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Speedometer way far off with shift points

Tiny
NOOTROPIC REVIEWER
  • MEMBER
  • 2005 FORD F-150
  • 5.4L
  • V8
  • 4WD
  • AUTOMATIC
  • 221,000 MILES
When driving the speedometer is reading eighty mph when doing about thirty mph. I have checked the speed sensor and gauge cluster, they are both fine. Rear gears have not been changed. What could throw it off this far?
Sunday, September 18th, 2016 AT 3:58 PM

10 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,643 POSTS
Hey,

The speedometer has a microprocessor that goes bad in the cluster which feeds info to the transmission. I would try replacing the cluster first, here is a wiring diagram to show the cluster wiring. Check out the diagrams (Below). Please let us know what happens.
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Tuesday, April 6th, 2021 AT 5:27 PM
Tiny
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Hi Ken,
actually a friend of mine has the same truck and he let me switch gauge clusters with his to see if it would change anything and it didn't.
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Tuesday, April 6th, 2021 AT 5:28 PM
Tiny
KEN L
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Okay, here is the wiring for the VSS is goes into the BCM so it's either bad or there is a bad connection somewhere causing high resistance in which case you will need to do a pin to pin check here is a guide to help you.

https://www.2carpros.com/articles/how-to-check-wiring

Let me know hat you find

Best, Ken
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Tuesday, April 6th, 2021 AT 5:28 PM
Tiny
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It's definently not a bad vss sensor as i've tried 3 different ones and none have changed anything. I'll look into the wiring link you gave. Also, do you know where I could find the wiring diagram for the electric 4x4 actuator motor?
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Tuesday, April 6th, 2021 AT 5:28 PM
Tiny
KEN L
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Yep I saw that, I meant the BCM was bad, here is the rest of the wiring you needed, let me know what you find out.

Best, Ken
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Tuesday, April 6th, 2021 AT 5:28 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
Just to add to this one have you checked to see what the scan tool reads for speed compared to the speedometer?
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Tuesday, April 6th, 2021 AT 5:28 PM
Tiny
KEN L
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Good call
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Tuesday, April 6th, 2021 AT 5:28 PM
Tiny
NOOTROPIC REVIEWER
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Okay, so I've done some testing and learned some stuff. I used my obd 2 reader to check the speed and it corresponded to the gauge cluster so it's definitely an issue at the pcm or before it. Also I tested the 2 wires in the vss plug for ohms like you said above and they both came back with about.5 ohms so they should be fine. If these wires are fine then doesn't that only leave the pcm to be at fault? Or is there something I'm forgetting?
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Tuesday, April 6th, 2021 AT 5:28 PM
Tiny
KEN L
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Its sounds like you got everything and the PCM is the fault. It happens, please let us know if that fixes it so it will help others

Best, Ken
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Tuesday, April 6th, 2021 AT 5:28 PM
Tiny
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Got my new pcm in. Swapped it all out and now it is working thank you.
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Tuesday, April 6th, 2021 AT 5:28 PM

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