Ok everytime I am stopped at a light or even when my car is in park, my speedometer begins to jump up and down, usually it will jus eventually stop, yesterday the engine started to rev then the car just shut off while I’m just sitting at a light, then my headlights began darken right before it shuts off. Can someone give me an idea on what could the problem. 2008 Chevy Cobalt
Any warning lights on?
I take it you had the ignition switch replaced? If so, check battery connections are clean and tight. Also, if this is the original battery, get a new one.
Start by having the fault codes read. Many parts stores will do this for free. Post them here.
I’d bet lunch that your Vehicle Speed Sensor is bad… your speedo should not be jumping up and down when you’re parked and its doing so eliminates a lot of possibilities, and the VSS will affect engine operation, but don’t start buying parts based on my wild hunch. Start with the codes.
Sounds like the battery, alternator or corroded cables.
It’s normal for the headlights to darken a little as the engine comes to a halt. The voltage supplied by the alternator with the engine running is higher than by the battery alone. I doubt that is diagnostic.
The question is: What’s causing the engine to die while idling? I guess where I’d start is a quick visual check in the engine compartment for anything obviously wrong, then measure the idle rpm and compare to what it should be. After that, as mentioned above, check for stored ECM diagnostic codes, and review the actual routine maintenance schedule vs what Chevy recommends. Anything behind schedule is a suspect.
To my mind the speedo jumping when in park has to be a meaningful clue. Note that if the OP’s description is accurate, and I have no reason to believe otherwise, it’s the SPEEDO that’s jumping around and not the tachometer. The tach jumping around would be driven by the engine, but the speedo should sit at zero even if the engine is revving and dropping. A bad VSS could cause the engine to wander, but I know of no way a wandering idle could cause the speedo to go up and down when parked.
Anyway, that’s my logic.
I believe that mountainbike’s logic is sound and that the problem is most likely a defective VSS.