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
