Electrical 2000 Mustang Gt Speedometer Issue!

Dillon Terry

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Sep 18, 2016
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How's it going boys, this is my first post on stangforum, but I've been on for weeks reading other posts and searching and so far I didn't find anyone solving this problem.

2000 GT Mustang with a 5 speed manual.

When I accelerate down the road the speedometer doesn't start working until I'm going about 45 miles an hour and it only says I'm doing about 8.
The fastest I've gone down the road was about 80 and it said I was doing about 30 miles an hour.
Yes my car has stock gears..... I think but,
it shouldn't make that big of a difference anyway.
When I reset all the gauges by disconnecting the battery the needle resets goes all the way to 150 and back down to 0 and I've even taking the cluster out took the whole thing apart and checked the back of the speedometer where the armature is, checked all the wires, nothing looked out of place.
so the cluster is not my issue.
Next day I went down to O'Reilly's and got anew speed sensor that single bolts to the side of my transmission. The old one looked fine and the gear didn't appear to be worn AT ALL!
I did have to put the old gear on the new part, because it doesn't include one.
I've checked all fuses I traced the wiring back as far as I can without getting ridiculous and Ican't find any imperfections.. Is there a ground wire that I could check? Another fuse or relay?

I can't drive this car properly without aspeedometer.. IT SUCKS just having a tach.

Is there anyone that has had this problem or is seen it before that can help me!?
 
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Dillon Terry

New Member
Sep 18, 2016
7
0
1
24
How's it going boys, this is my first post on stangforum, but I've been on for weeks reading other posts and searching and so far I didn't find anyone solving this problem.

2000 GT Mustang with a 5 speed manual.

When I accelerate down the road the speedometer doesn't start working until I'm going about 45 miles an hour and it only says I'm doing about 8.
The fastest I've gone down the road was about 80 and it said I was doing about 30 miles an hour.
Yes my car has stock gears..... I think but,
it shouldn't make that big of a difference anyway.
When I reset all the gauges by disconnecting the battery the needle resets goes all the way to 150 and back down to 0 and I've even taking the cluster out took the whole thing apart and checked the back of the speedometer where the armature is, checked all the wires, nothing looked out of place.
so the cluster is not my issue.
Next day I went down to O'Reilly's and got anew speed sensor that single bolts to the side of my transmission. The old one looked fine and the gear didn't appear to be worn AT ALL!
I did have to put the old gear on the new part, because it doesn't include one.
I've checked all fuses I traced the wiring back as far as I can without getting ridiculous and Ican't find any imperfections.. Is there a ground wire that I could check? Another fuse or relay?

I can't drive this car properly without aspeedometer.. IT SUCKS just having a tach.

Is there anyone that has had this problem or is seen it before that can help me!?


It consistently reads about 50mph too slow..
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
2000 GT Mustang with a 5 speed manual.
Next day I went down to O'Reilly's and got anew speed sensor that single bolts to the side of my transmission. The old one looked fine and the gear didn't appear to be worn AT ALL!
Just wondering. Has there been a transmission swap done on this car? Perhaps a transmission from an older car?

I suspect this is the case. Why? Because the 1999+ model year mustang uses an OSS style sensor. An OSS sensor is a non contact sensor that uses a reluctor wheel trigger.

The 1998 and older model year Mustang uses a VSS style sensor. The VSS sensor is driven by a worm gear.

The the output of the VSS sensor is NOT compatible to the OSS sensor. The sensors are wired differently and are powered differently. The sensor type also needs to match what the PCM is expecting.

Seems possible to me that someone has re-pinned a VSS style speed sensor into a OSS application. Or has used a VSS transmission wiring harness in an OSS application.

Here's' some more information that may help you to understand the difference between a VSS and OSS sensor.

http://www.allfordmustangs.com/forums/4-6l-tech/140694-t-45-tail-shaft-swap.html
 
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Dillon Terry

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Sep 18, 2016
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Just wondering. Has there been a transmission swap done on this car? Perhaps a transmission from an older car?

I suspect this is the case. Why? Because the 1999+ model year mustang uses an OSS style sensor. An OSS sensor is a non contact sensor that uses a reluctor wheel trigger.

The 1998 and older model year Mustang uses a VSS style sensor. The VSS sensor is driven by a worm gear.

First off, Thank you so much for your info Burns!.

So do I need to buy a different gear or sensor? replace my pcm?
To help you tell exactly what I have, I can post some pictures today and show you my transmission and the sensor.
 

Dillon Terry

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Sep 18, 2016
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Oops still getting the hang of this, I replied on the bottom of the quote above.

Ill post some pictures today and show you.
 

Dillon Terry

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Sep 18, 2016
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When I went to O'Reilly's and told them the year of my car they gave me this sensor and it was the same as the old one I took out.
So are you saying that O'Reilly's gave me the wrong kind of sensor and the one that was in my transmission to begin with was wrong as well?. Wmburns
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
In your opening post you mentioned that "the gear was not worn at all". The OSS sensor is a non contact sensor the same as the ABS sensor and the crank sensor. As such, it will not have a gear on the end.

The VSS sensor is a contact sensor that is turned by worm gear on the tail shaft.

As far as figuring out who did what, there's already a problem if trying to fit a VSS sensor on a 2000 model year Mustang. It's up to you to figure out if it's a case of someone not understanding the difference or using the wrong transmission harness.

Note, After a certain model year Ford standardized on the number of OSS teeth across the product line. If a late model OSS style transmission is placed into an early model year Mustang, the speedometer will work but it will be off by a FIXED numeric RATIO.

Reading very low or very high is a prime symptom of a mis-wired speed sensor. The other clue is always being off by the some amount (50 MPH).

Here's some more information on the differences between a VSS and OSS sensor.
http://www.allfordmustangs.com/forums/4-6l-tech/140694-t-45-tail-shaft-swap.html
 

Dillon Terry

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Sep 18, 2016
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So do I have to replace my tail shaft with an OSS style to get my speedometer to work? Or can I just get a different VSS gear with more or less teeth to make it better?
Is it possible to remedy this by anything other than a tranny tail swap? ..
Wmburns
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
As the electrical output of the VSS sensor is NOT compatible to the OSS sensor, no combination of VSS gear will make the VSS sensor "just work" in an OSS application. Think about it. For a VSS gear change to stand a remote chance to work, the speedometer would have to be off by a numeric RATIO.

Suggest researching "Extended range Speedcal". But be forewarned the most versions of the Speedcal allow a late model transmission (OSS style) to be used in an older car (VSS style).
 
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