Speed problem

I don't know if your speedometer is mechanically connected or a wired connection to sensors on the vehicle or even the ECM etc. It is likely you would have to replace all of that.

 

That's a shop problem. PRobably a dealer issue where they can hook a big computer or even a dyno time for it to replicate the problem. I think it will help you to get it done. Or you might be exposed to a big ticket.

 

Where where you told to look? The speed should be taken from the rear output shaft/yoke of the transmission. If it’s mechanical there’s a cable there. If it’s electronic, there will be wires there.

Given the year of your truck I’m going to guess it’s electronic. And I’m also going to guess your needing a new speedo gauge. But I don’t know how to tell you to confirm that.

 

Likely a cabling issue.
Replacing will fix.
I've been wrong before ,though.

 

I could be wrong, but I'm thinking there is 2 speed sensors, and they both have to be hooked up. I know for a fact the one on the back of your tranny, by the yoke needs to be hooked up. I had my speedo mess up some on an international, but it would also turn the check engine light on any time I let off of it, it turned out to be the sensor on the throttle pedal. Mine worked but anytime you were giving it fuel, it would jump up and show 10 to 20 miles an hour fast, let off and it would show the right speed. The cruise would go wild when you tried to use it too. lol

 

My 13 spd in 2000 Freightliner has 2 , second one is on top at about 11 o clock. Controls the cruise also.I ran a long time without cruise, had no idea there was 2, odometer still doesn’t work, Figured I need a speedo, but wonder if a wires broken somewhere. Don’t know. Only time I had a similar problem as yours was after changing sensor,Too much gap. Don’t know exact setting, I think I bottomed it out and backed off 1 turn. I’m sure someone knows exact settings. Strange yours works/ wires unhooked? Maybe 2 nd one is working. Somethings wrong. Good Luck