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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[John N.]

Well Gentlemen, I can't thank you all enough for your depth of knowledge on these machines.
Nail on the head. I found a used-but-good TCM locally, put it in, and every single issue disappeared. All codes cleared themselves.
I test drove the truck on the highway, all shifting as per normal. Heck ya!! Thank you, Thank you, Thank you.

When you hear hooves running, don't expect to see a heard of zebras. I'll check this thing first next time...

 

[Lennydmaxguy]

Glad you fixed it. Always a good feeling.

I'm going to be the odd guy here and say I really like 6 speed conversions. But only if you can control the calibration yourself. I've done a couple myself and would do it again. If you're up to a little challenge, you can build your own setup. You just need:

1. A 6 speed valve body. Any will do, I like to keep it simple and use the LBZ/LMM design
2. Internal wiring harness connectors to match the VB. On the LB7 truck, you'll also need a LLY internal harness
3. Again LB7 specific, you need to run a wire from the TCM to the transmission for the G solenoid
4. A TCM calibration to make 6th gear happen. I have several unlocked versions if you're interested

 

[Bdsankey]

Glad you fixed it. Always a good feeling.

I'm going to be the odd guy here and say I really like 6 speed conversions. But only if you can control the calibration yourself. I've done a couple myself and would do it again. If you're up to a little challenge, you can build your own setup. You just need:

1. A 6 speed valve body. Any will do, I like to keep it simple and use the LBZ/LMM design
2. Internal wiring harness connectors to match the VB. On the LB7 truck, you'll also need a LLY internal harness
3. Again LB7 specific, you need to run a wire from the TCM to the transmission for the G solenoid
4. A TCM calibration to make 6th gear happen. I have several unlocked versions if you're interested

What VB mods are required?

 

[Lennydmaxguy]

What VB mods are required?

Other than swapping the complete valve body for a newer design from a 2006-19 truck, nothing needs changed. I'm not up on what exactly is different inside the VB, but I know for a fact that a LLY VB doesn't work, in spite of the fact that it has a G solenoid already there.

 

[Cougar281]

The G-Sol isn't needed.... It's an emissions thing that reduces line pressure and increases mileage by 0.000000002 mpg. Don't bother running a wire for it and just disable it in the tune.