95 Freightliner fld120 - speedo not working correctly

speedo sensor is on the back of the trans near the output shaft. check the wires you can pull it out and clean any gunk off the end of the sensor as well and see if that helps

 

X2 speed sensor

 

Check that the yoke on the rear of the transmission is tight. It sounds to me that you may have a wiring issue to the speed sensor, check for green in the connector. Speed sensors generally don't fail intermittently, possible, but they usally just stop working when they fail.

Intermittent issues are usually wiring or a loose trans yoke, or the speedo head itself could be bad.

You may have a dual sensor, meaning 1 sensor with 2 plugs. 1 is for the engine ECM, the other is for the speedo. If you do, just swap plugs, if the speedo starts working, it's the sensor. If the speedo works on the other plug, then the engine ECM will be getting a bad signal. That could cause a check engine light and depending on the perameter settings cause a derate.

 

I changed the yoke at transmission end and when I was leaving the shop I noticed my speedometer doesn't work I told the mechanic, he went down and switched the plugs I guess. Later it was raining a lot and my cruise started to have hickups. I switched plugs again and my speedo didn't work, so I put plugs again as before. Now I got check engine light,and again u switched plugs. Will see if cruise will work.
Also yesterday I got new sensors with two plugs it looks indetical tried to install it and the old one seamed loose in place and I noticed that my speedometer was moving bit erratically. Anyhow I changed it in rush, did few yards didn't notice my speedometer works so I put old one back, got it tied nicely and speedo worked but cruise has hickups, then now check light. Does it matter which plug goes where or I can plug it which ever?