Hi everybody--


New member here (and new to car maintenance/repair)-- I've been poring through the forums and they've been a great help in trying to get a problem resolved with my '08 Cobalt (~60,000 miles), however I'm currently at a loss.


About a week and a half ago, I noticed a lurching motion when the transmission would shift gears. I didn't think much of it but the next morning, I went to start the car on a cold morning and got an array of issue-- no reading on my speedometer or fuel gauge, no external temperature reading, the vehicle wouldn't shift out of first gear, and I had a power steering warning. I limped it to a nearby repair garage and after a check by the mechanics was told to take it to the Chevy dealership as they best they could figure was that it was a bad computer module.


I ended up poking around on my own, after looking here and elsewhere, and found that the vehicle speed sensor output wires had been completely severed. I went to the local dealership, got a new output plug with wire (the original plug's wire had severed so close to the plug itself that a solder didn't seem feasible). I connected the wires temporarily with twist-on wire connectors and took the car around the block-- speedometer was working again, all gauges and systems seemed to be functioning normally, the vehicle was shifting gears appropriately. I parked the car back in my garage and left it for a couple days, figuring I'd do a more permanent connection and clean up after Christmas.


I went to install new butt connectors yesterday and once I did so, I went to start the car. The vehicle wouldn't start-- when I tried to turn the Cobalt over, I get a single click, the headlights turned on, radio, etc, but no crank from the engine. Eventually, I was getting no power at all-- I took my battery to O Reillys and it tested for a bad cell. I installed a replacement battery and got power to the lights, radio, etc again, but still no crank.


I've checked power connections, cleaned acid from the negative battery terminal, tested various fuses related to the ECM and ignition systems, but haven't been able to dial down the issue. OBDII is showing a U0100 error. I'm close to giving up and having the car towed to the dealership, but was hoping there might be something I've missed. I'm kicking myself for not trying to start the car *before* I removed the twist-on connectors, so I'd at least know if something happened in the two day interval since it was working, or if I somehow loused something up when I put on the butt connectors (whatever that might have been).


A couple additional notes-- am not sure if these will be of any help, and--to reiterate-- I'm completely new to car repair, but thought I'd throw these out there in case they do mean something: I checked the fuses with a circuit tester. The ECM/Trans circuit gave a reading when the car was off, but not when the ignition was turned to the on position. In addition, the circuit tester was lighting up when I made contact with most metal components in the engine compartment except the metal section along the inside of the the throttle position sensor and also the PCM(?) (I'm having a bit of trouble delineating some of the systems. There's a rectangular metal box with a single connection along the front of it just in front of the fuse box. In front of that, and dropped a bit lower, is what looks like a second computer with two cable inputs attached. This second one has no power read from the circuit tester unless a ground wire is running from the ground chassis to it. Ditto the TPS).


Thanks in advance for any help.

 

Wow, an actually descriptive and literate post and nobody responded.

Ill try to help, I know you said you checked the fuses with a tester, and you mentioned one was only good under certain conditions, did you visually inspect the fuse? It should be easy to tell if the circuit is broken inside just by looking. If you can overlay a power/ground wire and things start working properly then you just found your issue.

How did the VSS wiring get damaged? It sounds like there is other potential damage to the wiring that was not seen on initial inspection. Got any critters getting in there to have a snack and stay warm for the winter maybe?

 

Hi Slowbalt--

Thanks for the response! Yes, like I said, I'm new to car repair so I figured as much specificity as possible would be good since I'm not sure what's notable and what's not.

Yes, I did visually inspect the fuses-- both in the under-hood fuse box and passenger-side interior box. No breaks that I could see, and I even swapped like-for-like (known working) fuses to double-check. I'll plan to do a check of all the fuses in the box-- not just the relevant ECM/Starter related fuses, just to do due diligence though.

I'm not sure how the VSS cable was damaged-- it was a clean cut right below the input plug, as if it had been severed, so it's possible that something chewed on it. I did a visual inspection of the other cables and was not able to see any additional damage, though I'll doublecheck that as well.

One additional note that I forgot to include in my original post: I get both low fuel (despite a full tank) and power steering warnings on the dash on attempted startup. My perusing of the forums seem to point to the no crank/only click, power steering, low fuel combo being a bad ECM fuse but am not sure what else to check after looking at fuses, swapping known good fuses into ECM spots, etc.

I have a fellow coming out next Tuesday to take a look at the car, but would certainly appreciate any further guidance prior to that point. Thanks again for your feedback and advice!

 

You could also check the ECM/powertrain main relay too by swapping with a fan or horn relay.

