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2001 GMC Sierra speedometer affect transmission shifting?

Tiny
JIM1048
  • MEMBER
  • 2001 GMC SIERRA
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 83,000 MILES
Just purchased this 6.5 turbo diesel truck -the speedometer is broken as it only registers about 1/2 of the actual speed you are traveling. And the tranny shifts really rough. Does the tranny shift by actual speed, RPM's or the speedometer registering speed? Could the abnormal shifting be caused by the malfunctioning spedometer?
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Sunday, November 22nd, 2009 AT 10:36 PM

1 Reply

Tiny
KHLOW2008
  • MECHANIC
  • 41,814 POSTS
Hi jim1048,

Thank you for the donation.

The speed sensor is an important part of the transmission shifting and if it malfuanctions, erratic shifting would occur.

The output from the Vehicle speed sensor is the main signal for the PCM to control transmissionm shifting.

RPM and throttle opening are other subcomponents of the sensors for the PCM to undestand the driving condition to provide appropriate gear shifting.
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Monday, November 23rd, 2009 AT 8:55 AM

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