08 Cummins 6 speed automatic.

Noticed last summer pulling fifth wheel slow on a steep grade that it didn't want to downshift into 1st and 2nd almost seemed to be slipping. Also when it doesn't want to downshift the jake doesn't work either. If I stopped, it would lock into 1st and pull fine. Shifting seems ok.

Now yesterday I was pulling two loaded wagons with it and when I took off I realized that it seemed to be slipping. I believe I was in 2nd. So I stepped on it a little more and sure enough it was. The odd part is, the speedometer sped up when I stepped on it but the pickup didn't. How is that possible?

2nd odd thing. So I eased into it up to about 25 mph and later decided to downshift and see if I could make it do it again. When I hit the downshift button it showed that I was in 4th. So I hit it again, it changed to 3rd but did not downshift. Hit it again it downshifted and showed 2nd. Hit it again it downshifted and showed 1st. However I was going too fast for it to be 1st. About 15 mph and between 1500 and 2000 rpm. I'm about positive it was not showing correct gear or was not shifting when it was supposed to.

Is there a clutch after the speed sensor? TIA

 

You have a transmission problem and are in limp mode. I suggest that you have the TCM scanned for codes.

 

What defines limp mode? Will it not give me a check engine or other warning? So far there's been nothing. Am running a smarty jr. Will it show codes?

 

1st and 4th gear only. Trans is trying to protect itself... You should have a CEL and at least get a P0700 code in the ECM that says there is some issue in the TCM. You can use the Smarty to scan the ECM and see if anything comes up there, but it won't show you the specifics in the TCM. You will need to have that scanned separately.

 

It's never went into 1st and 4th only and as I said earlier no code on overhead. Will check smarty and get TCM scanned when I get a chance.