Allison shifting

[Jackal1200]

2001 2500HD 4WD 52k Here's one.Service engine light on.Limp-home mode.Code#700 & #702 for gear select module.Restat truck a few times service light out shifts fine.How odd?

[madmatt]

Have you ever replaced the NSBU switch? It sounds as though you need one.

[Jackal1200]

Thats the direction I am heading. Thank you

[madmatt]

Get one from an Allison dealer and save yourself some cash. it'd be worth while to replace it w/ the updated switch if it hasn't yet been done even if that turns out not to be the problem. I'd start there anyway and save paying someone to diag it. I'm sure you can find instructions here on how to replace it.

[Driveshaft]

A little off topic, but it is shifting. Can anyone tell me how the 6 speed alli downshifts on 6 to 5 in tow haul? Does it "manual" downshift like the 5 speeds on 5 to 4? Thanks in advance.