Petericaza

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my speedometer doesn’t work but every other gauge does on my 99gt cluster my car was a 2004 v6 swapped with a 99gt everything I don’t know how to fix this speedometer problem my obd2 read the car is going 17 kph max even when we are on the highway
 
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tsemmett

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Have you checked the speed sensor as 08GT500 suggested? The fact that OBD doesn't have the speed means the ECU doesn't either.
 

Sluggie24

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Is it an automatic trans? If so 99-03 read vehicle speed differently than 04 did due to a change in the transmission.
 

tsemmett

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You said "my obd2 read the car is going 17 kph max even when we are on the highway", which is about 10 MPH, the lowest reading on the speedo.

Did I misread that, and OBD is showing the correct speed? If this is the case, check the sending unit on the transmission.
 
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tsemmett

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It may be that the speedo should move, but that is also right at the bottom of the speedo (mine starts at 10mph), and they aren't 100% accurate. Could be that it's just low enough not to actually show on the speedo.

In any event, the PCM controls the speedo; if it thinks you're only doing 17 kph at highway speeds, something between it and the speed sensor is off.

I do believe the speed sensor is on the driver's side, but it should be the only electrical connection on the tail of the trans; on my T45. You might confirm it's even the right type (if the trans is 98 or older it wouldn't be for instance).
 

Petericaza

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I’m not sure what trans I have that’s the problem I know the 98 and older have some type of cable
It may be that the speedo should move, but that is also right at the bottom of the speedo (mine starts at 10mph), and they aren't 100% accurate. Could be that it's just low enough not to actually show on the speedo.

In any event, the PCM controls the speedo; if it thinks you're only doing 17 kph at highway speeds, something between it and the speed sensor is off.

I do believe the speed sensor is on the driver's side, but it should be the only electrical connection on the tail of the trans; on my T45. You might confirm it's even the right type (if the trans is 98 or older it wouldn't be for instance).
right I will check that is that is there any reals ya to tell if it’s the sensor or a wiring issue or do I just replace it
 

tsemmett

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The 96-98s still have an electronic sending unit, the difference is it's driven by a gear in the transmission, where the newer ones uses hall effect sensor/toothed wheel.

I'd probably ID The trans first; there should be a metal tag on the transmission with the model and part numbers. Check this for more: Identify Transmission Code
 

Petericaza

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The 96-98s still have an electronic sending unit, the difference is it's driven by a gear in the transmission, where the newer ones uses hall effect sensor/toothed wheel.

I'd probably ID The trans first; there should be a metal tag on the transmission with the model and part numbers. Check this for more: Identify Transmission Code
If I have a 96-98 transmission would I have to get a 96-98 gauge cluster and have it reflashed ? Or...
 

tsemmett

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A 96-98 cluster is not PCM controlled, so it's not quite that simple. Find out what tranny you have first and let's go from there; it could be something much simpler. ;)
 

Petericaza

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I could not find the tag for the transmission however I did the next best thing a visual inspection and it looks like it’s a t45 transmission down there
 

wmburns

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For the 1999-2004 model year the output of the transmission OSS speed sensor goes directly to the PCM and only the PCM. The PCM de-codes the speed and the TELLS the cluster what speed to display via SCP data packets.

Soooooooooooooo if you want to trouble shoot this, the process begins with finding out IF the PCM "knows" the correct speed.

IF the PCM has an incorrect speed THEN the problem is between the OSS sensor and the PCM or in the PCM programming.

Otherwise the problem is in the cluster itself or in the data communication between the PCM and the cluster. OBTW, if the cluster displays the correct mileage and the rest of the cluster works, IMO it's reasonable to assume that the cluster to PCM data packets are working.

Here's some information on how to trouble shoot and/or repair a cluster. However, I recommend having the cluster professionally repaired if needed. The fee is not really that high and the guys that do this sort of work on a regular basis will be more accurate. I have had great luck using united speedometer and electrical.

My odometer goes on and off?

Here's also a case where a powerful ODB2 scanner with bi-directional capabilities can provide a ton of useful information. The ODB2 below can also talk directly to the cluster to "learn" what the cluster believes it's view of the world is.

ForScan ODB2 scanner w ELM327 USB

NOTE2. Mustangs older than 1999 use a VSS style transmission sender. The VSS sensor output is NOT compatible to an OSS sensor. The signal is different. The wiring to a VSS sensor is also different. If someone has mixed/matched parts from a 1998 and older Mustang with a 1999 Plus Mustang this could explain some of the problems. The type of speed sensor used in your application is easy to see. A VSS sensor is driven via a worm gear. Where as an OSS is a non contact sensor similar to the ABS tone ring.
 
Last edited:

wmburns

SN Certified Technician
Aug 14, 2009
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Re-post of information in another thread to keep all of the speedometer related trouble shooting together.

As stated in other posts, IF the speed is incorrect at the PCM, THEN it will be incorrect at the cluster. Since the PCM speed is obviously wrong, the focus should switch to:
  • The transmission speed sensor
  • the wiring between the speed sensor and the PCM
I have answered tons of posts over the years from people that have mixed/match various parts from different model years "assuming" that all Mustangs are the same (it bolts up fine so it must be OK). One of the frequent threads that come up is when someone swaps a transmission with a VSS sensor into an application that is expecting an OSS sensor.

The T45 does support both an OSS sensor or VSS sensor depending upon which tail shaft was used. You need to find out WHICH speed sensor you have. IMO one way to do this is by removing the speed sensor and looking down into the hole. IF you see a worm gear, then the sensor is a VSS style. If you see reluctor wheel teeth, then it's an OSS style. Here's is some more information to help you to understand the difference.

T-45 Tail Shaft Swap
T-45 Tail Shaft Swap - Ford Mustang Forum
T-45 Tail Shaft Swap 4.6L Tech
www.allfordmustangs.com

The next set of trouble shooting depends upon WHICH sensor is in the transmission.
 
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