[Chris8264]

Hello guys I need help with my car. I have a 2004 mustang v6 3.9 with an automatic transmission. My speedometer shows 120 mph when I’m only going around 30 mph. When the car hits 120 mph car feels like it wants to stall and won’t go any faster. I’ve checked for fuses and everything is good. I changed the transmission speed sensor and it still does the same thing. Any help would be appreciated Thank you!

 

[crjackson]

Welcome and thank you for joining ALLFORDMUSTANGS!
Please read the Site Rules if you haven’t already.
We encourage you to complete your Account Settings.
If you need help posting, please read this FAQ.

We’re happy you have chosen to be a part of our community.

Someone will be along shortly to offer advice.

 

[Cobrajet67]

Hello guys I need help with my car. I have a 2004 mustang v6 3.9 with an automatic transmission. My speedometer shows 120 mph when I’m only going around 30 mph. When the car hits 120 mph car feels like it wants to stall and won’t go any faster. I’ve checked for fuses and everything is good. I changed the transmission speed sensor and it still does the same thing. Any help would be appreciated Thank you!

If you replaced the speed sensor then maybe there's a wiring issue in the sensor circuit like a short to ground or power or else something is likely wrong with the PCM or instrument cluster. There's a stepper motor in the cluster that moves the needle. It sounds like the PCM thinks that you're going 120mph so it triggers the speed limiter. I would think that the speed sensor would be the issue in most cases like this but I would look over the wiring going from the sensor to the PCM before sending out the PCM and cluster to an automotive module repair service. I would send them both out to have them tested and repaired if necessary. Automotive Circuit Solutions, Autoecm, Upfix, and Module Master are reputable places worth considering. They'll test the PCM and cluster and if needed then repair them as well.
The only other thing that I can think of here is that you have a bad power or ground issue in the system somewhere but you should have other weird electrical issues going on as well if this were the case. Do you know if this car has an aftermarket tune in it?