Those messages appearing on the dash are because the bus has no communications from the ECM to tell it what the fuel level is, those warnings are common with can bus issues.

 

Tried swapping the ECM/PT main relay but no dice there.

One thing I have noticed-- and again, this may be nothing-- but I tested the two computers racked up in front of the fuse box using a multimeter, with negative lead hooked to ground. The rear computer measured a steady 12V or so. The front computer (with the two input clips and the ridged surface) is spiking all over the place. Again, may be nothing but I thought I'd note that as well.

Thanks!

 

The one with two connectors is the engine controller, the single connector is the transmission controller.

Is the circuit you are testing for sure the main power to the ECM? It sounds like you may have some resistance in your ECM power wire. If you add a new(preferably fused) wire from battery positive to the ECM it should fix the issue. If it does, then you can go about replacing the broken/damaged factory wire.

Also just a note, make sure there are no damaged pins both on the ECM itself and on its connectors, especially for the circuit in question.

 

To clarify what I'm doing-- I'm touching the negative lead of the multimeter to ground and touching the positive lead directly to the metal portion of the computer. TCM reads a steady ~12 V, the ECM is bouncing all around, anywhere from 4 V up to 10 V. So it sounds like I may be testing this in the wrong way since I'm not going directly to a circuit?

I've previously tried to remove the ECM connectors to check the pins, but they're not providing a lot of give when I try to unclasp them. Concerned about breaking the plastic on the connectors when I try to remove them, but I can try again.

Thanks!

 

Youre touching the TCM outer metal casing and getting 12v when the other lead is touching, say, the cylinder head? That should definitely not be happening. If im understanding correctly, it sounds like something could be shorted in the TCU which is sending electrical power to its case... strange for sure.

To disconnect the ECM connectors you have to slide that red tab out first, then press the black tab with your thumb to release.

 

Posting this here so the conversation flows right.

Okay, that is definitely not the proper way of testing, but I think you are on to something. If you are running from the power post on the fuse box to the ecm case and getting 12v, that must mean the ecm has a good ground. If the TCM seems flaky, overlay a new wire on the actual ground circuit to chassis ground and you may find your solution. Maybe Kolt can help out with some wiring diagrams? You may even be able to run a jumper wire from the TCM case to chassis ground, but I wouldnt recommend it because im not sure if this could damage the TCM. Do at your own risk.

 

@Kolt can you help?

 

To clarify-- TCM is consistent, ECM is not (I think we got our wires crossed up there). I did run a wire from the ground chassis and tucked it behind the ECM. Now, positive lead to jump post+negative lead to ECM gives me a mid-11 V, steady reading.

Tried to start vehicle again-- still just a click. Possibly of interest-- the Power Steering warning remains, but the Low Fuel warning is no longer appearing.

Thanks!

 

A follow-up--

Checked the ECM this morning and was reading a voltage of just under 12V. Still no start. I went ahead and removed the ground wire, then removed the front connectors one at a time, testing voltage on the ECM both times-- still spiking in both cases (since I'd removed the ground wire). I checked voltage from the large (power?) pin on both connectors, and got a steady 12V flowing *out* from those.

 

If youre getting a click isnt that better than before? If youre reading under 12volts now, you need to charge your battery.

 

No, I've been getting a single "click" on startup throughout this whole process.

Getting a 11.95-12.04 V on everything *except* the ECM (TCM, ECM input cable power pins all read at a full 12V) using the process I noted yesterday.

I'm wondering at what point I'd need to consider the possibility that the ECM itself had gone bad. I notice that when I turn the key to the On position, the lights on the console all flash briefly except for the check engine light (I thought a flash on startup was normal). I feel that it would certainly be odd for the ECM to go out commensurate with that bad VSS cable but am reaching the point, after 7-10 days where I may need to just have it hauled to a dealership to be examined.

 

A fully charged battery should read 12.7-12.8 volts, I think thats your problem.

 

I just used the multimeter to check the voltage on the battery-- checking against the positive and negative terminals, I'm getting about 11.8V-- this is on a brand new battery I purchased less than a week ago. The previous battery (unsure of its age, but it was in use at the time the no-start issue first began) had a dead cell according to the auto shop so I purchased a new one. Still no start when I brought it home from the shop and hooked it up. Unable to start doing a jump either.

The previous battery did have what looked like severe corrosion along the negative terminal and cable. I cleaned the battery cable connector with corrosion cleaner and also warm water/baking soda. Testing voltage from the terminal to the connector cables showed a consistent flow of power though, as we're discussing, it's not as high as it should be.