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Speedo not going back down to zero

 
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Old 01-17-2012, 01:24 PM
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Default Speedo not going back down to zero

I am having an issue with my 2003 X-type where the speedo is not going all the way back to zero, and as I stop it continues to not go back all the way. This makes the zero point creep up. It had been creeping up to be about 25mph high, but this morning it decided that 70mph would be my starting point, so it said I was doing 130 down the freeway.

When I turned the key off at work it went to 70, then I kept turning the key on and off and it slowly stepped back down to 0, after 8-10 on and offs. Whatever is causing this does not seem to affect the trip computer, the odometer seemed to go up like it should.

Any ideas what is causing this?
 
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Old 01-17-2012, 04:28 PM
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Rodney, it is sounding like you are having an issue with the computer that controls the instrument cluster. It sounds like to me that the biasing for the speedo is getting thrown out of whack which is causing what you are seeing. Think of it this way. The computer doesn't know the difference if you have the speedo at 0 mph for a 0VDC indication or if the computer fails and it is giving a 0 VDC indication. So, to allow the computer to be able to sense if it has a failure, it is set up to make say a 1 VDC signal the 0 MPH mark (hence why you will see the speedo drop below zero when starting the car some times). Then the computer adds a certain amount more voltage to the 1 VDC to give you your MPH as you are moving. If for some reason the 1 VDC signal starts drifting, it will cause the 0MPH point to move too. Unfortunately, this is all controlled inside of the instrument cluster.
 
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Old 01-18-2012, 10:53 AM
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Thank you for the explanation. I take it from what you are saying that to fix this issue would require replacing the instrument cluster, which presumably is not the easiest or least expensive thing to have to do. Could there alternatively be an issue with the wiring from the gauge to the computer?
 
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Old 01-18-2012, 10:07 PM
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Rodney, if there was a problem with the computer output, you should be seeing your mileage jump way up when the speedo is acting funny. So, you may want to reset the MPG for say "Trip B" and then see what the mileage does. If you are pulling something like say 40 MPG, then that would be a hint that the problem is with the computer, not the instrument cluster.
 
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Old 01-19-2012, 10:59 AM
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As far as I can tell at the moment, the odometer seems to be correct, tach is good too. I have been monitoring the trip info and seeing as how far the speedo has been off these last few days I am pretty sure I would be able to tell by now if this was wrong. I do not know how the odometer, vs the speedo, vs the tach get fed their information, but I was just wondering if I could have a poor ground somewhere that is preventing the full voltage drop on deceleration. It's weird because the speedo increases as I accelerate correctly, and the speed as compared to the starting point is correct, it is just that as I slow down I can see the needle start to drop then it gets a hit where it jumps back up a fraction, and then continues down, but every time I slow to a stop this little hit pushes up the baseline zero point.
 
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Old 01-19-2012, 04:00 PM
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Rodney, the instrument cluster has a common ground. So, if it is a ground issue, you would be seeing a problem over the whole cluster, not a single gauge. As for the speedo, there are 4 signals that are generated by the ABS sensors in the wheels. These are sent to the ABS module where the signals are processed, averaged, and then converter to a signal to be put on the CAN bus. Once on the CAN bus, the various other computers can look at the information and pluck off what information they need and when. In this case, the speed information is sent to the instrument cluster, the ECU, and the TCM (tranny computer). Since you are not talking about the car shifting funny (it would be really funny), the data on the CAN bus is good. Therefore that pretty much only leaves an issue with the instrument cluster. You can take the car to a dealership and they can do a diagnostic on the instrument cluster and see what they tell you, but plan on a $100 bill for that service.
 
 
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