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.
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.
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.
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
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.
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.
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.
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.
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.
