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Just wondering if anyone else has had this issue with their g6.. Every time I'm on the interstate and I'm going downhill the speedometer goes haywire and starts bouncing on me. By bouncing i mean it'll show that I'm going 70 at the beginning of the hill and while I let off the gas my speedometer goes to 20 mph.. It'll go back up if I either hit the brake or hit the gas. And once it does this my cruise control kicks off. Any thoughts on what it could be?


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Moderator Member Thingy
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That'd be my guess, but I'd wait for someone else to chime in.
 

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Every time I'm on the interstate and I'm going downhill the speedometer goes haywire and starts bouncing on me. By bouncing i mean it'll show that I'm going 70 at the beginning of the hill and while I let off the gas my speedometer goes to 20 mph.. It'll go back up if I either hit the brake or hit the gas. And once it does this my cruise control kicks off. Any thoughts on what it could be?
I'm thinking that may be a problem with the needle motor in the IPC. Since the vehicle speed input is a critical input to the ECM & you are not complaining about any drive-ability issues (except the cruise control disabling), I suspect that the VSS input to the ECM is true. Now if you were also complaining about trany shifting & converter engagement issues I'd say it was definitely the VSS. Maybe you aren't seeing these other issues due to the problem being so intermittent. Easy way to check is with a scan tool that can read real-time sensor data. A simple code reader can't do this. Any repair shop has this capability. You would want to check the VSS value during an intermittent event...i.e.-while recreating the issue during a drive. Remember, even if a scan tool reading shows a faulty speed sensor signal, you will still need to check wiring and connectors before condemning the VSS.

If it is the needle motor, you are looking at a new IPC unless you are a very resourceful electronic tech. IIRC, the dealer gets in the $400 neighborhood for them. IIRC you may be able to get them directly from GM's supplier, a company called Specmo, for somewhat less.

If it's the VSS, they are less than $25.

I would first want to know what the real-time speed sensor data shows before doing anything. Since the ECM receives the speed sensor data & passes it to the BCM which in turn passes it to the IPC, it's a complex chain of events & I could be wrong about the IPC as the culprit. There are rarely easy answers on computer-controlled cars.
 
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