05YellowGT

Guy's , I got onto I95 last night and kicked it to merge with traffic. I got over into the fast lane and I looked at my speedometer and it said I was doing 100+ , but I knew there was no way I was by then , I might have been at first but not then. I have the aeroforce scan gauges and I pulled up the mph on it and it said I was about 65 mph as it should have been. When I would slow down the speedometer would drop down , but never go below about 40 mph even though the scan gauge would show the correct value. When I got home and shut the car off , the needle stayed around 40 mph instead of dropping down. Same thing when I left this morning , but after driving around for a while the needle finally started registering the correct speed and it now drops to zero when I stop. I looked at the shop manual and it says the computer sends a single to the speedometer to tell it how fast the car is going and since the computer knew the correct speed , I'm assuming it must be the speedometer. Any ideas ?

sook

Sounds like it just needs to be recalibrated. If the speedo thinks 40 is "0", then it would say that you were doing 100mph when you were only doing about 65. This is just an observation since its never happened to me. If I were at home, i would poke around the service manual to see if there is a procedure to recalibrate it. Maybe someone else can help with a solution.

krnpimpsta

How does your scan gauge pick up its data? Is it through the OBD-ii port by any chance? If so, that probably has something to do with it. Next time it happens, try disconnecting everything from your OBD-ii port and see if the gauges return to normal.

I have my SCT XCal2 "permenantly" connected to my OBD-ii so I can look at my engine specs at anytime through my computer. Once in a blue moon (sometimes weeks apart, sometimes months apart) my gauges will all go crazy or off. I have to disconnect my XCal2 from the OBD-ii, then disconnect it from the USB to my computer (to completely power it down), before I can plug it back in. My gauges return to normal immediately after I unplug the device from my OBD-ii.

SoFlaBoss

That's really odd what your speedo is doing. I have an aeroforce scan guage plugged into my OBD II port and I've even pulled my speedo needle off once to change my gauge faces, and I've never had the same problem. I quess it's possible that your needle is loose, but I'd tend to think it was more of an internal problem with the speedo. Sorry

05YellowGT

Well , it has returned to normal and this is the only time it has ever happened and it was an instantaneous thing when it happened. One second it was fine and the next time I looked it was reading high and wouldn't drop to zero. The scan gauges are connected to the PBD II port and yes they read what the computer thinks is the vehicle speed , which was correct. I know from problems an after market alarm system in conjunction with my S/C created how sensitive the computer is to changes in the electrical system. If it occurs again , I take your suggestion and unplug the gauges. I too replaced my gauge faces several years ago and had to remove the needle and replace it and I didn't quite get it at exactly zero. It reads about 3 mph above what the computer says the speed is and now that it's working again it's the same. So I don't think the needle is loose , it either had to be internal to the speedometer or the computer was sending the wrong signal. I'll have to wait and see if it happens again , here's hoping it doesn't ...

cave diver

I had the same thing happen in my 1999 Ranger and 2001 Olds. In both cases it was the speed sensor, a $13.00 part. In the Ranger it also caused my transmission to quit shifting and the ABS light to come on.

Arrow

I can never tell. My speedo goes "whacky" from light to light.

If you have another speedometer, put it in and see what happens. If it goes nuts too (and it's not your foot) then possibly the computer? I would assume that if the speed sensor was messed up, it would be messing up the computer, and yer 'Stang would be doing all kinds of weird things. So if the car is driving fine, but just giving you the wrong speed on the speedo, it's either the speedo or a fault in the computer's output.

05YellowGT

Dave , I assume the computer uses the speed sensor and it was showing the correct speed on the scan gauge , so I doubt if it's bad. Either the computer was sending a bad signal or the speedo was malfunctioning seems to be the two choises. Anyway , I'm hoping it doesn't occur again and just remains a mystery ...

Burke0011

Man that's weird - never heard of that happening before

Hope it doesns't pop up again man!

OneCoolGT

I have a strange problem with my speedo also, the needle never goes back to zero at a stop or when shut down. It goes down to the 2mph spot.. It's been this way since new and the dealer said it was normal...?!?! I want to pull the needle off and fix it, how do I go about taking it apart without breaking anything?

Thanks!

05YellowGT

The face plate assembly just pops off and then the gauge faces are exposed. If I remember correctly there are 4 screws that hold that assembly in place , then you just remove them to pull it out and disconnect the harness from the back. You could try doing it without removing the assembly , but it might be easier if you do. When I replaced my faces the kit came with a little plastic piece that you use to pry the needle off , it looks like a little flat fork with two prongs. Then reposition the needle where you want and push it back on. Mine is a little off right now , it sets slightly above zero but not enough I want to pull it off again to fix ...

steevr

Mine is also at 2mph and always ready 2 mph faster than the GPS. The odo is dead on though. I sure would like to have it at zero and also read correctly.

OneCoolGT

I am glad I'm not the only one with this problem... is it worth taking apart and fixing?

Dougxox

there is a spot on my way to work for about a half mile where my speedo on the Z2 goes nuts... increases but does not decrease when i slow down, so will hit about 100 to 120... resets after a certain curve. just the GPS not getting signal through the trees i guess.
Have had no problems with the in dash speedo.