Speedo / Shift Problem
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More Turbo
Joined: Jun 2007
Posts: 537
Likes: 3
From: Carvel, AB, Canada
Speedo / Shift Problem
Well I just finished replacing my front axle u-joints and needle bearings. Took the truck for a test drive and noticed the truck shifting late. Looked at the speedo and it's only showing about half of the actual speed I was travelling. I know during the service I disconnected and reconnected the ABS sensors in the front, but not the vehicle speed sensor - that I always thought was on the rear axle housing. Now the truck shifts like @#$% and my speedo is off. Anyone know if the ABS sensors are part of the signal to the speedo/transmission and how to test the circuit. I know they send a pulse signal, but what kind of voltage range should the output be?
Post Fiend
Joined: May 2006
Posts: 10,007
Likes: 4
From: Central Florida
If the ABS sensors in the front didn't match the signal from the rear sensor, it should throw an ABS code and light ABS/Brake idiot light. I have to believe they are not related. The VSS is on the pumpkin, and should be the only place the ECM reads speed. If you have the availability, use a code reader to see what speed your ECM is getting and compare it to the speedometer.
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Joined: Jun 2007
Posts: 537
Likes: 3
From: Carvel, AB, Canada
That's what I figured, but I've got no idiot lights on. The strange thing is the half speed reading on the speedo. I figured that if it's checking for a pulse signal then voltage wouldn't really matter as long as it registers. Seeing the speedo move shows that that is happening.
Post Fiend
Joined: May 2006
Posts: 10,007
Likes: 4
From: Central Florida
Seeing the speedo move only means that the computer is telling it to move that much. It's not like the old days when the cluster got it's information directly from the sensors, now the computer reports every to the cluster, if a wire is shorted or you have a ground problem in the cluster it will show different than the ECM reads.
So the VSS maybe giving the correct info to the computer, but the cluster maynot be receiving the same signal.
So the VSS maybe giving the correct info to the computer, but the cluster maynot be receiving the same signal.
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Joined: Jun 2007
Posts: 537
Likes: 3
From: Carvel, AB, Canada
I don't think it's the cluster. A bad signal to the cluster wouldn't make the tranny shift like that. They should be separate signals. The only common one should be the feed to the ECM from the VSS.
