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?
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Sunday, September 18th, 2016 AT 3:58 PM

17 Replies

Tiny
MHPAUTOS
  • EXPERT
A different differential will cause the speed reading to alter and will larger tyres, have these been changed?
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Tuesday, September 20th, 2016 AT 2:06 AM
Tiny
NOOTROPIC REVIEWER
  • MEMBER
The tires are the stock size and so is the diff. I'm starting to think that it's an electrical issue, but I don't know how to check this and I can't find a wiring diagram for this anywhere online.
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Tuesday, September 20th, 2016 AT 4:47 PM
Tiny
KEN
  • ADMIN
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

Please let us know what you find so it will help others.

Best, Ken
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Thursday, September 22nd, 2016 AT 5:20 PM
Tiny
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  • MEMBER
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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Thursday, September 22nd, 2016 AT 5:40 PM
Tiny
KEN
  • ADMIN
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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Thursday, September 22nd, 2016 AT 6:08 PM
Tiny
NOOTROPIC REVIEWER
  • MEMBER
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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Thursday, September 22nd, 2016 AT 6:13 PM
Tiny
KEN
  • ADMIN
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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Thursday, September 22nd, 2016 AT 6:18 PM
Tiny
SATURNTECH9
  • EXPERT
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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Thursday, September 22nd, 2016 AT 6:33 PM
Tiny
KEN
  • ADMIN
Good call
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Thursday, September 22nd, 2016 AT 7:05 PM
Tiny
NOOTROPIC REVIEWER
  • MEMBER
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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Friday, September 23rd, 2016 AT 3:36 PM
Tiny
KEN
  • ADMIN
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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Saturday, September 24th, 2016 AT 12:22 PM
Tiny
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  • MEMBER
Got my new pcm in. Swapped it all out and nothing changed. What a bummer. Any idea on what I can do next?
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Thursday, October 6th, 2016 AT 6:43 PM
Tiny
KEN
  • ADMIN
It sounds like you have high resistance or a bad ground in a wire somewhere, please use wring diagrams and recheck the system doing a pin to pin check again, we are missing something.
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Thursday, October 6th, 2016 AT 9:22 PM
Tiny
NOOTROPIC REVIEWER
  • MEMBER
So one thing I've noticed from driving is that once started it doesn't read any speed for a few second. Then it says trans fault and gives a bunch of alerts. It gives these same alerts when there the speed sensor is unplugged. After driving for 30 seconds it recognizes the speed sensor and shoots up to like 80 mph. I don't know what this tells me.
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Sunday, October 9th, 2016 AT 6:07 PM
Tiny
SATURNTECH9
  • EXPERT
Are there any codes being set from all the warning lights coming on?
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Monday, October 10th, 2016 AT 7:03 AM
Tiny
NOOTROPIC REVIEWER
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Yeah. It throws codes p2106 and p720.
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Monday, October 10th, 2016 AT 6:26 PM
Tiny
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  • MEMBER
Still haven't found a solution. Does anyone know if the abs speed sensor in the front wheels could affect the speedo?
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Saturday, October 22nd, 2016 AT 7:45 PM

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