Allison 5sp: Having speedo/tranny issues??

[harmison81]

I have some issues with my truck's speedometer jumping. Driving on the hwy get to speed and the speedometer starts to jump. About that time the ABS light and the brake light come on. Some of the codes are P0721, P0237, P0238. Put a new output sensor in gets better for about 20 to 30 miles and then the speedometer starts jumping again. Put new front abs wires in. No change. I also thought that I might of got a bad sensor so I got another one. Same thing speedometer still jumping. Wired a new plug in where the output speed sensor is. Got a new plug from GM. No change again. ???????? Need all the help I can get... Thanks

 

[TrentNell]

I have some issues with my truck's speedometer jumping. Driving on the hwy get to speed and the speedometer starts to jump. About that time the ABS light and the brake light come on. Some of the codes are P0721, P0237, P0238. Put a new output sensor in gets better for about 20 to 30 miles and then the speedometer starts jumping again. Put new front abs wires in. No change. I also thought that I might of got a bad sensor so I got another one. Same thing speedometer still jumping. Wired a new plug in where the output speed sensor is. Got a new plug from GM. No change again. ???????? Need all the help I can get... Thanks

I had a truck come in recently with similar issues but more trans related codes , after diagnosing it; it came down to a bad TCM . The P0237&P0238 are boost sensor codes , not related to the trans. For the speed sensor code if the speed sensor ohms out OK then check that the harness hasnt opened up or has a bad or damaged connector , a ohm test at the TCM can check this if you have the diagram . I have also seen some t-case problems cause out put speed sensor problems as well , but not sensor performance codes .