LB7 Trans issue

[John N.]

I've had some shifting issues start happening, infrequently at first, now the truck won't shift into gear at all. Limp mode, no reverse, shift range inhibited, random up and down shifts.
2003 LB7 with Allison transmission. Codes PO700, 711, 748, 778. Seems all electrical. I've changed the NSB switch and found oil in the 20 pin connector, that I cleaned out. Still the same issue. I can't move the truck at the moment. Does anyone have a flow chart, wiring diagram, any suggestions for this old girl?? Need to get her back on the road soon! Help!

 

[kidturbo]

I've seen a couple of those early TCM's flake out in ways that will make ya chase your tail. Checked one on a buddies 03 a while back, and swore it was a wiring issue. He stumbled onto a good TCM local, and quickly called to tell me that fixed it. He had replaced the switch, valve body harness, sensors, while I saw TCM spouting data pointing to open circuits...

 

[John N.]

Ok, I'll see if I can get my hands on another TCM just to test. I've got a new switch, 20 pin connector seal, NSB wiring harness ready to go in next, hopefully not just a waste of effort, if the TCM is bad..

 

[Bdsankey]

Unplug the transmission at the 20pin, does the truck move?



If yes, it's electrical of some type.
If no it's internal/hard parts related.

 

[PureHybrid]

Like Brad said, if you unplug the main harness it will go to a default mode and give you 3rd / Reverse if everything mechanically is in order.



And I've also seen recently where a TCM was causing open circuit codes and shifting problems

 

[John N.]

Wow! Great test! Yes, the truck goes into both reverse and 3rd gear with the 20 pin disconnected.

I literally just changed the pressure switch and internal filter while I was there and had all the same symptoms. Then I saw this.
The temperature was bouncing like a ball immediately after start up. TCM must have called it quits as you guys mentioned. Are they still available or just from the wreckers? I saw the update to the LLY 6 speed. Is this a better alternative for a totally stock truck?

 

[PureHybrid]

6 speed conversions are hardly worth the price. No new TCM, might find a module repair specialist or get a reman from someone

 

[TheBac]

Look on car-part.com in your area under "transmission computer".

 

[Bdsankey]

I have a few TCMs. With that, I would highly recommend against a 6spd conversion.

Personally I’ve never been happy with the shift quality. They do not financially pay for themselves with MPG improvement, they improve cabin noise that’s about it.


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