely your culprit.. I found a company that I highly recommend who sells TCMs pre programmed to your VIN number for an extremely reasonable price. They also ship super fast. Let me know if you are interested. I've referred two other people to them at this point because of how awesome their service is. It's as simple as plugging it in and driving. No programming needed.

 

[Helixal]

Partial progress?


I replaced the ignition coil module with a Delphi brand unit even though it was sold as an ac delco. But then the one I took out was a Delphi (probably original).


So now I can hear the fuel pump rev up and get a strong spray at the schrader valve. Still turns over but doesn’t start.


Just to test things I disconnected the external connector to the ignition control module, I then turned the key to the on position and I could hear the fuel pump rev up. So here the fuel pump was working even though the new coil module and icm were out of the system.


The only thing that I can deduce from this is that the the new coil module reset something somewhere to allow the fuel pump to engage.


Now I only need a spark.


Any insight would be appreciated.


Alq

 

[Itjustruns]

Still scratching your head? Because I can tell you that I went through this exact situation about 6 months ago, and a bad TCM can in fact cause every one of your symptoms and is most likely your culprit.. I found a company that I highly recommend who sells TCMs pre programmed to your VIN number for an extremely reasonable price. They also ship super fast. Let me know if you are interested. I've referred two other people to them at this point because of how awesome their service is. It's as simple as plugging it in and driving. No programming needed.

Yeah I would be interested. I parked the car for a minute as other projects popped up, but yesterday I did some more diagnostic. I can still get the starter to crank by jumping the solenoid but now the car won't start. I jiggled the car in park and neutral, as well as played with the key in the cylinder while having someone jump the solenoid, still to no avail. The lights don't dim or blip when you try to start the car, so I know for a fact that the circuit is failing before the relay, that I ensured is good. I'm confused about the TCM code but I am suspicious my Neutral Safety Switch might be bad, either that or my Ignition Switch. Both are cheap junkyard buys so I'm probably at least going to try, however I don't see how either could cause a TCM connection code, the only scenario I can see that happening is if my ignition switch was only partially powering the computers, and I don't know if that's even possible, or my neutral safety switch is bugging my transmission and TCM out, and preventing the car from starting. Right now that's my best guess but I have never dealt with a problem similar to this one before, usually it's more cut and clear when one fails, this is just a confusing game of chasing problems.

So a failing TCM can cause a no start no crank, and will prevent the car from starting even jumping the solenoid? If all else fails I'm definitely interested in this service you mentioned. And if anyone has any idea, it would be much appreciated. I should have scrapped this car but honestly I love a good challenge and a part of me wants to fix it even if I lose money on the car.

 

[Dennis Schiller]

Re check all the grounds. Test for + AND - at the coil module.
Look at the grounds all clustered together at the fron of engine.
You may need to add a ground to ECM aluminum case, internet states jump starting a Cobalt fries the internal ground.

fyi, I'm trying to sell mine before IT craps out!

 

[Itjustruns]

Re check all the grounds. Test for + AND - at the coil module.
Look at the grounds all clustered together at the fron of engine.
You may need to add a ground to ECM aluminum case, internet states jump starting a Cobalt fries the internal ground.

fyi, I'm trying to sell mine before IT craps out!

Thanks for the info, I'll double check the grounds and check the positive, I did add an extra ground to the tcm as I saw that was a common problem.

 

[Itjustruns]

Yeah so I've been working on other projects debating getting a new TCM for the car. Checked every wire in the harness and everything checked out, ign switch checked out neutral switch checked out. I found a good service that does pre programmed TCM's to match your vin for 100 bucks. Going to install it and let anyone who comes across this know if it's plausible that a bad TCM can tank your car and cause the issues I was having.

 

[Spiike]

I'm having one of your symptoms. Where I drive some where, turn the car off and the car won't start, battery is fine. So every 10 minutes I try again and usually 30 minutes later it's fine and starts up. Also threw a p0300 code. Took to GM and they replaced the ignition pack. Cleaned up the computer connections with dielectric grease spent over 700. At this point 220000 km on the car im ready to trade it in. Occasionally it will lose power and seconds later or minutes later the engine power comes back and we are good to go. With the cel blinking. But having a reliable car is my main concern. I can't be stuck in traffic with no engine power. And I can't be half hour late to appointments.

 

[Itjustruns]

Alright so the verdict is HALFWAY in, replaced the TCM and nothing changed, added the extra ground to the new TCM and the car would crank, but not start. Old TCM has been thoroughly tested as bad, and my grounds for my TCM are good, so the need for an added ground is absolutely crazy to me. No idea what happened or is happening to cause that. Also, car will STILL not start. Cranks like a champ though. Security light was on for first key on, then it went away. Gonna go out and check the spark and fuel to see what's going on.

 

[Itjustruns]

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:


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

 

[Jabber]

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]

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]

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:


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